|Disease (causal agent)
||Survival of pathogen and effect of environment
||Comments on control
|Black root rot
|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.
|Infections girdle plant stems.
Cottony fungal growth or large, black sclerotia develop inside stems. Dead stems take on a bleached, white color.
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. more info *
|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 mefenoxam in preplant treatment or use
later as a drench to protect against Pythium and Phythophthora spp. more info *
|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. Do not replant in fields where disease has been severe. Steam
treat to kill resting spores. Protect foliage with mancozeb. more info *
|Brown, water‑soaked decay of
flowers, leaves, and stems. Woolly 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. more info *
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. more info *
|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).
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. more info *
|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 methyl bromide-chloropicrin combination or solarize soil. more info *
|Water mold root rots
(Pythium 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 mefenoxam. More info: Pythium
Root Rot, Phytophthora Root and Crown Rots *
are also susceptible to anthracnose (Colletotrichum antirrhini), 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.