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Summary :
Downy mildews can spread quickly, causing severe damage, especially during cool, rainy weather. The infection marks leaves with irregular lesions and covers affected tissues with whitish down. Severely affected plants dry out and die. In Ireland, downy mildews caused an epidemic in potatoes, leading to a terrible famine in the mid-19th century.
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Host plants :
Various trees and shrubs, including azalea, currant, grape, rose and viburnum.
Various herbaceous plants, including aster, begonia, chrysanthemum, helianthus, lily, lupine, peony, petunia, phlox, primula, snapdragon and sweet pea.
Certain vegetables, including eggplant, potato, sweet pepper and tomato.
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Signs and symptoms :
- The infection often starts with the appearance of small, dark, irregular scorch marks (lesions) on upper leaf surfaces and stems. A greyish-white down (mycelium) forms around the edges of and under the infected zones.
- When weather conditions are favourable, the lesions grow and the infection spreads very quickly to leaf and flower stalks and fruit, which are covered in dark, glossy patches.
- In the final stage, affected tissues turn brittle, leaves and flowers drop, fruit rot and severely affected plants die.
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Description and life cycle :
Downy mildew fungi are microscopic members of the class Oomycetes, oospore fungi.
They produce thread-like tubular mycelia that form branching sporangia: small sacs containing asexual or sexual spores, depending on weather conditions.
These obligate parasites overwinter inside infected leaves, stems and fruit in the form of spores and mycelia.
In spring, the mycelia resume growth and the spores are dispersed by wind and rain, causing an initial infection.
During the growing season, when weather conditions are favourable, the mycelia proliferate and produce zoospores, asexual spores with flagella that make them free-swimming. A film of water on plant tissues promotes their dispersal and germination.
This is the most active phase, when the downy mildew multiplies quickly and consolidates its infection of the plant.
In late summer, the fungi produce oospores, sexual spores that are able to survive during unfavourable conditions and serve to maintain the genetic diversity of the species.
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Favourable conditions :
Downy mildews grow quickly when temperatures are cool or warm, but not too hot (optimum 20 °C), and when the air is very humid. A film of water on plant tissues promotes infection.
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Identification :
Regularly inspect the foliage of susceptible plants and those growing in less than ideal conditions, especially when weather conditions are favourable to the disease.
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Prevention :
- Plant susceptible species in sunny, well-aerated spots; maintain adequate spacing between plants.
- It is best to water in the morning and to avoid wetting the foliage.
- Rotate vegetable crops over a four-year period.
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What to do :
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Physical Control :
- Rake up and dispose of infected leaves and fruit to reduce the source of contamination.
- Cut off and dispose of all the affected parts during dry weather, to avoid spreading the disease.
- Disinfect pruning tools regularly with a 70% isopropyl (rubbing) alcohol solution.
- Never compost infected plant litter.
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Biological control :
None available.
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Chemical control :
The Montréal Botanical Garden does not recommend the use of pesticides to control this disease.
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Credit : Jardin botanique de Montréal
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