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Alternaria Leaf Spot

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.

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. Harvesttotable.com has more than 10 million visitors each year.