Powdery mildew, one of the most easily recognized of all plant diseases, is characterized by the presence of a whitish, powdery mildew growth on the surfaces of leaves, stems, and sometimes petals. The fungal threads and the spores (which develop on short, erect branches) are visible with a strong hand lens. Under some conditions, however, the threads are so sparse that the mildew can be detected only by examination under strong light with a good lens or dissecting microscope. In some cases, the mildew develops only in small areas in which the leaf cells are killed and turn black.

The mildew spores are easily detached and carried by air currents to surrounding plants where they initiate new infections. On some plants, such as grape ivy, rose, and delphinium, the young foliage and stems often become severely distorted in addition to being covered by the whitish mildew growth.

Seriously affected plants may be of little value as cut flowers or potted plants. Poinsettia crops are now threatened by a powdery mildew disease. The disease develops rapidly during the fall. While scouting for whiteflies on poinsettias, also watch for powdery mildew colonies on the upper or lower surface of older leaves. At times a yellow spot on the upper leaf surface may indicate a mildew colony growing on the undersurface. Pick off affected leaves and initiate fungicide treatment immediately.

Bioenvironmental Control

Unlike the spores of nearly all other fungi, powdery mildew spores can germinate and initiate infections at humidity levels far below those commonly encountered in the greenhouse. Development of mildew following infection, however, may be more rapid and luxurious at higher humidities. As a deterrent to mildew in greenhouses, ventilation and heating should be adjusted to avoid high-humidity conditions. Irrigate plants early in the day. Heat at least one hour before sunset, and provide adequate ventilation. Horizontal airflow systems assist in management of powdery mildew.

Chemical Control

Under some conditions, fungicides are essential for mildew control. Systemic and nonsystemic protectant materials are available for spray application (see Table 6.1).

5.1.2 Botrytis Blight

The common gray mold fungus, Botrytis cinerea, attacks a wide variety of ornamental plants, probably causing more losses than any other single pathogen. The fungus causes a brown rotting and blighting of affected tissues. It commonly attacks the stems of geranium stock plants and wounds on cuttings. As a result of Botrytis infection, very small seedlings can be rotted; stems of poinsettia, snapdragon, zinnia, exacum, or lisianthus can be girdled; and petal tissues of many plants, including carnations, chrysanthemums, roses, azaleas, and geraniums, can be spotted and ruined. The fungus is usually identified by the development of fuzzy grayish spore masses over the surface of the rotted tissues, although such sporulation will not develop under dry conditions.

Spores of Botrytis are produced on distinctive dark-colored, hairlike sporophores and are readily dislodged and carried by air currents to new plant surfaces. The spores will not germinate and produce new infections, however, except when in contact with water, whether from splashing, condensation, or exudation. Only tender tissues (seedlings, petals), weakened tissues (stubs left in taking cuttings, tissues infected by powdery mildew), injured tissues (bases of cuttings), or old and dead tissues are attacked on most crops. Active, healthy tissues, other than petals, are seldom invaded. Petals shed from crops in hanging baskets may encourage Botrytis leaf infections on the crops grown below.

Bioenvironmental Control

Because high humidity is required for spore production and actual condensation is necessary for spore germination and infection, Botrytis can usually be controlled by watering early and by heating and ventilating to prevent any condensation on the plant surfaces. Because the fungus readily attacks old or dead tissues and produces tremendous quantities of airborne spores, the importance of strict sanitation cannot be overemphasized. All old blossoms and dead leaves should be removed, and all fallen leaves and plant debris on or under the benches should be gathered and burned.

Chemical Control

Fungicides may be required under some greenhouse conditions, especially with highly susceptible crops such as exacum, geranium, poinsettia, lisianthus, bacopa, osteospermum and fuchsia. Fungicide resistance is reported for Botrytis cinerea (benzimidazole and dicarboximide materials).

5.1.3 Root Rot Diseases

Rhizoctonia, Phytophthora, and Pythium not only cause damping-off of seedlings but together with Thielaviopsis are very important in causing root and basal stem rots of older plants. These pathogens are common inhabitants of soil and attack a wide range of plants. They are spread by the mechanical transfer of mycelia, sclerotia, or resting spores in infested soil particles (on flats, tools, pots, baskets, or on the end of the watering hose) or infected plant tissue.

Whereas sanitation measures are effective against all the root rot fungi, as well as water molds, fungicides are more specific in their control benefits. The most important control measures are (1) the use of a light, well-drained mix, (2) thorough pasteurization of soil mix as well as disinfestation of the containers, tools, and benches that come in contact with the plants (see Tables 3.3.1 and 3.3.2), (3) the use of clean plants, (4) the enforcement of a sound sanitation program, and (5) the use of supplementary fungicide drenches to minimize recontamination. (See Table 3.3.2.)

5.1.3.1 Pythium Root Rot

Pythium causes a dark brown to black wet rot that makes roots soften and disintegrate. It typically attacks below the soil surface and may extend up into the base of the stem. For most effective management, because fungicide resistance has been observed, alternate two fungicides during production of crops such as poinsettia, geranium, and lily.

Bioenvironmental Control

Pythium is favored by cool, wet, poorly drained soils. Using a well-drained mix with sufficient air pore space and avoiding excessive levels of ammonium or soluble salts will minimize Pythium losses.

Chemical Control

Many Pythium strains are not sensitive to metalaxyl or mefenoxam (in Subdue and Subdue MAXX). Switch to other fungicides if Subdue appears to be ineffective (see Table 6.1).

5.1.3.2 Phytophthora Root and Stem Rot

Phytophthora may cause root rots, but often it attacks the stem base as well, girdling the stem and leading to wilting and death. In recent years Phytophthora diseases have been especially common on fuchsia, calibrachoa, verbena, pansy, gerbera and poinsettia.

Bioenvironmental Control

Phytophthora is favored by deep planting and by wet, poorly drained growing mixes. Avoid growing Phytophthora-sensitive plants in ebb-and-flood benching because the zoospores can easily spread the disease from pot to pot. Remove symptomatic plants promptly.

Chemical Control

The same fungicides that work against Pythium will work against Phytophthora because both are oomycetes (commonly called “water molds”). Resistance to mefenoxam/metalaxyl (in Subdue and Subdue MAXX) has occasionally been observed in Phytophthoras isolated from greenhouse flower crops in recent years, so always be careful to rotate among materials for the control of the disease. Do not rely exclusively on metalaxyl/mefenoxam for Phytophthora control. See Table 6.1 for choices.

5.1.3.3 Rhizoctonia Root Rot

Rhizoctonia causes a drier root or stem rot. Affected tissues are brown or tan. It is favored by an intermediate range of moisture, neither too wet nor too dry. Cankers formed by Rhizoctonia usually appear at the soil line; roots are also sometimes affected in peat-lite mixes.

Bioenvironmental Control

Rhizoctonia disease is often favored by warm temperatures, so losses will typically occur during spring bedding plant production and summer pot plant propagation. Good sanitation practices are the primary defense against Rhizoctonia.

5.1.3.4 Thielaviopsis Root Rot

Thielaviopsis causes a drier stem lesion than Rhizoctonia, one that soon turns black because a large number of black spores of the fungus are produced in the lesion. It may also cause a very black root rot. In recent years most losses have been seen in pansy, vinca, and calibrachoa. High pH and poor drainage encourage Thielaviopsis root rot. Losses have also occurred in hanging baskets of fuchsia grown at high pH (6.5–7.0) and in poinsettia crops. Pansies, violas, and calibrachoas are very susceptible.

Bioenvironmental Control

The disease is rarely a problem in growing media adjusted to pH 4.5 to 5.0.

5.1.3.5 Damping-off Disease

Damping-off of seedlings, which is caused mostly by water molds and fungi, can be a complex of several diseases occurring separately or simultaneously. Most commonly, either Rhizoctonia or Pythium is involved. Botrytis, Sclerotinia, and Alternaria are also occasionally responsible for damping-off.

Preemergence Infection

Rotting of seeds or attacks on seedlings before emergence are commonly caused by a water mold such as Pythium or Phytophthora.

Postemergence Infection

Rot developing at the soil line after emergence, which causes the seedling to topple, is most commonly caused by Rhizoctonia. This is the conspicuous type of damping-off most frequently reported by growers. Older seedlings may be infected at the soil surface and yet remain upright. Transplanted seedlings remain hard and stunted and eventually die. In some cases, water molds (such as Pythium) invade the rootlets at the tips and progress upward to the stem, whereupon the plant dies.

Cultural Control

For all practical purposes, Rhizoctonia and Pythium do not have an airborne stage. Therefore, spread of both pathogens depends primarily on the mechanical transfer of mycelia, sclerotia, or resting spores in infested soil particles (on flats, tools, baskets, or in the end of the watering hose) or infected plant tissue. Thus, if soil or another medium is steamed or chemically treated and care is taken to prevent recontamination, damping-off should be of little significance. Sowing seed in a layer of screened sphagnum, vermiculite, perlite, peat-lite mix, or other sterilized material also helps. However, some peat moss used in peat-lite mixes may carry these pathogens, and the seed itself may occasionally carry damping-off pathogens. Fungus gnats and shore flies are also able to spread damping-off pathogens within the greenhouse.

Chemical and Biological Control

Fungicide-treated seed is available for some crops. To avoid plant injury, it is best to rely on careful sanitation practices rather than on fungicide drenches to protect crops until after the seedlings emerge. Preplant mix incorporation of granular fungicide formulations may lead to phytotoxicity problems with some seedling species, particularly if ingredients are not well distributed through the soil mix. Use of biological controls in the plug trays may help with disease prevention.

If experience has shown that particular plant species are plagued by damping-off, make spot applications of appropriate fungicides to just those species (see Table 6.1).

When making preventive fungicide treatments, pay careful attention to the appropriate dosage delivery for the size of the container and recognize that fungicides labeled for ornamentals are often not registered for use on vegetable seedlings.

5.1.4 Bacterial Blight of Geranium

Bacterial blight is caused by Xanthomonas campestris pv. pelargonii, which can cause leaf spots as well as systemic infections in geraniums. Leaf symptoms are either an overall tiny spotting (1/16 –1/8 in. diameter) or a wedge-shaped yellow area often followed by leaf wilting. The disease can cause black dieback of growing points and stem cankers at the base of the petioles. In hot, humid weather, the bacteria spread from infected leaves into the stem, becoming systemic and killing the plant.

Zonal and ivy geraniums (Pelargonium X hortorum and P. peltatum) are most likely to develop symptoms of this disease; a few cases of leaf spots on Regal geraniums have been observed. Hardy Geranium species may be a source of bacteria that can cause disease on greenhouse crops of Pelargonium species. Geraniums grown from seed can become badly diseased if they are grown with an infested cutting crop. Plants in families other than the Geraniaceae are not susceptible.

Xanthomonas-free material for cuttings is assured through careful culture indexing. Culture indexing is performed by specialists and involves removing thin slices obtained aseptically from the base of a cutting and placing the slices in a nutrient medium. Cultures of nutrient media showing any fungus or bacterial growth are discarded along with the cuttings from which the slices were removed.

Cultural Control

Grow culture-indexed cuttings only, and grow stock plants using individual tube watering systems. The organism is easily spread by splashing water. Subirrigation may spread the disease from root system to root system. Keep stock from different suppliers separate, and grow seedling geraniums separate from cutting crops. Do not hang ivy geraniums over a bench or floor crop of geraniums. Do not grow hardy (perennial) Geranium species near greenhouse crops of Pelargonium spp. Rogue out symptomatic plants immediately.

5.1.5 Southern wilt (Ralstonia solanacearum)

Ralstonia solanacearum is a bacterium that has various races, including Race 1, which is endemic in the southern United States and has a very wide host range. Race 1 has been reported to cause disease on many flowering plants, including geranium (Pelargonium x hortorum). It is seen occasionally in greenhouse production, particularly in the South. Another type of R. solanacearum, Race 3 biovar 2, is not known to occur in the United States or Canada and is of great concern to the potato industry. It is widely distributed in warmer climates elsewhere around the world, where it causes a very serious disease on potatoes. Race 3 is also a pathogen of tomato, eggplant, and several weed species. In recent years, geranium cuttings produced in Kenya, Costa Rica and Guatemala were found to carry this race of R. solanacearum, resulting in a massive eradication effort in U.S. greenhouses under a federal quarantine in 2003. To keep this pathogen out of the United States, a joint effort has been made by USDA, the geranium propagators, and the greenhouse industry. Clean stock production procedures have been carefully refined in offshore propagation facilities. Careful sanitation practices are also important for each individual grower. To minimize risk, keep geraniums from different suppliers and from different ship dates separate. Avoid hanging geraniums over other crops. Do not grow geraniums in the same greenhouse with tomato transplants. Do not subirrigate geraniums. Test kits are available to check on-site for the presence of Xanthomonas or Ralstonia bacteria in geraniums that have suspicious wilting symptoms. Identification of the particular race of Ralstonia may only be done at a USDA-approved testing lab; the grower-friendly test kits will only identify the organism as Ralstonia solanacearum, detecting Race 1 as well as Race 3. Yellowing and wilting of leaves is the only aboveground symptom caused by Ralstonia in geraniums, whereas Xanthomonas campestris pv. pelargonii will cause tiny, round, brown leaf spots (if the bacteria have been splashed onto the leaves), as well as wilting. Remember that wilting in geraniums is most often caused by Pythium attack on the root systems: seek help with diagnosis from a qualified laboratory rather than assuming that your crop is infected with Ralstonia.

5.1.6 Verticillium Disease

Verticillium spp. are fungi capable of infecting a wide variety of ornamental plants; some of the more important are chrysanthemums, China asters, snapdragons, roses, geraniums, and begonias. Symptoms vary with the host.

Snapdragons can appear completely healthy until blossoms develop; then the foliage can suddenly wilt completely. The conductive tissues of some varieties can turn brown or purple, particularly within woody stems.

With chrysanthemums, there is usually a marginal wilting of the leaves, followed by yellowing and eventually death and browning. Leaves remain attached and hang down against the stem. These symptoms commonly develop at first on only one side of the plant and only after blossom buds have formed. Young, vigorous plants usually remain symptomless.

The buds on one or two branches of red-flowered varieties of greenhouse roses turn blue and fail to open; the leaves and the green stem tissues may become mottled, and when the stem is shaken, the leaves fall from the plant and the stem dies. Additional shoots can develop from basal buds and go through the same sequence, though eventually a shoot may remain healthy. Usually no vascular discoloration occurs.

With semituberous-rooted begonias, some yellowing of leaf margins can occur, but the most distinctive symptom is the development of an extremely shiny lower leaf surface.

The symptoms thus are quite variable, but the most characteristic ones are one-sided development, wilting and yellowing of leaf margins progressing upward from the lowest leaves, lack of leaf and stem lesions, and normal-appearing roots.

The fungus causing the disease invades the soil and may persist there for many years. Initial infection usually occurs through normal roots, and the fungus grows upward through the water-conducting (xylem) tissues. Infected plants of some types (for example, chrysanthemums) are usually not killed by the fungus and, during periods of rapid vegetative growth, can appear symptomless.

Cultural Control

Cuttings taken from symptomless diseased plants can carry the fungus internally and introduce the disease to new areas. Obtain planting stock only from a reliable dealer, and purchase chrysanthemums and geraniums from propagators who culture index all nucleus stock (see “Bacterial Blight of Geranium” for a description of culture indexing).

Chemical Control

Plant only in soilless mixes or in soils that have been steamed or treated with chloropicrin to eliminate Verticillium (see Table 3.3.2).

5.1.7 Viruses

Viruses are submicroscopic infectious agents that consist of particles composed of protein surrounding genetic material (RNA or DNA). Because plants do not produce antibodies, they neither recover from a virus infection nor become immune. Once a plant is infected, it may remain infected for life, even though the symptoms of disease become masked. Thus perennial plants and vegetatively propagated greenhouse plants carry the virus from one crop to the next with continuing loss to the disease.

Currently, impatiens necrotic spot virus (INSV) is the most common and most damaging virus in the greenhouse industry. The virus has an extremely broad host range, and its insect vector, the western flower thrips, is widespread and hard to control. Tomato spotted wilt virus (TSWV) is also vectored by this thrips. For control suggestions, see the bedding plant section under “Diseases of Specific Florist Crops” and the descriptions given under the sections covering diseases of African violet, begonia, calceolaria, chrysanthemum, cyclamen, and gloxinia. INSV is also a problem on foliage plants as well as most crops other than roses and poinsettia. Geraniums have only rarely shown symptoms of the disease. Tuberous dahlias and chrysanthemums, along with many other flower crops, are susceptible to both INSV and TSWV.

Symptoms

The most common symptom of virus infection is stunting or dwarfing. Leaves may also show distinctive signs, most commonly color changes. Leaves may show spots, streaks, blotches, and rings of light green, yellow, white, brown, or black, or they may develop uniform yellow or orange coloration. Leaves also may change in size or shape, either puckering or developing rolled margins. Flowers may be dwarfed, deformed, streaked, or faded. These are only a few of the more obvious symptoms caused by viruses.

Spread

Generally, viruses are not transmitted through seed with the exception of tomato ring spot and tobacco ring spot, which affect geraniums. A bedding plant crop grown from seed can suffer serious loss if a virus disease (e.g., INSV) is introduced by an insect vector and has an efficient means of spread. The next year, however, the crop will again start clean.

Although viruses can spread unaided from cell to cell in one plant, they require active assistance to spread from one growing plant to another as well as a wound through which to enter the plant. Most frequently, viruses are spread by insects feeding on a healthy plant after feeding on an infected one, by grafting with a scion from an infected plant, or by using infected stock plants as a source of cuttings.

Insect control can be critical to virus control in greenhouses. The current difficulty in controlling western flower thrips, for example, creates a potential danger of widespread INSV or TSWV infections.

Indexing Program

The use of pathogen-free propagating material is extremely important in any disease control program. Virus indexing is used to eliminate viral pathogens from propagative material of chrysanthemums, carnations, geraniums, orchids, lilies, hydrangeas, and foliage plants. Culture- and virus-indexed material is initially free of internal pathogens that the index is set up to check; such plant material is not disease resistant, however, and requires a growing medium free of pathogens and good cultural practices if the full potential of healthy plants is to be realized.

Virus indexing makes use of indicator plants or serological assays for each specific virus because many cultivars can act as “sleepers,” that is, virus carriers that show no external symptoms. Sleeper varieties or cultivars are a tremendous threat because they can be responsible for a large amount of virus spread before the grower realizes there is a serious problem. When the plants are about to flower, the seriousness of the problem is realized in uneven plant growth and flowering time along with a reduction in flower quality.

Diseases of a few crops for which indexing programs have been developed are listed below.

Chrysanthemum Carnation

Viruses Viruses

Chrysanthemum stunt Carnation mottle

Chrysanthemum mosaics Carnation ring spot

Chrysanthemum aspermy Carnation mosaic

Chrysanthemum chlorotic mottle Carnation streak

Tomato spotted wilt Carnation etch-ring

Necrotic fleck

Vascular wilts Vascular wilts

Verticillium wilt Fusarium wilt

Bacterial blight Phialophora wilt

Bacterial wilt Bacterial wilt

Slow wilt

Geranium

Viruses

Tomato ring spot

Tobacco ring spot

Pelargonium flower break

Vascular wilts

Bacterial blight (Xanthomonas)

Southern bacterial wilt (Ralstonia)

Verticillium wilt

5.1.8 Nematode Diseases

Nematode problems affecting plant root systems are now quite rare, except in cases where soil is used as a component of the growing medium.

5.1.8.1 Root-knot Nematode

Root-knot nematodes may cause plants to appear stunted and unthrifty and to wilt on warm days. When the root system is examined, galls are generally conspicuous and easily recognized. Root-knot has been seen primarily on herbaceous perennials in recent years. On some crops, root-knot nematodes may cause crop loss even when only a few galls are evident. The presence of root-knot nematodes may also increase the amount of plant injury from bacterial and fungal diseases, or it may break the resistance of plants to these diseases.

Galled plants will not perform as well as healthy ones, but adequate moisture and fertility may mask the difference in vigor between nematode-infested and healthy plants.

Six kinds of root-knot nematodes are recognized in the United States today. All have been identified on greenhouse crops in New York State, although only the northern root-knot nematode, Meloidogyne hapla, survives outdoors. Thus the other five kinds are shipped into the state on plant material. The host ranges and host-parasite relationships may vary, but all have essentially the same life history.

Eggs of Meloidogyne are about twice as long as they are wide. They are usually found in a gelatinous mass about the posterior end of the female. Eggs hatch into small, slender worms (larvae) about 1/50 inch long. The larvae migrate through the soil seeking new roots, which they enter near the tip. Once inside the root, with its head located in what will become the vascular cylinder, the nematode does not change position. Stimulated by the nematode’s saliva, nearby root cells develop into giant cells, which provide nourishment. Other cells adjacent to the nematode enlarge and increase in number, forming the familiar gall or knot. After the giant cells are functioning, the nematode goes through three molts (sheddings of cuticle), becomes an adult female, and starts the cycle over. A female can lay as many as 2,000 eggs during her life, but the average is probably 200 to 500.

The temperature of the soil is critical in the development of the nematode. It takes about 17 days at 29° C (85° F) for females to develop from infective larvae to egg-laying adults, 21 to 30 days at 24° C (76° F), and 57 days at 16° C (60° F). Females fail to reach maturity at temperatures above 33° C (92° F) or below 15° C (59° F).

Spread within a greenhouse occurs through movement of infested soil or plant debris by workers, water, and possibly wind. Migration of larvae through the soil is limited to perhaps a few feet per year.

There is no known cure for root-knot nematodes. With continued care, infected bed or bench plants can produce a good crop. Discard infected potted plants carefully to prevent spreading the nematode. Preplanting treatments of steam or fumigants (see Table 3.3.2) effectively eliminate nematodes from soil, but be sure that infested crop residues are thoroughly decomposed.

5.1.8.2 Other Nematodes Affecting Roots

Other root-attacking nematodes can cause chlorosis and stunted and unthrifty growth of aboveground parts of the plant. Affected roots may be shortened, thickened, excessively branched to the point of becoming matted, and occasionally killed.

Control of Root-Attacking Nematodes

Steam disinfestation or chemical fumigation may be used to free soil from nematodes before planting.

5.1.8.3 Foliar Nematodes

Leaf (foliar) nematodes cause deformity of young growth, leaf spots, and defoliation. The spots are first discernible on the lower leaf surface as yellowish or brownish areas, which eventually turn almost black. Although the lesions are small at first, with favorable temperature and moisture they may spread until much of the leaf is destroyed. Unlike other nematodes, foliar nematodes do not persist in the soil in the absence of living host-crop tissues.

On chrysanthemum plants, the leaf veins retard the spread of the nematodes through the leaf, causing the lesions to be V-shaped or angular patches Infection begins on the lower leaves and progresses upward.

On Peperomia, Gloxinia, African violet, and Elatior begonias, the lesions are less definite in outline and infection may occur on any leaf.

Plants with foliar nematode infestations should be discarded promptly.

5.2 Diseases of Specific Floral Crops

Keeping floral crops free of disease requires constant care and planning. Prevention is the basis of freedom from disease and should be an integral part of the general cultural program. The symptoms of the diseases of major floral crops are described here. Instructions for use of commercial foliar fungicides and bactericides are given in Table 6.1.

5.2.1 African Violet (Saintpaulia ionantha)

· Corynespora leaf spot: round brown leaf spots with yellow haloes

· Root-knot nematode: Swollen, knobby areas on roots.

· Foliar nematode: Distortion and yellow areas on foliage.

· Powdery mildew: White dusty patches on foliage or flowers.

· Impatiens necrotic spot virus: Yellow mottle or brown spots in foliage or stunting of center leaves; may resemble cyclamen mite feeding injury.

5.2.2 Azalea

· Septoria leaf spot: Angular leaf spots, brown to purple in color.

· Foliar nematode: Angular leaf spots, which are easily mistaken for a fungal disease.

· Ovulinia petal blight: Brown discolored areas on flowers, beginning as tiny, water-soaked, round spots and progressing to flower collapse. More aggressive than Botrytis but could be mistaken for Botrytis flower blight.

· Rhizoctonia stem rot: Cankers form at soil line; particularly likely during propagation.

· Phytophthora root rot and wilt or branch dieback: A highly contagious and aggressive disease. Young shoots that are directly invaded wilt, eventually turn brown, and die. If infection is through the roots or crown (as is more common in New York), roots are softened and brown and/or a reddish-brown canker develops at the soil line. All or part of the plant eventually wilts and dies. A reddish-brown canker or vertical streak may be evident just beneath the bark on the lower stem.

· Cylindrocladium blight: May cause purple or brown leaf spots, as well as lethal root rot or cutting rot. Affected stem tissues turn reddish brown in the cankered areas. High moisture and temperatures of 27–29° C (80–85° F) are very conducive to disease.

5.2.3 Bedding Plants (Annuals and Perennials from Seed)

· Bacterial leaf spots: Bacteria (Pseudomonas sp.) can cause irregular water-soaked areas on leaves of plants including impatiens and New Guinea impatiens under wet conditions. Pseudomonas leaf spots on impatiens are variable in size and may blight the entire leaf; spots often start at the leaf edge. Zinnias are susceptible to a different bacterium causing angular tan spots with yellow haloes (Xanthomonas zinniae).

· Damping-off (pre-emergence): Seedlings fail to emerge because seeds are attacked after imbibition (usually caused by Pythium or another water mold).

· Damping-off (post-emergence): Seedlings collapse because of attack at the soil line (often caused by Rhizoctonia) or root rot (often caused by Pythium).

· Root rot: Plants are small in size, foliage has grayish or purplish cast, and roots are soft and brown. Most often caused by fungi that cause damping-off, or by Thielaviopsis basicola.

· Botrytis gray mold: Damping-off (seedling collapse) or tan to brown leaf spots, often irregular in outline and often zonate. Stems are also attacked.

· Alternaria leaf spot: Common on dusty miller, marigold, impatiens, zinnia, and geranium. Small purple to brown leaf lesions develop under conditions of warmth and high moisture. On impatiens, spots have a purple to black rim and a light center and are only a few millimeters in diameter. They are easily confused with bacterial leaf spot, or impatiens necrotic spot virus (INSV), but their uniformly small size and the tendency of the leaf to turn yellow help distinguish them.

· Impatiens necrotic spot virus (INSV): This tospovirus disease, spread by the western flower thrips, has caused devastating losses to some bedding plant crops in cases where the grower was unaware that the thrips population had skyrocketed. Young seedlings are more susceptible than older plants. Almost all crops are susceptible, but losses have been most dramatic in impatiens, particularly double-flowered varieties. New Guinea impatiens, begonia, coleus, browallia, and many other bedding plants are susceptible. Impatiens show black leaf spots as well as blackening of sections of the stem. INSV-infected plants may also show ringspots, mottling, or browning along the veins. Many species of infected seedlings are stunted by INSV infection. Vegetatively propagated flower crops in hanging baskets have often provided inoculum (and thrips vectors) for a seed crop grown below them. Flowering pot plant crops or weeds in the same greenhouse may also provide inoculum of INSV that may injure bedding plants. Begonias are often a source of the virus. Tomato spotted with tospovirus (TSWV) may also affect bedding plants, but it is far less common.

· Tomato and pepper transplants are especially vulnerable to the INSV relative, tomato spotted wilt tospovirus (TSWV). Keep vegetable transplants separate from flower crops, and grow plants from cuttings well separated from plants from seed. Not all infected plants show symptoms; symptomless plants (including weeds) may be the source of virus for crops that are highly susceptible. Discard symptomatic plants immediately and guard constantly against thrips population buildup to avoid INSV or TSWV losses. Monitor the thrips population with yellow or blue sticky cards, and initiate treatment if more than 10 thrips are caught per card per week, assuming three cards per 1,000 sq. ft. greenhouse. Maintain a strict weed control program. Beware of carrying over virus-infected stock plants from one production season to the next.

5.2.4 Begonia

· Damping-off: Collapse of young plants may be caused by Pythium, Rhizoctonia, or Botrytis.

· Botrytis leaf spot: Large, irregularly outlined brown leaf spots, particularly common on large plants given insufficient spacing; stems may also be invaded, leading to wilt of the cankered portion.

· Powdery mildew: Fuzzy white patches on leaves or flowers. In some cases, leaf tissue shows dark, greasy-looking spots beneath a sparse colony of powdery mildew. Begonias are highly susceptible.

· Bacterial leaf spot (Xanthomonas campestris pv. begoniae): Dark, greasy spots appear on leaves, or brown V-shaped dead areas develop at leaf margins. Spots are surrounded by a speckled or chlorotic zone of leaf tissue. With high temperatures, disease may progress until plants collapse. Certain Elatior begonias are particularly susceptible; Non-Stop begonias may show less extensive leaf spotting.

· Foliar nematodes (Aphelenchoides fragariae): Sunken gray-green blotches that turn reddish brown or black. Infected leaves may wilt, die, and hang limply from the plant. Elatior begonias are extremely susceptible to foliar nematodes.

· Impatiens necrotic spot virus (INSV): Yellow variegation or round brown spots in leaves, chlorotic mottling, brown streaking along veins, and brown patches in the leaf at the petiole end. Control of the vector, the western flower thrips, is essential.

5.2.5 Bulb Crops for Forcing

· Botrytis: Occasionally, when aeration is insufficient during forcing, Botrytis may cause lesions on foliage and cripple the expansion of leaves. Good cultural conditions prevent this problem. Bulbs and roots may also be affected.

· Tulip fire (Botrytis tulipae): Foliar lesions on tulip may be caused by a host-specific species of Botrytis introduced via sclerotia on diseased bulbs. Destroy infected plants and protect others with appropriate fungicides during foliage expansion.

· Iris ink disease (Mystrosporium adustum): Scales are blackened. Destroy bulbs in which fleshy scales are affected. Not common in North America.

· Bulb rots (Penicillium sp., etc.): Portions of bulbs are discolored, most often resulting from invasion of stressed tissue by weak pathogens. Avoid bruising or overheating during handling or storage. Phytophthora spp. may cause rotting of the stem, basal plate, and roots; bulb scales are not affected.

· Gray bulb rot (Sclerotium tuliparum): A dry rot with sclerotia sometimes evident.

· Flower break or mosaic: Interruption of flower petal color by white streaks or yellow mottling of foliage. Caused by virus infections. Aphid control is important for preventing spread.

5.2.6 Calceolaria (Calceolaria herbeohybrida, C. integrifolia)

· Impatiens necrotic spot virus (INSV): Pale patches in lower leaves enlarge and brown with time; young plants may be more severely affected. Control of western flower thrips is essential.

· Stem or crown rot: Stem collapse, possibly resulting from infection by Botrytis or Sclerotinia sclerotiorum.

5.2.7 Calibrachoa (Calibrachoa hybrids)

· Pythium root rot: Plants are stunted and may wilt. Roots are soft and mushy.

· Phytophthora root rot: Plants may grow normally until they suddenly wilt at flowering. The Phytophthora often attacks at the stem base.

· Thielaviopsis root rot: Plants are stunted, yellowed or purplish. Roots are also stunted and may be discolored. Infected plants will wilt.

5.2.8 Calla

· Soft rot (Erwinia carotovora or E. aroidae): A soft, mushy, foul-smelling rot of rhizomes or stem bases. Cut out rotted spots before planting. Place rhizomes in separate containers and water individually if possible to reduce disease spread. Use a well-drained growing medium. Water sparingly.

· Root rot (Phytophthora or Pythium spp.): Roots are decayed back to the rhizomes.

· Viruses: Small plants, chlorotic foliar streaking, flower distortion. Discard affected plants.

5.2.9 Carnation (Dianthus caryophyllus)

Note: Use culture-indexed plants, and steam-pasteurize or fumigate growing media and beds.

· Fusarium wilt (Fusarium oxysporum f. sp. dianthi): Stunting, wilting, foliar yellowing or browning, purplish vascular discoloration.

· Rhizoctonia stem rot: Brown canker at the soil line.

· Alternaria leaf spot (Alternaria dianthi): Ashy white spots with dark fungus spore structures on the center of older spots.

· Bacterial wilt (Pseudomonas caryophylli): Sudden wilting and drying of the top of the plant or of one branch. Vascular system shows yellow streaks. Root system also decays, and sticky cankered areas may appear at the base of the stem. Symptoms appear at high temperatures. Discard infected plants immediately. Fortunately, this disease is eliminated by culture indexing.

· Fusarium stem rot (Fusarium roseum): Roots and stem base may be rotted and discolored.

· Botrytis flower spot: First shows on outer petals on outside of bud.

· Rust (Uromyces caryophyllinus): Reddish-brown spore pustules on leaves and stems.

· Fusarium bud rot (Fusarium tricinctum poae): Fungus spores are carried to the buds by grass mites. Petals inside bud decay first.

· Greasy blotch (Zygophiala jamaicensis): Spots with radiating, spider web–like margins.

5.2.10 China Aster (Callistephus chinensis)

· Fusarium wilt (Fusarium oxysporum f. sp. callistephi): Stunted growth, yellowing or scorching of foliage, collapse. Vascular system discolored.

· Verticillium wilt: Same symptoms as Fusarium wilt. Disease not specific to China aster and may be severe in soil where other crops susceptible to Verticillium have been grown previously.

· Aster yellows: Caused by a phytoplasma carried by the aster leafhopper. Plants may be chlorotic and misshapen; flowers may be entirely or partially virescent (green).

· Rust: Pustules filled with orange-colored powdery spores on leaf undersides.

· Botrytis: Spotting of petals or leaves.

5.2.11 Chrysanthemum (Chrysanthemum grandiflorum)

Note: Use culture-indexed stock.

· Verticillium wilt: Interveinal chlorosis beginning on lowest leaves, followed by leaf death, or general wilting and death of the whole plant. Not common.

· Fusarium wilt: Stunting, purpling, scorching, or chlorosis of foliage; wilting and death follow. Vascular discoloration in the stem.

· Foliar nematodes (Aphelenchoides ritzema-bosi): Young growth at top of plant may be distorted by nematode feeding. More typically, brown wedge-shaped dead areas appear on lower foliage and the lower leaves turn brown and dry. Discard infested plants.

· Septoria leaf spot (Septoria obesa and S. chrysanthemella): Brown oval leaf spots; now fairly rare.

· Bacterial stem blight (Erwinia chrysanthemi): Wilting at top of plant; discoloration of vascular system. Break out cuttings instead of using a knife.

· Bacterial leaf spot (Pseudomonas cichorii): Brown to black irregularly outlined blotches on leaves may expand rapidly under moist conditions and spread to stems, buds, and other leaves if plants are crowded and watered from overhead.

· Ascochyta blight (Mycosphaerella ligulicola): Leaf spots, stem cankers, flower bud infection. Easily confused with symptoms of bacterial leaf spot or tomato spotted wilt virus, but less common than these two diseases.

· Botrytis: Petal spotting.

· Rhizoctonia: Brown canker at stem base.

· Powdery mildew: White fuzzy patches on foliage under humid conditions.

· White rust (Puccinia horiana): Yellow spots on upper leaf surface and, on the underside, raised tan spore pustules that are powdery white when mature. This disease is under federal quarantine regulation in the United States; report suspected cases to your state horticultural inspector immediately. Destroy infested plants.

· Brown rust (Puccinia tanaceti): Yellow spots on the upper leaf surface and brown pustules on the leaf underside. Not common in New York but known in the United States.

· Tomato spotted wilt and impatiens necrotic spot tospoviruses: Some cultivars are symptomless when infected; others show blackened midstem sections, chlorotic mottle or necrosis of leaves, or even wilting of the entire plant. Control of the vector, the western flower thrips, is essential for control of the disease.

5.2.12 Cineraria (Pericallis x hybrida)

· Impatiens necrotic spot and tomato spotted wilt tospoviruses (INSV or TSWV): Cause yellow spotting or blotching on lower foliage that may progress to necrotic spots that coalesce before leaf wilt. Blackened areas may develop on petioles. Leaves sometimes show a mosaic or ringspot pattern. Control of the vector, the western flower thrips, is essential.

· Botrytis: Petal blight or leaf spotting in excessively wet environments.

5.2.13 Coleus (Solenostemon scutellarioides)

· Impatiens necrotic spot virus (INSV): Round, brown leaf spots, zonate spots or rings

· Downy mildew: A new disease that is having serious effects on coleus in greenhouse production and in landscapes. Spotted, flecked, twisted, wilty or dropping leaves. Top growth may be stunted. Whitish growth of conidiophores sometimes visible on leaf undersurface.

5.2.14 Cyclamen (Cyclamen persicum)

· Anthracnoses (Gloeosporium and Cryptocline): round, brown spots on foliage or petals that spread via splashing.

· Botrytis: Petal spotting or petiole collapse from crown rot.

· Cylindrocarpon root, corm, and petiole rot (Cylindrocarpon sp.): Elliptical brown cankers at petiole base; brown discoloration inside corm; partial or entire wilting of plant. Remove infected plants. Sanitation is important.

· Fusarium wilt (Fusarium oxysporum f. sp. cyclaminis): Chlorosis of older leaves, followed by wilting and collapse of plants. Corm is firm but shows brown to purplish discoloration of vascular system when sliced open. Remove infested plants. Raise pH to 6.0, and avoid ammonium nitrogen fertilizers. Cultural controls to reduce wilt losses include using media with a pH of 6.0 or above, composted growing mix, and fertilizers low in ammoniacal nitrogen.

· Impatiens necrotic spot or tomato spotted wilt tospoviruses (INSV or TSWV): Leaves may show round brown or black spots, “fingerprints” (ring spots of brown or yellow), or brown patches at the base of the leaf. Stems may show black bands. Control of the vector, the western flower thrips, is essential.

· Soft rot (Erwinia carotovora): Soft, mushy decay of corm leading to plant collapse. Control fungus gnats, and plant shallowly in a well-drained mix.

5.2.15 Foliage Plants

Root-knot nematodes: Swollen, knobby areas on a stunted root system. Discard affected plants, pots, and soil.

Foliar nematodes: Dark brown dead patches bounded by major leaf veins. Frequently seen on species and varieties of Peperomia and bird’s nest fern.

Soft rot (Erwinia sp.): Mushy decay of fleshy plant parts. Discard affected plants; space remainder. Keep foliage and stems dry; avoid splashing water. Sterilize soil and pots before reusing.

Fungal leaf spots (Leptosphaeria, Colletotrichum, Myrothecium, Fusarium): Brown dead spots of various sizes on foliage, typically exhibiting fungus sporulation on the dead area. Generally controlled by keeping foliage dry to prevent infection. Wet plant surfaces (e.g., water in the whorl of dracaenas) promote fungal infection.

Stem rot at soil line (Rhizoctonia, Sclerotium rolfsii and Sclerotium delphinii): Brown stem discoloration, sometimes showing black or brown sclerotia or white to tan mycelium of the causal fungus on the dead area.

Water mold root or leaf rot (Pythium, Phytophthora): Black to brown portions of plant in a state of soft, wet decay. Pothos is especially susceptible to Phytophthora.

Impatiens necrotic spot virus (INSV): Swedish ivy may show dark rings or spots, while peperomia may develop black stems or ringspots; additional foliage plants may also be hosts.

5.2.16 Freesia (Freesia refracta)

Fusarium wilt: Plants turn yellow, wilt, and die. Corms show pink to brown discoloration.

Fluoride injury: Dark streaking and tip dieback in foliage of plants, especially when grown at low pH. Increase pH to 6.0 to 6.5 for symptom reduction, and eliminate sources of fluoride (fluoridated water, perlite, etc.).

5.2.16 Fuchsia (Fuchsia x hybrida)

Botrytis: Leaf infections during propagation can cause brown stem cankering and wilting and loss of cuttings. Keep Botrytis under control on stock plants to avoid losses during propagation.

Phytophthora nicotianae: Particularly during the moist conditions that prevail during propagation, stems are rotted and leaf bases show brown decay.

Rust: Bright yellow-orange sporulation is obvious on the underside of affected leaves, and the upper surfaces show tan leaf spots with a purple rim. Free moisture is necessary for infection, so losses are greatest during propagation.

Thielaviopsis: Wilting of all or portions of a good-sized plant is typical. Roots may be quite stunted and show areas of very black discoloration. The problem is most likely under conditions of high pH (pH below 5.5 discourages disease development) and in growing mixes that include some mineral soil.

5.2.17 Gardenia (Gardenia jasminoides)

Canker (Phomopsis gardeniae): Swollen stem base; yellow discoloration beneath the bark.

Bacterial leaf spot (Pseudomonas gardeniae): Leaf spots ranging from pinpoint dots to rounded brown spots (1/4 in. diameter) with water-soaked margins. Remove infested leaves.

Root-knot nematodes: Swollen, knobby areas (galls) on a stunted root system. Discard infested plants.

5.2.18 Geranium (Pelargonium hortorum)

Bacterial blight (Xanthomonas campestris pv. pelargonii): Tiny (1/16 in. diameter) round brown leaf spots, often surrounded by a chlorotic zone. Spots form when bacteria have been splashed onto the leaf surface. Subsequent systemic invasion of the plant leads to the development of a yellow or tan wedge-shaped area at the leaf edge and then to wilting of the leaf. Further progression of the disease may lead to brown stem cankers at nodes, brown to black vascular discoloration inside the stem, and tip dieback or wilting of all or part of the plant. Roots usually remain healthy-looking. Disease symptoms develop most readily under warm (spring) greenhouse temperatures. Spread is rapid during the handling and overhead irrigation associated with propagation.

Only geraniums are susceptible to bacterial blight.

P. hortorum (zonal) and P. peltatum (ivy) both show symptoms; P. domesticum (Martha Washington or Regal) is less likely to show symptoms. Hardy Geranium species may also be a source of infection. Infected plants should be destroyed; there are no chemical controls. Although culture-indexing procedures should have eliminated this disease from modern geranium production, it still occurs sporadically in the industry today, causing large financial losses to geranium growers. Plants with symptoms suggesting bacterial blight should be sent to a lab for diagnosis.

Verticillium wilt: Yellowing, browning, and wilting of lower leaves, possibly with some internal vascular discoloration in the stem. Culture indexing has largely eliminated this disease.

Botrytis: Large brown zonate lesions on leaves, browning of flowers, dieback of stubs on stock plants after cutting harvest, and infection of wounds on cuttings.

Bacterial fasciation (Rhodococcus fascians): Clumped stubby shoots form at or below soil level. May also affect lily, gladiolus, viburnum, chrysanthemum, dahlia, sweet pea, buddleia, hollyhock, forsythia, phlox, primula, delphinium, and petunia. No treatments are available; infected material should be discarded.

Edema (physiological disorder): Tiny water-soaked blisters that later turn brown and corky. Most severe on ivy geraniums. To discourage edema, ivies should be grown at a pH of 5.0 to 5.5 and given light levels of no more than 4,000 foot candles. Remove saucers from hanging baskets and water early in the day. Keep nitrogen and iron levels high in the foliage. Use a hand lens to look carefully for spider mites when edema is noted—mite feeding may be responsible.

Rust (Puccinia pelargonii-zonalis): Yellow spots show on the upper leaf surface, and pustules of reddish-brown spores appear in concentric rings on the underside. Pustules on the leaf underside may not be evident until two weeks or more after exposure to spore inoculum, whereas pale spots on the upper surface are visible within 7 to 10 days. Only P. hortorum, the florist’s geranium, is affected (ivy and Martha Washington/Regal types are not susceptible). This is a very serious disease. Incoming shipments should be closely examined for signs of rust. Spores are spread easily by splashing water or strong air currents. Plants infected in the greenhouse may decline rapidly in outdoor plantings. Use both contact and systemic fungicides for sporicidal and plant protection benefits.

Pelargonium flower break virus (PFBV): Chlorotic mottle or ring spot patterns form in foliage, and white streaks develop on the back of petals. Although geraniums are susceptible to many different viruses, most of the disfiguring viruses have been eliminated from geranium stock by virus indexing. In recent years, PFBV symptoms have frequently been observed in geranium foliage, as this virus has not been included in the standard virus-indexing programs. Fortunately, symptoms of disease may be obvious during winter growing conditions in the greenhouse, but symptoms are not evident by the time the crop is sold in the northeastern United States.

Other viruses: Symptoms may include yellow spots, rings, petal distortions, leaf curl or crinkle, dark green patterns along veins, white or yellow splotchy patterns on the leaves, yellow veins, and cupping of leaves. Tomato ring spot virus and tobacco ring spot virus may be carried in seed or by nematodes. Because of the benefits of virus-indexing programs, viruses are generally not a problem in modern geranium production.

Southern wilt (Ralstonia solanacearum): This disease is not as common as bacterial blight caused by Xanthomonas campestris pv. pelargonii, but it is a very important disease. One strain of Ralstonia solanacearum that has been detected in a few instances in geraniums (Race 3 biovar 2) also causes a very serious disease in potatoes and tomatoes. If you detect wilting in geraniums that does not appear to be due to Pythium root rot, it is important to contact a diagnostic lab to make an accurate diagnosis of the problem. Simple in-house quick tests for Ralstonia are also available for grower use. The symptoms of Ralstonia infection on geraniums are a diffuse yellowing, wilt, and death of leaves, usually beginning with the oldest leaves. Brown wedges may develop in leaves as they decline. This disease is differentiated from bacterial blight caused by Xanthomonas in that Ralstonia does not cause leaf spots. The systemic symptoms caused by the two bacteria look extremely similar. Plants with Southern wilt due to Race 3 biovar 2 will need to be eradicated from the greenhouse: this disease is regulated by federal quarantine. The endemic strain Race 1 is not under quarantine regulation.

5.2.19 Gloxinia (Sinningia speciosa)

Phytophthora crown rot (Phytophthora nicotianae): A brown, wet discoloration develops at the center of plants, which wilt and collapse completely. Laboratory diagnosis may be necessary to determine whether this browning and collapse is caused by Phytophthora or by tomato spotted wilt virus.

Impatiens necrotic spot and tomato spotted wilt tospoviruses: On the most susceptible young plants (four to six weeks old), browning and total collapse is typical. On more mature plants, look on the older leaves for round brown spots, chlorotic ring spots, or brown zigzag patterns. Control of the vector, the western flower thrips, is essential to halt the spread of the virus to other crops.

Foliar nematode (Aphelenchoides spp.): Necrotic leaf lesions, which progress until the whole leaf is destroyed.

5.2.20 Hydrangea (Hydrangea macrophylla)

Botrytis storage mold: Bud rot. Proper ventilation during storage is important. Fungicide sprays before and/or during storage may be helpful.

Powdery mildew: White fuzzy patches on foliage.

Root rot (Pythium sp.): Stunted or wilting plants with root discoloration.

Virescence (phytoplasma): Green flowers or witches’-broom growth pattern. Destroy affected plants.

5.2.21 Kalanchoe (Kalanchoe blossfeldiana)

Basal stem rot (Rhizoctonia): Brown canker at stem base, grayish discoloration, and wilting of foliage.

Stem rot (Rhizopus sp.): Collapsed stem portions bearing sporulation typical of the Rhizopus fungus.

Powdery mildew: Dark bronze patches on the foliage; the white mycelial growth that is typical of powdery mildew infections on other plants is generally less evident.

5.2.22 Lily

Root rot (Pythium sp.): Roots become softened and have a brown discoloration. Particularly likely to occur in poorly drained mixes.

Root rot (Rhizoctonia sp.): Roots show a brown discoloration and may appear stunted. May be present in addition to Pythium.

Virus (mottle, streak): Foliage shows pale streaking or spotting; flowers may be distorted or small. Rogue out infected plants; control aphids (potential vectors). CMV and potyvirus have been found in symptomatic lilies in recent years.

Leaf scorch (physiological): Seen primarily in Croft lilies; tip scorch on leaves. Keep calcium levels and pH high (use a pH of 6.8 to 7.2) and avoid sources of fluoride such as superphosphate and perlite.

Upper leaf necrosis (physiological): Seen primarily in the hybrid lily ‘Star Gazer,’ this appears as dead areas in the leaves just below the flower. The cause of this problem is a calcium deficiency that occurs most often with larger bulbs. Keep plants well ventilated to encourage transpiration.

5.2.23 Nemesia

• Bacterial soft rot: Wet collapse of portions of leaves or of whole plants under warm, wet growing conditions

• INSV: Dead, brown areas develop in the foliage and growth is stunted.

5.2.24 Orchid

Botrytis: Petal spotting.

Virus (Cymbidium mosaic, Odontoglossum ring spot, and others): A wide range of symptoms may result from virus infection, including dark brown or purple spotting on foliage. Spotting sometimes closely mimics infection by fungi or bacteria. Some of the most damaging orchid viruses are easily transmitted by routine handling of the plants, so the orchid collector should become familiar with special handling procedures that avoid transmission of viruses.

5.2.25 Pansy (Viola tricolor)

Myrothecium crown rot (Myrothecium roridum): Plants yellow and collapse, often after transplanting.

Phytophthora crown rot (Phytophthora nicotianae): Softening and discoloration of stem base leads to browning of lower leaf bases and plant collapse.

Thielaviopsis root rot (Thielaviopsis basicola): Scattered individual plants in plug tray or packs are stunted and yellowed; roots are stunted and may be blackened. Disease development is discouraged by pH levels below pH 5.5.

Cercospora leaf spot (Cercospora violae): Leaf spots begin as small purple spots and enlarge to form large 1/4 in. diameter dark spots without a sharp circular outline (usually seen on lowest leaves). Rogue out immediately.

Downy mildew (Pernospora violae): Pale areas of leaves, with sporulation on leaf undersurface.

5.2.26 Poinsettia (Euphorbia pulcherrima)

Powdery mildew: This disease has caused major losses to growers who detected it late in the crop. White powdery spots appear on either the top or bottom surface of leaves or bracts. A yellow leaf spot may indicate the presence of a mildew colony on the undersurface. Monitor for powdery mildew colonies through the entire production season. Early detection will improve the effectiveness of control.

Stem rot (Rhizoctonia solani): Brown stem cankers at the soil line or brown discoloration at the base of the cutting. Usually a problem during propagation.

Root and stem rot (Pythium aphanidermatum): Soft brown decay of roots sometimes extending up into the stem to cause a brown or black basal canker. This species of Pythium is favored by hot summer growing conditions.

Root and stem rot (Phytophthora drechsleri): Root rot and dark brown or black cankers at the stem base may be caused by Phytophthora as well as by Pythium. This species has been the most common problem in recent years.

Stem rot and wilt (Phytophthora nicotianae): Tips of shoots wilt and blacken. Black lines may be observed running up the stem of younger plants. Reddish-brown cankers may be observed at the soil line even though roots generally appear healthy.

Bacterial stem canker (Clavibacter poinsettiae): Black, water-soaked canker along the stem and spots and blotches on leaves. Uncommon under the cooler temperatures typical of poinsettia production in the northern United States.

Scab (Sphaceloma poinsettiae): Round or elongated tan lesions with a purple margin on stems; puckered brown spots on leaves. This disease has been a problem in the northeastern United States again recently: scout for symptoms.

Thielaviopsis root rot: A blackened, stunted root system, longitudinal cracks at the stem base, and stunted or wilting plants. Use a soil pH of 5.5 or below to deter this disease. Poinsettias in growing mixes that do not contain mineral soil are unlikely to develop Thielaviopsis root rot.

Crud (physiological disorder): Dried drops of plant sap (latex) occurring in the bracts may interfere with normal bract development and cause disfigurement of the flower head.

Alternaria leaf spot (Alternaria euphorbiicola): Brown spots with yellow haloes; damage not usually significant in the northern United States.

Xanthomonas leaf spot (Xanthomonas campestris pv. poinsettiicola): Brown spots with yellow haloes; damage is not usually significant in the northern United States.

5.2.27 Rose (Rosa hybrida)

Powdery mildew: White fuzzy patches of mycelial growth and sporulation on leaves, stems, and flowers.

Nematodes: Stunted growth or symptoms of nutrient deficiency or swollen, knobby areas on the root systems. Because roses for cut flowers are still grown in ground beds, they are one of the few crops in New York that may be troubled by nematode infestations.

Cankers (Coniothyrium fuckelii and other fungi): Brown discolored sections of canes often die back from a pruning cut or may be centered on a wound, sometimes around the graft union. Cankers generally bear sporulation of the causal fungus. Prune out the cankers, sanitize shears between cuts, and avoid both water stress and overhead watering of freshly pruned plants to minimize the chance of new infections.

Black spot: Large black leaf spots with a fringed margin, especially concentrated along veins. Extensive spotting leads to leaf yellowing and leaf drop. Shoots may also be infected. This problem is generally not severe in greenhouses where overhead watering of the foliage can be avoided by subirrigation techniques; outdoors it is a major problem.

Verticillium wilt: Wilting of leaves at shoot tips and yellowing or interveinal scorching of lower leaves; dieback. Vascular discoloration is usually not obvious within the stem, but some sparse black flecking may be observed. Symptoms appear when the plant is under stress, such as that caused by drought.

Downy mildew: Angular dark patches between the leaf veins, usually surrounded by yellow areas. Defoliation may be extensive. Spots frequently resemble spray injury. Under high relative humidity, sporulation will be produced on the underside of the leaf lesions or on stem lesions. Incubation in a moist chamber may be necessary to encourage this growth for identification purposes.

Botrytis: May cause petal spotting

Pest Management Guidelines A Cornell Cooperative Extension Publication	Cornell Guide for Pest Management of Greenhouse Floral Crops
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