Downy Mildew

Downy mildew typically produces pale green to yellow patches on the upper leaf surface, often bounded by the leaf veins so that they appear angular rather than rounded. On the corresponding underside, especially in humid or damp conditions, a fuzzy grey, purplish, or white sporulating growth develops, most visible early in the morning while dew is still present.

Downy mildew oomycetes produce sporangia on infected leaf undersides that are carried by wind and rain splash to new leaves and plants, where they can release swimming zoospores under wet conditions that actively move toward and infect fresh tissue. This cycle can repeat every few days when humidity stays high and leaves remain wet for extended periods.

• Foliage-infecting species (e.g. on grape and cucurbits) survive between seasons mainly in infected plant debris
• Some species, such as the sunflower downy mildew pathogen, also produce long-lived oospores in soil that can infect roots and cause systemic seedling infection
• Cool, humid nights with dew formation favour sporulation and new infections

Because downy mildew species are largely host-specific, the crops most commonly affected include grapevine, cucurbits such as cucumber, lettuce, sunflower, and members of the brassica and onion families, among others. The presence of downy mildew on one crop does not indicate risk to unrelated crops nearby, since the causal oomycetes rarely cross between very different host groups.

• Prolonged leaf wetness from rain, dew, fog, or overhead irrigation
• High humidity combined with moderate-to-cool temperatures
• Dense canopies with poor air circulation that trap moisture
• Susceptible varieties and continuous cropping of the same host
• For soil-borne species, wet soils around germination and establishment

Because downy mildews spread quickly under favourable weather, management focuses on reducing leaf wetness, choosing resistant varieties, and detecting infection early.

• Choose resistant or tolerant varieties where available
• Improve air movement through appropriate spacing and canopy management
• Avoid overhead irrigation practices that extend leaf wetness, where practical
• Improve drainage and avoid waterlogged soils for species with a soil-borne stage
• Scout regularly, especially during humid, wet weather