Pest Of The Month: Alternaria Leaf Spot of Cabbage
In addition to cabbage, cauliflower, broccoli, collards, mustards, and other brassicas are commonly infected. Alternaria also can affect the edible flower parts of broccoli and cauliflower. Symptoms on broccoli and cauliflower appears as brown-black discolorations on the heads (sometimes referred to as brown rot). Spots on leaf blades vary in size from small dark circular spots to large black, brown, or tan spots 2 to 3 inches in size as infections progress. Leaf-spotting on margins of leaf blades can be confused with black rot symptoms. Old leaf spots become papery in texture and may tear and display a shot-hole effect.
Affected leaves may have a purplish cast. Larger spots often have a dark green-black coloration of fuzzy growth in the spots, usually concentrated in the center. This dark, fuzzy growth consists of the fungus spores (conidia) and spore stalks (conidiophores). The spore masses may be interrupted, forming concentric rings within the leaf spot as the result of interruptions in weather favorable for spore production.
Survival And Spread
The fungus can survive on or in crop debris, seed, and possibly cruciferous weeds. Spores are produced on old crop debris and leaf spots. In culture, the optimum temperatures for spore production are between 75°F to 82°F. If leaf wetness is prolonged, spore production may occur outside the optimum range of temperatures.
Spores are released during the day as the relative humidity diminishes. Movement of equipment or work crews in the field and wind aid in spore release and disease spread. Spores can blow more than one mile downwind from infected crops.
After landing on susceptible plant tissue, spores germinate in the presence of free moisture in the form of dew, rain, or irrigation water.
With longer periods of leaf wetting due to prolonged dew periods or heavy or frequent rains, the number of infections increases drastically and lesion size increases.
The fungus also may be seedborne.
An integrated approach is most effective in managing Alternaria leaf spot of cabbage.
• Purchase quality seed, preferably hot water-treated.
• Seed treatment with a broad spectrum fungicide will help reduce any inocula carried on the seed.
• Rotate crops and avoid planting crucifers on the same land in succession or in successive years.
• Purchase disease-free transplants.
• Begin a preventative fungicide spray program during transplant production and after plants are set in the field. More frequent applications may be necessary if wet conditions prevail. If control efforts are started after symptoms are widespread, control will be difficult.
Harvested fields should be disked in shortly after harvest to bury old crop debris.