Lesions caused by Alternaria brassicae, Alternaria tenuis and Stemphylium botryosum commonly affect leaves already weakened by other causes such as virus.

 

Identification

Infection begins as small dark spots in already yellowed areas of leaves, especially at leaf margins. Lesions are dark brown to black and can appear velvety black with heavy sporulation. Old leaves weakened by age, disease or stress are colonised first

 

Alternaria sp. lesions on a virus-infected beet leaf.

 

Importance

These fungi exacerbate damage from virus infections and deficiencies, significantly reducing green leaf and DM yield.

 

Spread

Spores produced on the lesions are readily dispersed by wind. Cool, humid conditions favour infection.

 

Management

Alternaria and Stemphylium mostly occur in mature crops with older, senescent foliage, so management is difficult. Monitor irrigation and fertility to reduce plant stress and limit potential disease development. Some fungicides are registered for Alternaria control.