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Alternaria Leaf Spot

Alternaria leaf spot describes a broad group of fungal diseases that infect several common garden plants. As with many common fungal diseases, several species of closely related fungi cause this leaf-spotting disease. Some target specific plants, but others strike entire plant families. In the case of brassica vegetables such as cabbage, broccoli, cauliflower, and Brussels sprouts, several forms of the disease affect a single family.

Alternaria leaf spot thrives with warm temperatures and prolonged periods of high humidity. Overcrowded gardens encourage its development, as spores spread via wind, splashing water, garden tools, and even gardeners' hands and clothes. The fungi survive from year to year on infected plant debris left to overwinter in the garden.

Alternaria Leaf Spot Identification/Symptoms: Alternaria leaf spots often have different common names based on the plant and fungal species involved. The diseases that target brassicas include those commonly called brown spot, black leaf spot, and shot hole disease. Early blight in tomatoes and late blight in carrots are closely related to these brassica diseases.

As with early tomato blight, the first brassica symptoms are small, circular leaf spots that develop dark, target-like rings with a yellow outline. As the disease progresses, the spots merge and entire leaves become yellow or brown and scorched. Spot centers die and fall out, leaving a shot-hole behind. Spots may develop on stems and stalks as well as leaves.

In some plants, Alternaria leaf spot problems are mostly cosmetic, but brassicas sustain damage that leads to deterioration and rotting, storage problems, and general crop loss.

How to Control Alternaria Leaf Spot: Preventive treatments and regular applications of effective fungicides help protect brassica crops against Alternaria leaf spot. Daconil® fungicides from GardenTech® brand offer highly effective three-way protection to prevent, stop, and control Alternaria leaf spot and more than 65 types of fungal disease. These products treat broccoli, Brussels sprouts, cauliflower, and cabbage right up to harvest day:

  • Daconil® Fungicide Ready-to-Use comes in an easy-to-use sprayer bottle that's perfect for treating small areas and individual plants. Shake the bottle and it's ready. Spray until all plant surfaces are thoroughly wet.
  • Daconil® Fungicide Concentrate offers an economical alternative for garden areas treated with a hand-held, hose-end or tank-style sprayer. Measure the recommended concentrate amount with the convenient measuring cap, add water, and mix well. Spray all plant surfaces to the point of runoff.

Alternaria Leaf Spot Tips: Avoid overhead watering to limit leaf wetness, and allow enough space at planting time for good air circulation and ample sun penetration. Keep your garden free of fallen leaves all season, and especially thorough at fall cleanup time.

Always read product labels thoroughly and follow instructions, including guidelines for treatable plants, application rates and frequencies, and pre-harvest intervals (PHI) for edible crops.

GardenTech is a registered trademark of Gulfstream Home and Garden, Inc.

Daconil is a registered trademark of GB Biosciences Corp.

​Photo Credit:

Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org (CC BY 3.0 US)

Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org (CC BY 3.0 US)

Robert Wick, University of Massachusetts, Bugwood.org (CC BY 3.0 US)