| Disease (causal agent) | Symptoms | Survival of pathogen and effect of environment | Comments on control |
|---|---|---|---|
| Fusarium wilt * (Fusarium oxysporum f. sp. callistephi) |
Plants yellow and wilt, often on one side. Brown discoloration of vascular system develops. Disease also causes damping-off of young seedlings at soil temperatures of 75° to 80°F. | Commonly seedborne. In soil for many years. Disease is most severe when soil and/or air temperatures are high. | Use disease-free seed. Fumigate the seedbed with chloropicrin or a sequential application of chloropicrin/1,3 dichloropropene and metam sodium or solarize soil. Grow on clean land, or only once every 5 years on infested land. Treat seed with a fungicide. |
| Gray mold * (Botrytis cinerea) |
Brown, water-soaked decay of flowers. Fuzzy gray fungus spores form on rotted tissues. Fungus also attacks base of plant. | In plant debris. Favored by cool, wet conditions. | Avoid overhead irrigation. Mist blooms with iprodione or fenhexamid. |
| Leaf spots (Stemphylium callistephi) |
Circular, irregular, brown spots appear on lower leaves. Leaves may die. | In plant debris. Airborne spores require long (48 hrs), damp periods for infection. | Avoid low-lying areas where air movement is poor. Do not use overhead irrigation. Protect foliage with a fungicide such as mancozeb. |
| Root rot (Pythium *and Phytophthora spp. *) |
Plants wilt or suddenly collapse. Roots decay. Blackish discoloration of leaves, stems, and roots occurs. Also causes damping-off of seedlings. | In soil. Favored by heavy, waterlogged soils. | Avoid planting in poorly drained fields. Plant on raised beds. Do not overirrigate and keep hose ends off the ground. Drench seedlings with oomycete (water mold) specific fungicide. |
| Rust * (Coleosporium asterum) |
Orange pustules of powdery spores form on undersides of leaves. On living plants and possibly from spores from alternate host (three-needle pines). | Favored by free moisture from rain, dew, or fog. | Avoid overhead irrigation. Treat at the first signs of rust and continue until conditions are no longer favorable for the disease. Grow seedlings away from main crop. |
| Cottony rot or Sclerotinia rot (Sclerotinia sclerotiorum) | Infection girdles stems. Cottony, white fungal growth and large, black sclerotia develop on and inside stems. Stems take on a bleached-white color. | Airborne spores produced by sclerotia in soil, but infection more common from growth of hyphae from sclerotia. Favored by wet weather. | Avoid overhead irrigation. Treat planting area with PCNB. Spray plants with iprodione or thiophanate-methyl before rainy periods and at 2- to 4-week intervals during wet weather. Remove plant debris from field. |
| Stem rot (Rhizoctonia solani, Botrytis cinerea) |
A brown decay develops at the soil line and affects the basal leaves and stem. | Soilborne and in plant debris. Gray mold (B. cinerea) favored by cold, damp conditions. Disease development can be rapid under high temperature conditions. | Before planting or transplanting, mix PCNB or Trichoderma spp. into top inch of soil. Spray bases of seedlings with thiophanate-methyl, iprodione, or Trichoderma spp. |
| Verticillium wilt * (Verticillium dahliae) |
Symptoms are almost identical to Fusarium wilt. Not a common disease of asters in California. | In soil for many years. Symptoms most severe during warm weather that follows a cool period. | Avoid planting in fields where fungus has occurred or fumigate soil as described for Fusarium wilt. |
| Virus or viruslike diseases | Symptoms | Host range and natural spread | Comments on control |
| Aster yellows * (Aster yellows phytoplasma) |
Infected plants produce an upright basal rosette of yellow shoots. Sometimes one-sided. Flowers are deformed and remain green. Sporadic disease of asters in California. | Aster yellows phytoplasma has a wide host range. Vectored by leafhoppers. | Locate seedbed away from weedy areas. Control weeds and leafhoppers in noncropped areas. |