Downy mildew is a serious disease on Cucurbits. It may be especially devastating to cucumbers where complete defoliation may occur only a few days after the appearance of initial symptoms.
Downy mildew is a fungal disease that leads to a multitude of symptoms that vary with the cucurbit type. On cucumbers, symptoms can be spotted on the upper and lower side of the leaves. Distinct water-soaked lesions often accompanied by gray mycelium are first observed on the lower side of the leaf. Yellow irregularly shaped lesions are found shortly after, confined by the small leaf veins on the upper foliar side, and eventually turn brown causing the drop of the leaf. To better identify the lesions, a signature “checkerboard” arrangement is characteristic of cucumber downy mildew.
Symptoms normally appear 4 to 12 days after infection. On cantaloupe, the somewhat angular lesions tend to have a yellow halo around them. On watermelon, the spots may or may not be angular, normally turning brown or black with the leaf developing an upward curl. On pumpkins and winter squash, the symptoms may resemble powdery mildew, causing yellow spotting that tends to brown out. As the lesions age, they usually become necrotic on all types of cucurbits and the leaves often senesce. This dieback is normally first noticed on the oldest leaves near the center of the plant. While the petiole, stem, and fruits are not attacked, fruits tend to be reduced in size, involving significant yield reductions.
Biology of the causal organism
Downy mildew is caused by Pseudoperonospora cubensis, an oomycete pathogen more closely related to water molds, such as Phytophthora, than to true fungi. This organism is called an obligate parasite, meaning that it requires a living host tissue to grow and reproduce.
Sporangia, the reproductive structures, spread the disease on wind currents. They are produced on the undersides of the leaves when conditions are optimal: high humidity and nighttime temperatures ranging between 13 and 24 degrees C. While the spread and survival of sporangia are highly dependent on weather conditions, high plantation density under the greenhouse and infected debris increase Downy mildew infections.
Only the presence of Sporangia is not enough to cause the infection. Upon deposition of the sporangia on the leaf surface, the absence of free moisture may prevent the occurrence of an infection. 2 to 6 hours of free moisture are required to induce contagion. In the same context, temperatures outside of the acceptable range 5 to 27 degrees C may also prevent infection.
The control measures below are suggested under the Integrated Pest Management – IPM umbrella, whether crops are grown conventionally or organically as they may help reduce or delay the chances of initial infection.
- Select growing sites with good air drainage, full sunlight, and low humidity.
- Allow enough row spacing to permit plant canopy aeration
- Avoid overhead irrigation to prevent leaf wetness over a prolonged period.
- Insure adequate, but not excessive fertilization.
- Frequently monitor the crop and keep an eye on the weather forecast
- In the early stages of a downy mildew epidemic, remove infected plants to help slow the spread of the disease.
- Beware of spreading the inoculant further by hand or by infested equipment, and always make sure to sanitize after getting into contact with any infested body.
- Adopt disease-resistant or tolerant varieties
When conventionally growing, it is strongly advised to apply preventive fungicides on a 7 to 10 day schedule upon the emergence of the seedling or during the transplantation phase.
When the first symptoms of downy mildew are detected in the area, a curative fungicide should be sprayed on a 5 to 7 days schedule.
Downy mildew can very rapidly develop resistance to fungicides. Therefore, resistance management strategies should be followed by alternating fungicides from different chemical families with distinct modes of action and by including protectant fungicides, such as Mancozeb into the tank mix.
Amongst the effective fungicides available, Cuprosate C and Rival Duo enclose ideal active ingredients CYMOXANIL 50 G/L + PROPAMOCARB HYDROCHLORIDE 400 G/L specific for oomycetes and are thus considered the best chemical fungicides for Downy Mildew control in cucurbits.
As for organic practices, several alternative fungicides are labeled effective against cucurbit downy mildew, including copper-based fungicides. Growers should be cautious in applying copper, though, as it can be phytotoxic to cucurbits. In addition, high levels of this element in the soil can be toxic to earthworms and other beneficial organisms contributing to the overall soil health and plant growth promotion.