Helianthus annuus
Charcoal Rot (fungus – Macrophomina phaseolina): The symptoms are premature ripening of stalks with a poorly filled head. At the base, the inside of the stem has a shredded appearance, and the fibers are covered with numerous, tiny black resting bodies of the fungus that resemble pepper flecks. Varieties may differ in resistance to this disease.
Rhizopus Head Rot (fungi – Rhizopus spp.): The head turns brown and becomes soft. A gray mycelial growth will later develop. Wet weather following flowering favors disease development, particularly if the heads are damaged by hail, birds, or insects. In Texas, head rot has been associated with larvae of the sunflower moth, Homoeosoma electellum. There is no control, although varieties with upright heads are more frequently infected than varieties with bending heads.
Southern Blight (fungus – Sclerotium rolfsii): Plants wilt, then die. A bright, white mycelial growth can be seen at the base of the stalk. The diagnosis is confirmed by presence of round, tan resting bodies of the fungus, which resemble mustard seeds. This fungus is soilborne.
Downy Mildew (fungus – Plasmopara halstedii): The symptoms are pale green or yellowish areas spreading out from the midrib of leaves. With moist weather, there is the presence of a downy growth on the underside of the leaves. As the plants mature, there may be some wrinkling and distortion of the leaves and plants are stunted. Angular leafspots can also be produced. The disease is seed and soilborne. Resistant varieties are available.
Rust(fungus – Puccinia helianthi): Small, dark brown, powdery pustules on the underside of leaves are surrounded by chlorotic areas. Many of these pustules will cause the leaf to take on a brown appearance. These heavily-infected leaves will die. A severe infection will reduce seed quality and yield. The use of resistant cultivars is the best control measure. In addition, residue from previous crops should be buried and volunteers should be controlled.
Mosaic (virus): Mottled areas are visible on newer leaves. This virus symptom was seen in the Lower Rio Grande Valley. The virus has been tentatively identified as a member of the Carlavirus group. It occurs infrequently.
Powdery Mildew (fungus – Erysiphe cichoracearum): The symptoms are an extensive growth of white, powdery fungal mycelium on the upper leaf surface. The disease usually occurs too late in the season to affect yield.
Sclerotinia Wilt (fungus – Sclerotinia sclerotiorum): Plants wilt, then die. A bright, white mycelial growth can be seen at the base of the plant, along with considerable shredding of the stalk. Black, irregularly-shaped resting bodies of the fungus are found within the stem. A long crop rotation with non-susceptible crops, such as corn, sorghum or wheat, is the recommended control.
