However, the presence of the characteristic downy fungus spores on the bottom of leaves quickly identifies the disease as blue mold and distinguishes it from other problems. Blue mold may affect plants in the field throughout the growing season. Often the spots coalesce to form light brown, necrotic areas. Leaves become puckered and distorted, large portions disintegrate, and the entire leaf may become unusable. Under continued favorable weather conditions the fungus can destroy all leaves at any growth stage.
Plants sometimes exhibit wilt symptoms with narrow, stunted, mottled leaves.
Systemic stem infection, resulting in partial or overall stunting of the plant, is common in some regions. Leaves may become twisted or distorted. Vascular discoloration occurs inside the stems, and lesions are visible as brown streaks. If this occurs near the base of weakened stems, the plants often lodge or snap off. The most commonly used name for the blue mold fungus is Peronospora tabacina Adam.