Downy mildew

Downy mildew is a disease caused by the fungus-like water mold, Pseudoperonospora cubensis, which attacks only cucumbers and related crop species (gourds, squash, pumpkin and melons). This disease primarily affects foliage and can cause severe yield losses in a short period of time.

The basics of downy mildew

Symptoms are usually seen first on the lower, older leaves. Initial symptoms of downy mildew typically consist of angular yellow spots on the upper leaf surfaces.

Lack of photosynthetic tissue results in stunting of plants, reduced fruit size and poor fruit set.

Pseudoperonospora cubensis is an obligate parasite requiring living host tissue to survive. Infections in greenhouses likely originate from another type of spore (sporangia) that enters the facilities from the outside.

Moisture on the leaf surfaces is necessary for infection to occur. When spores land on a wet leaf surface, they can either germinate and infect through the breathing pores (stomates) on leaves or release many smaller spores, called zoospores, that swim in the film of water on leaves during humid or wet conditions and enter and infect leaves through stomates.

Optimum temperatures for infection range between 61° F and 72° F, with infection occurring more rapidly at warmer temperatures. For example, the periods of wetness needed for infection on cucumber leaves are about 12 hours at 50° F to 59° F, six hours at 59° F to 66° F and two hours at 68° F.

About four to five days after infection, new spores are produced and released into the air, primarily in the morning. Spores can quickly spread within the greenhouse via moist air currents, contaminated tools, equipment, fingers and clothing. Fortunately, the spores become less infective under conditions of high temperatures and low humidity in the greenhouse.

Downy mildew management strategies

Avoid dew formation by providing adequate heating and ventilation. This is critical to reducing the conditions for infection and disease development. Pay special attention to purging moist air out of the greenhouses during the evenings and keeping the plant foliage dry, particularly during the night. At normal greenhouse temperatures, maintain a maximum of 70 to 75% relative humidity (approximately 6.5 to 7.5 mbar or 0.65 to 0.75 kpa).

Remove all sources of infection, such as infected leaves and plants, and discard them in garbage bags away from the site. Disposal can include burial of plant debris, but ensure plants are buried properly so that spores do not escape and infect nearby commercial cucurbit fields (pumpkin, zucchini, cucumber and squash). Remove all weeds, including all cucurbit plants surrounding the greenhouse, because these may serve as reservoirs for the downy mildew fungus and for plant viruses. Disinfect surfaces in infested greenhouses.

Ensure that plants are sufficiently spaced and that the canopy is well pruned and thinned to provide for adequate air circulation.

Over-watering not only leads to overly soft, more vulnerable plants, but also to production of droplets of moisture at the margins of leaves (guttation) early in the morning that provide perfect infection sites for the downy mildew pathogen.

Any cultural practice (e.g., misting) that increases surface leaf moisture will increase disease development when downy mildew spores are present in the air.

If only a few spots are evident on a few leaves of one plant, the disease is in the early stages of development. Implement appropriate control procedures immediately, because the spores are readily dispersed by air currents, and further disease development may be very rapid under favorable environmental conditions. Ensure good coverage, particularly with protectant fungicides.

Where possible, rotate fungicides to reduce the chances of developing resistance in the fungus to the material applied. Generally, use systemic fungicides in combination with protectant fungicides. As for all crop protection chemicals, always read and follow label recommendations.

Source: Ontario Ministry of Agriculture, Food and Agribusiness