| Disease (causal agent) | Symptoms | Survival of pathogen and effect of environment | Comments on control |
|---|---|---|---|
| Crown gall * (Agrobacterium tumefaciens) |
Overgrowths or galls occur on stems and roots. | In soil for as long as 3 years; bacteria enter through wounds and can survive on infected roots. Gall development favored by rapidly growing host. | Avoid wounding plants. Do not grow plants in infested soil for 3 years, or fumigate soil. Discard infected roots. Rotate with non-woody crops, such as cereals and legumes. |
| Fusarium wilt * (Fusarium oxysporum) |
Symptoms are almost identical to those of Verticillium wilt. Not common in California. | Soilborne for many years. Disease most severe when soil temperatures are high. | Destroy infected plants. Grow plants in new areas or fumigate soil; see below for control of Verticillium wilt. |
| Gray mold * (Botrytis cinerea) |
Brown, water-soaked spots appear on petals. Fuzzy gray fungus spores form on soft, brown, decayed tissues; fungus may invade plant tissue that touches infected petals. | Plant debris. Favored by cool, wet conditions and condensed moisture on plants. | Remove withered or diseased flowers promptly. During cool, wet weather, spray with iprodione, mancozeb, or fenhexamid. |
| Powdery mildew * (Golovinomyces cichoracearum) |
White, powdery fungus principally grows on older leaves and stems. Severely affected leaves dry up and may fall. | On living dahlia leaves and as small, dark, resting structures (chasmothecia) on old leaves. Free water is not necessary for infection. | Protect foliage with myclobutanil or sulfur. |
| Root knot nematode** (Meloidogyne spp.) |
Knots or small swellings, caused by root knot nematodes, occur on feeder and fleshy roots. | In soil and on the roots of many plants. Favored by warm, sandy soils. | Use a nematicide to treat infested soil before planting or solarize soil. Destroy infected roots. |
| Sclerotinia or cottony rot * (Sclerotinia sclerotiorum) |
Plants wilt and die suddenly. Water-soaked stem cankers appear near the soil line. Cottony, white fungal growth; later, large black sclerotia are found on insides and outsides of stems. | In soil as sclerotia, which produce airborne spores that infect only inactive or weak tissues. Sclerotia also produce hyphae, which infect plant tissue. Favored by cool, moist conditions. | Avoid soil where disease has occurred (common disease of many vegetable crops). Treat soil with PCNB before planting. Protect plants with iprodione or thiophanate-methyl. |
| Smut (Entyloma dahliae) |
Yellowish, circular to irregular spots appear on leaves. Leaves later become brown and dry. | Plant debris. Favored by wet weather. | Do not use overhead irrigation. Mancozeb applied to control gray mold should help control smut. |
| Storage rots (Botrytis cinerea, Pectobacterium (=Erwinia) carotovorum, Fusarium spp.) |
Roots rot in storage. | Plant debris. Favored by high temperature and humidity. | Avoid plant injuries. Maintain a storage temperature of 40°F and avoid high humidity. In mild climates, leave roots in soil. |
| Verticillium wilt * (Verticillium dahliae) |
Basal leaves wilt and turn yellow. Frequently, only one branch is affected at first. Later, the entire plant dies. Dark discoloration of the vascular system occurs. | In soil for many years. Symptoms most severe during warm weather after a cool period. Fungus has a wide host range. | Destroy infected plants and roots. Fumigate soil with chloropicrin or a chloropicrin combination (tarped). Fumigation also controls most other fungi, bacteria, weeds, nematodes, and soil insects. Soil solarization might be considered in sunny climates. |
| Virus or viruslike disease | Symptoms | Host range and natural spread | Comments on control |
| Dahlia mosaic (Dahlia mosaic virus) |
The normal green color of leaves develops irregularly. Bands adjacent to the veins remain pale green (vein-clearing). Leaves may be distorted. Shortening of internodes (stunt) occurs in some cultivars. Flower color is usually normal. | Aphids. Spread vegetatively by cuttings and roots. In some cultivars, the virus is almost symptomless. | Destroy infected plants. Control aphids. |
| Mosaic (Cucumber mosaic virus) |
Mild leaf mottle accompanied by little or no leaf distortion. Some varieties are symptomless carriers of the virus. Not common in California. | Aphids. Spread vegetatively by cuttings and roots. Many plants: cucurbits, tomato, pepper, legumes. | Destroy infected plants. Control aphids and weeds. |
| Ringspot (Tomato spotted wilt virus) |
A well-defined mosaic mottle or irregular concentric rings or wavy lines in leaves. No leaf distortion or stunting occurs. Symptoms seen on older leaves. | Thrips. Spread vegetatively by cuttings and roots. Many plants. | Eliminate reservoir hosts and weeds. Destroy infected plants. Control thrips. Eliminate virus by taking small, stem-tip cuttings from rapidly growing plants. |
| Dahlias are also susceptible to southern blight * (Sclerotium rolfsii) and foliar nematodes (Aphelenchoides spp.).** | |||
| * For additional information, see section on Key Diseases. |
| ** For additional information, see section on Nematodes. |