Affected leaves develop large patches that are dark green water-soaked. These are visually identical to the early stages of foliar Phytophthora and Botrytis leaf blight. The symptoms of Alternaria infection in hot humid conditions resemble the symptoms of foliar Phytophthora blight and early Botrytis infection. This lesion began as a dark green water-soaked area and progressed eventually to a dark, dry, thin lesion. In a seedling garden symptoms of Alternaria infection in hot weather can be alarming.
Plants quickly go down and are black from the straw up.
The leaves collapse and are thin, wet and dark at first then they dry out and become blackened. These areas usually radiate out from a disease focus and form rough circles. The disease spreads on air currents throughout the garden. It is these conidia that cause the earliest infections in gardens. Any adjacent stems and leaves will develop Alternaria lesions.