Downy mildew on cucumber: symptoms, causes, control

Downy mildew is a fungal disease that affects cucumber leaves, causing yellow, downy spots that lead to poor plant growth. This article from the “WORLD OF PLANTS” website provides an explanation of the symptoms of downy mildew on cucumbers and methods of prevention and control.

  Cucumber downy mildew disease family

Name of the disease: Downy Mildew 
 Scientific name: Pseudoperonospora cubensis, Plasmopara viticola

It is one of the deadly diseases spread in cucurbits throughout the Arab region, especially those grown inside greenhouses. 

 Symptoms of downy mildew on cucumber

Spots appear on the upper surface of the leaves. They initially appear pale green, and later turn yellow. Their diameter may reach about 1 cm. They take on a many-angled or irregular shape and are defined by the veins of the leaf. These spots are offset on the lower surface by fluffy growths of a gray color.

The first symptoms appear on the old leaves in the middle of the plant. These spots dry up and turn brown. Then the infected leaves fall to the ground. The infection results in stunting and may be accompanied by total death. 

 Causes of downy mildew disease on cucumber

The disease is caused by Pseudoperonospora cubensis (Berkeley and Rostovzv) Curtis, an oomycete fungus.

 Suitable conditions for the spread of downy mildew on cucumbers

• Requires relative humidity more than 96%.
• Requires relative humidity more than 96%.
• The optimum temperature ranges from 16-22. 

 Development cycle of downy mildew on cucumber

The fungus   P. cubensis is a parasitic fungus, meaning it requires the living tissue of the host in order to survive and reproduce.  Therefore, the mushroom needs to spend the winter period in an area that does not suffer from frost, and where there are wild cucurbit cultivations. Spores are spread by wind over long distances and reach nearby plants and fields. Symptoms appear 4–12 days after infection.  The fungus thrives in cool, moist conditions, but the optimal conditions for infestation are 15°C with 6-12 hours of humidity, often in the form of morning dew.

 Losses of the spread of downy mildew on cucumbers

The infection results in dwarfing of the plant and may be accompanied by total death. The disease may lead to the immaturity of the fruits, and if they do mature, they will be weak, distorted, and have an unpleasant flavour. In cases of severe infestation, it sometimes leads to plant death in cucumber and cantaloupe crops, and is less severe in watermelon, zucchini, and squash crops. 

 Strategy for controlling downy mildew on cucumbers

To limit or combat downy mildew and limit its spread is to reduce humidity in agricultural crops, improve air circulation in them, get rid of dead parts inside them, and increase the distances as much as possible in the planted rows.

 Preventive measures to prevent the occurrence of downy mildew on cucumbers

Continuous ventilation of greenhouses if the cultivation is protected, adherence to the preventive spraying program, and the use of blue plastic reduces the penetration of the downy mildew fungus and its proliferation in protected crops. 

 Recommendations for organic and chemical control of downy mildew on cucumber

1. The disease requires a preventive spraying program to avoid infection, especially when humidity is high. It is preferable to use pesticides according to the proportions recommended by the manufacturers, which are Bravo, Mancozeb, Red and Mel, Daconil, and Divolatan, and spraying continues weekly until the disease stops.
2. Ventilate the greenhouse to reduce the humidity inside it by opening the windows, as well as reducing the irrigation water to the extent that ensures that the plant gets the water it needs, to avoid water evaporation and high humidity, and not using the irrigation method with sprinklers. 
3. Plant resistant varieties, if any, or tolerant ones, such as the R-C-14 cucumber variety, the Hana variety, and the Beit Alpha variety.
4. Plant plants at an appropriate distance, to avoid shading and moisture accumulation around the leaves. 

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