|Disease (causal agent)
||Survival of pathogen and effect of environment
||Comments on control
||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. more info *
||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 above for control of Verticillium
wilt. more info *
||Brown, water‑soaked spots
appear on petals. Woolly 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. more info *
||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 (cleistothecia) on old leaves. Free water is not necessary for infection.
||Protect foliage with myclobutanil,
fenarimol, or sulfur. more info *
|Root knot nematode
||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
||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. more info *
||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.
(Botrytis cinerea, Erwinia carotovora, Fusarium spp.)
||Roots rot in storage.
||Plant debris. Favored by high
||Avoid plant injuries. Maintain a
storage temperature of 40°F and avoid high humidities. In mild climates, leave roots in soil.
||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-methyl bromide combination (tarped).
Fumigation also controls most other fungi, bacteria, weeds, nematodes, and
soil insects. Soil solarization might be considered in sunny climates. more info *
(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.
(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.
(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.**
|* For additional information, see section on Key Diseases.
|** For additional information, see section on Nematodes.