Brassicas, Alternaria Leaf Spot
Three species of Alternaria cause serious damage to brassicas: Alternaria brassicicola, A. brassicae, and A. raphani. Alternaria brassicicola and A. brassicae infect broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kohlrabi, kale, rutabaga, and turnip. A. raphani is most often found on radish, but can infect other brassica crops. Disease development is favored by cool temperatures and long periods of leaf wetness or high relative humidity, and Alternaria leaf spot can be a limiting factor in the production of brassica vegetable and seed crops in regions where these conditions are common. In the Northeast, we usually see Alternaria infections start during humid summer weather, and continue into the fall when brassica leaves are remaining wet for long periods of time due to dew. Infection can cause reduction in crop quality and yield through damage to seedlings, leaves, and heads. Brussels sprouts can be rendered unmarketable by Alternaria lesions on the buds. The disease can spread in storage, so management is especially important for storage crops like cabbage and crops should be inspected for early symptoms before storing.
The initial symptoms of Alternaria leaf spot are small black dots surrounded by chlorotic haloes. As the disease progresses, lesions expand into characteristic, dark brown to black circular leaf spots with target-like concentric rings. The centers of lesions often turn brown and crack or fall out, giving the leaf spots a shot-hole appearance. Individual spots coalesce into large necrotic areas and leaf drop can occur. Symptoms typically appear on older, lower leaves first and then move up the plant as the disease progresses. Lesions can develop on cauliflower curds and broccoli heads in addition to the leaves, as well as on petioles and stems.
Alternaria species are simple parasites that survive saprophytically outside the host. Diseased crop debris is the primary site of survival from year to year. Resting spores (chlamydospores, microsclerotia) have been reported. Spore production is favored by exposure to high humidity (~87%) and temperatures between 68 and 86°F. Spores are released during warm, dry periods after a rain. The optimal conditions for new infections are temperatures between 55 and 75°F and high relative humidity or periods of long leaf wetness. Spores produced in the summer heat will proliferate once the weather cools going into the fall.
Alternaria species overwinter primarily in diseased crop debris. The pathogen can also survive on seed, both internally and on the seed surface. The main means of introduction into new areas is on infested seed. However, Alternaria fungi produce a profuse amount of airborne spores that can spread easily from field to field and farm to farm within an area once the disease is established. In addition to spreading via wind, spores of Alternaria are spread within fields by splashing water as well as insects, equipment, and workers moving through fields.
Alternaria infection can be mainly cosmetic, but results in crop rejection by distributors and customers. Infection of broccoli and cauliflower heads can lead to complete deterioration of the heads and result in total loss of marketability. Affected cabbage do not store well as lesions provide entry for secondary soft-rotting organisms. Control of Alternaria leaf spot on cabbage heads in the field is necessary for long-term storage. Heavy infections of foliage reduce plant vigor and yield. Pod infections cause distortion, premature shattering, and shriveled, diseased seed that germinates poorly.
Cultural Controls & Prevention:
- Buy certified, disease-free seed, or treat seed with hot water to eliminate any Alternaria spores. UMass Extension can treat most brassica seeds, see UMass Hot Water Seed Treatment Service for more information.
- Select disease-tolerant cultivars. There are no cultivars of any brassica crop that are truly resistant but some cultivars may have characteristics that reduce incidence. In broccoli, look for cultivars with tight, well-domed heads.
- Practice long rotations (at least 3 years) with non-cruciferous crops.
- Keep seedbeds disease-free to prevent the spread of disease and locate seedbeds so as to avoid wind-borne inoculum.
- Incorporate diseased plant debris into the soil immediately after harvest.
- Work in young, uninfected plantings first, and older, infected plantings last.
- Eliminate cull piles and manage compost piles well in order to promptly break down infected crop residues and prevent them from becoming sources of inoculum.
- Control brassica weeds, which can serve as alternate hosts.
- Minimize the length of leaf wetness periods by reducing plant density, orienting rows with prevailing winds, and irrigating in the morning when leaves can dry quickly.
- Avoid overhead irrigation, especially during broccoli, cauliflower, and cabbage head development. Allow sufficient drying time before nightfall if overhead is used.
- Avoid planting succession crops in close proximity to each other and rotate late-season crops to new fields away from early-season brassicas.
- Control flea beetles, which can spread Alternaria spores as they move through crops. For information on controlling flea beetles, see the article in the May 26, 2022 issue of Veg Notes.
Chemical Controls & Pesticides:
For more information on this trial and an example of an Alternaria fungicide spray program, see Hoepting’s full article article in the August 19, 2020 issue of Cornell Veg Edge.
General tips for controlling Alternaria with fungicides:
- Spray fungicides preventively before disease establishes itself because lower frame leaves serve as inoculum to infect heads.
- Once the canopy fills in, aeration is reduced, and leaf wetness is prolonged. Begin application of systemic/translaminar fungicides with very good to excellent activity on Alternaria at this time.
- Use an adjuvant with fungicides that have translaminar or systemic activity for improved efficacy.
- Do not apply a copper bactericide in the same tank mix with an adjuvant; excessive leaf burn injury may occur.
- Many pesticides are at risk of Alternaria developing resistance. See specific product labels for rotation restrictions and seasonal maximum use rates. Be mindful of pre-mixes that have more than one FRAC group per fungicide that need to be managed for fungicide resistance.
- Do not apply a product more than 1-2 times before alternating to another FRAC group.
- Do not use more than 2 applications per FRAC group per crop.
For Current information on disease recommendations ins specific crops including information on chemical control & pesticide management, please see the New England Vegetable Management Guide.
Crops that are affected by this disease:
-UMass Extension Vegetable Program, with chemical recommendations from “Fungicide Recommendations for Alternaria Leaf Spot & Head Rot in Broccoli” by Christy Hoepting, Cornell Cooperative Extension, originally published in Cornell Veg Edge Vol. 16 Iss. 20, August 19, 2020. Additional source: Alternaria Leaf Spot of Brassicas – Cornell Cooperative Extension.
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