Small yellow spots that appear first on older leaves are a symptom of this fungal blight. These spots gradually enlarge and become dark-colored areas filled with concentric rings.
Alternaria leaf blight is particularly problematic for plants in the cabbage and squash families. Strawberries and carrots are also susceptible. When alternaria infects broccoli and cauliflower plants, it may cause brown areas on the heads. As with most fungal diseases, alternaria thrives in warm, wet weather. The spores are spread by wind and are able to enter leaf tissues when foliage has been consistently moist for 24 hours. The fungus overwinters on plant debris in and around the garden, and can also be transmitted by seed.