Multiple growers in St. Johns County have been contacting me about their “spotty” cabbage. As it turns out, it’s been a favorable season for Alternaria leaf spot. Read on to find out more about this fungal disease of brassica crops.
What is Alternaria Leaf spot?
Alternaria leaf spot is a common, yet significant fungal disease of cabbage, collards and other brassicas in Florida. The disease is caused by several Alternaria species including A. brassicicola, A. brassicae and A. raphanin. Alternaria brassicicola and A. brassicae infect cabbage, cauliflower, broccoli, Brussels sprouts, kohlrabi, kale, rutabaga, and turnips. A. raphani is most often found on radish, but can infect other brassica crops as well. Alternaria leaf spot occurs sporadically, and tends to show up in northern Florida more often than it does in the southern part of the state.
Common symptoms of Alternaria diseases are dark brown to black circular leaf spots. As the lesions age, the center may fall out, giving them a shot hole appearance. Lesions can coalesce into larger necrotic areas followed by leaf drop and eventually death. Lesions can occur on all plant parts including leaves, stems, flowers, and seed. The leaf spots produced by Alternaria spp. are much more common than the stem lesions in Florida. Concentric bands or a solid mass of fuzzy dark green-to-black growth within leaf spots develop as a result of spore production.
Alternaria species survive saprophytically outside of their hosts. The primary site of survival is diseased crop debris; however it is possible the fungus can survive on brassica weed species. Spore production usually occurs at night and released during the day. Prolonged periods of overcast weather are favorable for continuous spore production. Optimum temperatures for spore production are between 75 and 82 degrees F, and under favorable temperature conditions, new spores can be produced within seven to ten days of infection. The number of infections and lesion size increases substantially with longer periods of leaf wetness. Prolonged dew periods and frequent rainfall are favorable for disease development
Although the infection can be mainly cosmetic, rejection by distributors and customers may occur from Alternaria leaf spot affected crops. Infection of broccoli and cauliflower heads can lead to total deterioration and loss of crop marketability. Heavily infected foliage will reduce overall plant vigor and yields. Infected cabbage should not be stored, as the lesions provide entry ways for soft-rotting bacterial pathogens.
Cultural Controls & Prevention:
- Plant only certified disease-free seed
- Practice crop rotation with non-brassica crops
- Eliminate cull piles
- Control brassica weeds
- Avoid overhead irrigation
- Rogue and discard infected plants
- Handle plants when dry and have workers wash their hands before resuming regular duties to prevent spread
Chemical Controls & Pesticides:
Refer to the Vegetable Production Handbook of Florida for application information on products labeled for use on brassica crops for the management of Alternaria leaf spot in Florida: http://edis.ifas.ufl.edu/pdffiles/cv/cv12200.pdf
Always read and follow all labeling and instructions.