5.1 Common Diseases of Floral Crops

5.1.1 Powdery Mildew

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.

 

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.