Disease occurs worldwide in temperate and cool growing regions.
Typically the first symptom observed is the brownish-purple velvet-like sporulation of the pathogen on healthy green leaves. As the disease progresses lesions which are slightly paler than the normal leaf color, enlarge and may girdle the leaf. These lesions progress to a pale yellow followed by brown necrosis resulting in collapse of the leaf tissue. Infected seed stalks tend to remain pale yellow and, as with the foliage, are often invaded by other fungi, typically Stemphylium or Alternaria species. Field infections usually begin in small patches and progress rapidly throughout the field. Bulbs can be infected and may either rot in storage, or if planted, give rise to pale green foliage.
Conditions for Disease Development
The fungus survives in volunteer onion plants, onion sets, plant debris or in the soil. The fungal spores are disseminated onto plants by winds and splashing rain during cool wet weather, which is essential for disease development. Rain, dew or high humidity (>95%) is required for fungal spore germination and infection. The fungus grows internally and continues to produce spores as long as the weather remains cool and wet.
A regular fungicide spray program based on climatic conditions can reduce crop losses. Avoid planting onion sets that are contaminated with the fungus. Eliminate plant debris and cull piles. Plant rows in the direction of the prevailing winds and use furrow irrigation rather than sprinkler irrigation. A 3-4 year rotation out of onions in areas where the disease is present can help reduce losses.