Brassica Alternaria leaf spot is a fungal disease of brassica crops caused by Alternaria brassicicola. Symptoms begin as small, black lesions on leaves that enlarge to form target-like concentric rings of necrotic tissue surrounded by a yellow halo. When humidity is high, these lesions produce black spores on the leaf surface.
Primary Source: Infected seed, infected overwintering plant debris, infected brassica weeds
Spread: Windborne spores, rain and irrigation splash, insects like flea beetles
Favorable Conditions: Spores are produced in high humidity (>87%) and moderate-to-high temperatures (68-88ºF). Optimal temperatures for infection are 55-75ºF.
Infection & Disease Cycle
Alternaria brassicola can overwinter on plant debris in the soil. Once temperature and humidity rise, spores (conidia) can spread from plant debris via wind, rain, and irrigation. These spores will inoculate healthy plant tissue, usually on lower leaves, causing lesions that will produce spores (sporulate) in high humidity. Lesions may also occur on plants coming from infected seeds, as well as brassicaceous weeds. Spores will disseminate to new healthy plant material, continuing the disease cycle.
Scouting regularly allows early identification of disease before significant spread and damage. The following practices can also help prevent disease development:
- Plant disease-free seed
- Manage host weeds
- Manage insect vectors (ex. flea beetles)
- Rotate away from crucifers (3 years)
- Maintain proper spacing between plants
- Avoid overhead irrigation
- Avoid working in fields when plants are wet
- Steam or fumigate seedbed soil
- Destroy infested plant debris
For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (D0062), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.
- Commercial Vegetable Production in Wisconsin (A3422) from the UW Extension Learning Store. This guide offers the latest recommendations for disease, insect, and weed management in Wisconsin’s most common commercial vegetable crops. Also included are lime and fertilizer recommendations as well as insect identification information and keys.
- UW Plant Disease Diagnostics Clinic. The University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic (PDDC) provides assistance in identifying plant diseases and provides educational information on plant diseases and their control.
Written by Amanda Gevens, Ariana Abbrescia, Russell Groves, and Ben Bradford. Last updated Nov 2023