Causal Agent: Peronospora destructor
Distribution: Worldwide, where cool, moist
conditions occur during the growing season.
Symptoms:
Typically the first sypmtom 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 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 voluteer 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 (greater than 95%) humidity
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.
Control:
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- to 4- year rotation out of
onions in areas where the disease is present can help reduce losses.
