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Beginner

Scientific Names
Alternaria brassicae and A. brassicicola

Identification

• On older leaves, small, yellowish lesions expand and become tan coloured
• Spots resemble a target with concentric light and dark areas
• On cauliflower and broccoli, black, sunken spots develop on individual florets
• Infections on cabbage heads range in size from pinpoint to several centimetres in diameter

Often Confused With
Downy mildew
White leaf spot

Period of Activity
Alternaria is spread by wind, splashing rain or on equipment throughout the summer season. Prolonged periods of leaf wetness at 15- 25°C (59- 77°F) promote Alternaria infection. High humidity and temperatures greater than 14°C (57°F) favour black spot development.

Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field. Observe both the older and newer leaf growth. Record the percentage of leaves infected and the average number of lesions per leaf. Also note any black spotting on florets or heads when inspecting mature plants.

Thresholds
None established.

Advanced

Scientific Names
Alternaria brassicae and A. brassicicola

Alternaria black spot affects most crucifers in Ontario. This disease can be caused by two species of the fungus Alternaria. While both species are present, Alternaria brassicicola is the most common. Losses can occur from damping-off of seedlings, spotting of leaves and heads of cabbages and spotting of the curds of cauliflower and the heads of broccoli.

Identification
The most obvious symptom of Alternaria is the spotting on cauliflower curds. Black, sunken spots develop on individual florets and expand to include large areas of the head. Similar spots occur on the heads of broccoli. On cabbages, spots range in size from pinpoint to several centimetres in width.

Symptoms appear first on the older leaves as small, yellowish lesions that expand and become tan coloured. As the spot develops it takes on a target appearance with concentric light and dark areas. If the lesion covers more than 50% of the leaf surface, the leaf will then drop off the plant. Fallen leaves can become entirely covered with the dark brown-black spores of this fungus.

Often Confused With
Downy mildew
White leaf spot

Biology
Seeds of many crucifer crops are infested with Alternaria. However, not all seedlings become infected as a result of infested seeds. Many cruciferous weeds also harbour this fungus, spreading the disease to crops by wind.  Physical contact with infected plants by equipment and people can also help to spread the spores. As soon as lesions are evident on older leaves, spores can spread to marketable portions of the crop.

Alternaria develops once a spore lands on a leaf or head tissue if conditions are favourable. The dark areas in the target spot are areas where spores are being produced. Spore chains can be seen when these areas are examined with a hand lens.

Period of Activity
Alternaria is spread by wind, splashing rain or on equipment throughout the summer season. Prolonged periods of leaf wetness at 15- 25°C (59- 77°F) promote Alternaria infection. High humidity and temperatures greater than 14°C (57°F) favour spore development.

Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field. Observe both the older and newer leaf growth. Record the percentage of leaves infected and the average number of lesions per leaf. Also note any black spotting on florets or heads when inspecting mature plants.

Thresholds
None established.

Management Notes

• Space plants to allow for adequate air circulation. This is especially important for broccoli. Water droplets that bead and remain on broccoli heads can provide the fungus with the moisture it needs to initiate disease.
• Avoid overhead irrigation once the disease has been diagnosed in a field.
• It is essential to ensure that cabbage is free of Alternaria before going into storage. If Alternaria is present in the field it will be necessary to protect the plants with a fungicide prior to harvest.  
• In storage, temperatures below 4ºC (39ºF) inhibit the development of new infections.
• The fungus persists on crop debris and wild crucifers and on or in seed. Hot-water seed treatment will eliminate both internal infection and external infestation of seed, while fungicide seed treatment will only control spores on the seed.
• Rotate with less-susceptible crops (cereals or corn) and control chewing insects.