| Disease (causal agent) | Symptoms | Survival of pathogen and effect of environment | Comments on control |
|---|---|---|---|
| Black root rot * (Thielaviopsis basicola) |
Roots are girdled by decay; tops slowly die. In less severe cases, elongated, black lesions occur on roots. Disease is particularly damaging to seedlings. | Soilborne fungus; produces dark, resting spores. Spores are spread in water. Favored by cool, wet soils. | In greenhouse, steam or chemically treat soil. Before planting, incorporate thiophanate-methyl into top 3 inches of soil. |
| Cottony rot (Sclerotinia sclerotiorum) * |
Infections girdle plant stems. Cottony fungal growth or large, black sclerotia develop inside stems. Dead stems take on a bleached, white color. | As sclerotia in soil. Airborne spores produced by sclerotia, which infect dead or weak tissues. Sclerotia produce hyphae, which infect plant tissues. Favored by wet weather. | In greenhouse, steam treat or fumigate soil. Treat soil with PCNB before planting. Protect plants with iprodione, or thiophanate-methyl. Mancozeb also helps control this fungus. |
| Damping-off and stem rot * (Rhizoctonia spp., Pythium spp., Phytophthora spp.) |
Seedlings killed. Stems rot at soil line. If problem is caused by Pythium, it usually begins at root tips. | Soilborne organisms. Favored by conditions unfavorable for plant growth. | Steam treat or fumigate soil. Incorporate PCNB into top inch of soil before transplanting, or spray bases of plants with thiophanate-methyl or iprodione to control Rhizoctonia. Include an oomycete- or water mold- specific fungicide in preplant treatment or use later as a drench to protect against Pythium and Phytophthora spp. |
| Downy mildew * (Peronospora antirrhini) |
Young tip leaves are dull green, severely stunted, and roll downward. Gray-purple fungus grows on undersides of leaves. Disease is common on seedling phase; large plants are less frequently attacked. Infected plants fail to produce flowers. | Thick-walled resting spores (oospores) in dead plant parts. Airborne spores. Favored by cool (40° to 60°F), wet weather. | In greenhouse, reduce humidity. Drench seedlings with mefenoxam or oomycete- or water mold- specific fungicide. Do not replant in fields where disease has been severe. Steam treat to kill resting spores. Protect foliage with mancozeb. |
| Gray mold * (Botrytis cinerea) |
Brown, water-soaked decay of flowers, leaves, and stems. Fuzzy gray fungal spores form on rotted tissues. Frequently found on stems of cut flowers. | In plant debris. Airborne spores. Favored by continued cool, moist conditions. | Reduce humidity in greenhouse. Clean up all plant debris. Protect foliage with iprodione or fenhexamid. Mancozeb also helps control gray mold. |
| Powdery mildew * (Golovinomyces orontii) |
White, powdery fungus grows on both leaf surfaces. Severely infected leaves may be killed. | On living leaves. Airborne spores; not in soil or on seeds. Favored by moist, shaded conditions, and dry foliage. | Protect foliage with triadimefon or sulfur. |
| Rust * (Puccinia antirrhini) |
Pustules of dark brown to purple powdery spores develop on leaves and stems. Rapid water loss from severely rusted leaves causes them to dry up. | On living snapdragon plants and spores on seed. Does not survive in soil, but does in plant refuse. Airborne spores. Favored by abundant dew, cool nights (50° to 55°F), and warm days (70° to 75°F). | In greenhouse, avoid wetting foliage and prevent moisture condensation at night by balancing heat and ventilation. Protect foliage with myclobutanil or triadimefon. Mancozeb also helps protect foliage from infection. Remove and destroy infected plants. |
| Verticillium wilt * (Verticillium dahliae) |
Plants wilt, frequently on one side. Wilting is more pronounced near time of bloom. Problem is most important in seed fields. | In soil for many years. Favored by cool weather. Plants wilt during hot weather. | Steam treat or fumigate soil with a chloropicrin or a chloropicrin combination or solarize soil. |
| Water mold root rots (Pythium spp.* and Phytophthora spp.*) |
Plants are stunted, wilt, or suddenly collapse. Roots decay. Plants fail to "push" after flowers are cut. Remaining stubs are more susceptible to gray mold. | Soilborne pathogen. Favored by heavy, waterlogged soils. | Avoid planting on poorly drained soils. Do not overirrigate. Steam treat or fumigate greenhouse soil. Drench plants with oomycete (water mold) specific fungicide. |
| Snapdragons are also susceptible to collar rot (Rhizoctonia solani), crown gall * (Agrobacterium tumefaciens), leaf and stem spot (Phyllosticta antirrhini), mosaic * (Cucumber mosaic virus), root knot nematode ** (Meloidogyne spp.), and stem rot (Phyllosticta antirrhini). Anthracnose (Colletotrichum antirrhini) is important elsewhere but is not found in California. | |||
| * For additional information, see section on Key Diseases. |
| ** For additional information, see section on Nematodes. |