The symptoms vary with plant age. On seed beds of small seedlings with leaves less than 2 cm in diameter, small patches of dead or dying seedlings with erect leaves provide evidence of the disease. Among older plants with leaves up to 4 cm in diameter, blue mold is first seen as circular, yellow areas of diseased seedlings. Plants in the center of the affected area may have distinctly cupped leaves. The upper surfaces of infected leaves can remain almost normal in appearance for 1-2 days before the plants begin to die and turn light brown.
Diseased leaves can become so twisted that the lower surfaces turn upward.
In such cases, the bluish color of the diseased plants becomes quite conspicuous, especially under wet conditions when sporulation is abundant. The blue mold progresses slowly at first. After 7-10 days, however, when sufficient secondary inoculum has been produced, a general epidemic occurs and the entire seedbed becomes affected almost overnight. When the weather is cloudy and cool, the fungus can kill plants in large areas of the bed. A characteristic foul odor of rapidly decaying vegetative matter develops.