Crucifer Diseases
 

Alternaria Leaf Spot
Alternaria brassicae, A. brassicicola

Found worldwide

Symptoms

Most crucifer plants are susceptible to Alternaria leaf spot and plants at all growth stages may be affected. Symptoms caused by the two pathogens are very similar and both may be found on the same plant. Infection of seedling stems may result in damping-off or stunted plants.

Leaf spots vary in size from pinpoints up to 5 cm in diameter. Typically lesions begin as small yellow areas that enlarge to about 1.5 cm in diameter and are dark colored spots with concentric rings. A brown discoloration of cauliflower and broccoli heads is caused by infections with these pathogens. The pathogen causes elliptical necrotic spots on seed stalks and seedpods. Pod lesions may extend into the pod interior and attack the seed causing them to shrivel. 

Conditions for Development

The pathogens are seedborne. They also sporulate profusely on debris from infected plants, and persist on susceptible weeds. Alternaria is readily spread by wind. The disease is enhanced by warm, wet weather with an optimum temperature between 25 and 30°C.

Control

Pathogen-free seed should be used. Hot water and chemical seed treatments are effective. Fungicidal applications may be necessary when crops are grown in conditions that are conducive for severe disease development. Avoid overhead irrigation particularly with cauliflower and broccoli in which head rot may occur.