Alternaria & Stemphilium leaf spots

Summary

Lesions caused by three fungi - Alternaria brassicae, Alternaria tenuis and Stemphylium botryosum - commonly affect leaves already weakened by other causes.

 

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.

 

Importance

These fungi exacerbate damage from virus infections and deficiencies, causing significant loss of 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.

 

 

 

 

Bacterial leaf blight (BLB)

Summary

Bacterial leaf blight (BLB) is caused by the bacterium Pseudomonas syringae p.v. aptata and can affect both leaves and bulbs.

 

Identification

Dark greasy leaf lesions, often with lighter brown areas and centres, commonly occur on leaf margins where bacteria have accumulated. Seed borne infection can cause seedling blight.

 

Importance

Not uncommon in fodder beet crops. Severe infection reduces green leaf area and bulb dry weight.

 

Spread

Primary infection often arises from seed-borne inoculum. BLB also enters crops via volunteer host plants and crops. Water splash, stock and machinery spread the disease. Bacteria can also survive on crop debris.

 

Management

Always sow seed from a reputable source.