Southern leaf blight

Bipolaris maydis (Helminthosporium maydis / Cochliobolus heterostrophus)

Credits:Biovision-Infonet

Symptoms first appear as small yellow dots that become elongated between veins. They later become brownish to creamy white in colour with reddish to purplish brown borders. Light brown leaf spots with a brown margin, at first elliptical, becoming rectangular, up to 25 mm long and 2 – 6 mm wide. The spots are at first restricted by the leaf veins, but later they may merge. Leaves dry out and die prematurely.

Silks, portions of the husks and cobs may turn black. A black mould may develop on cobs. Disease development is promoted by prolonged wetness on foliage, extended dew, RH (97-100%) and relatively warm temperatures (24-35degC).

Spread is by airborne spores; and the fungus is also seed-borne. Survival in soil occurs for up to 12 months.What to do:

• Use disease-free seed or treated seed (steam-air mixture at 53.9 – 55 degC for 17 minutes)
• Practise field sanitation, destroy crop residues and volunteer plants
• Practise crop rotation
• Use tolerant, resistant varieties if available