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).

·           Downy mildew: Occurs on alyssum, bacopa, bracteantha, coleus, geum, impatiens, osteospermum, rosemary, snapdragon, etc. Patches of pale or necrotic leaf discolaroation are seen opposite sporulation on leaf undersurface.

·           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, nemesia, lobelia 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 axonopodis 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 (Pectobacterium carotovorum or P. aroideae): 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 (Dickeya 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; report suspected cases to your state horticultural inspector immediately. Destroy infested plants. Overwintering in New York is now suspected.

·         Brown rust (Puccinia tanaceti): Yellow spots on the upper leaf surface and brown pustules on the leaf underside. Becoming more common.

·         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. Grayish 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 (Pectobacterium carotovorum): 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 (Pectobacterium and Dickeya spp.): 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.).

 

5.2.17 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.

 

5.2.18 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.

 

5.2.19 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 as well as many perennials. 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.

 

5.2.20 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.

5.2.21 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. Reddish spots on bracts.

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

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

 

5.2.22 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.

 

5.2.23 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.

 

5.2.24 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.

 

5.2.25 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.

 

5.2.26 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 (Peronospora violae): Pale areas of leaves, with sporulation on leaf undersurface.

 

5.2.27 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 become less common in recent years.

·         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 spp.): Brown spots with yellow haloes; damage is not usually significant in the northern United States.

 

5.2.28 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 or infection of young shoots leading to significant cankering.

 

5.2.29 Snapdragon (Antirrhinum majus)

·         Botrytis: Wilting and death are common when infection of lower leaves occurs on tightly spaced, bed-grown snapdragons, leading to stem cankers that girdle the plants. Snapdragons grown as bedding or pot plants are also fairly susceptible to Botrytis lesions under conditions of poor aeration. Leaf spots and cankers are light tan in color.

·         Downy mildew: Distorted foliage and/or leaf yellowing combined with easily visible patches of grayish sporulation on leaf undersides. Very damaging. Check incoming plugs carefully for suspicious distortion; incubate in a moist chamber to check for downy mildew.

·         Powdery mildew: White fuzzy patches on leaves.

·         Stem rot (Rhizoctonia): Brown canker at the soil line

·         Stem rot (Pythium or Phytophthora): Generally root rot precedes stem invasion by these water molds.

·         Nematodes: In ground beds, nematodes may be responsible for symptoms of stunting or nutrient deficiency or for swollen, knobby areas on roots.

·         Rust (Puccinia antirrhini): Dark brown spore pustules are strikingly visible on the undersides of leaves, while only a very faint chlorotic area may be seen from the upper surface. Symptoms develop first on the lowest foliage. Condensation or splashed water is necessary for infection, so environmental management can provide significant control of this disease. Systemic fungicides are also available.

·         Impatiens necrotic spot tospovirus (INSV): Round, white, or tan leaf spots up to 1/4 in. diameter are caused by this virus on snapdragon. Stem cankers and wilting are also possible.