Powdery mildew on cucumber: symptoms, causes, control

Powdery mildew is a fungal disease that affects cucumber leaves, causing white spots and reducing crop quality. This article from the “WORLD OF PLANTS” website reviews methods for preventing and controlling powdery mildew on cucumbers.

 Cucumber powdery mildew family

Name of the disease: Powdery mildew on cucumber  
 Scientific name: Erysiphe cichoracearum

A common disease in cucurbits, especially cantaloupe, cucumber, zucchini, pumpkin, squash, and sometimes watermelon. The susceptibility of each crop differs from the other in terms of infection with this disease. We find that the most susceptible are cucumbers, followed by cantaloupe, followed by zucchini, followed by watermelon and pumpkin, and the least affected are squash and pumpkin. 

 Symptoms of powdery mildew on cucumber

The infection begins to appear on the lower surface of the lower leaves, especially older leaves, in the form of a powdery-looking growth, in the form of white spots scattered on the upper surface of the leaves, and they grow to cover the entire surface of the leaf, and soon these leaves wither, dry out, and die. 

 Causes of powdery mildew on cucumbers

Erysiphe cichoracearum is an obligate fungus that reproduces on living plant hosts.

Suitable conditions for the spread of powdery mildew on cucumbers

• Prefers warm, dry climates. The fungus does not need water on the leaf surface for infection to occur. However, the spores need dew to germinate and can be carried by the wind. 
• Relative humidity may reach less than 20%, and infection can occur at a relative humidity of about 46%, but the infection rate increases if the humidity is higher than that.
• The optimal degree of infection is between 20-27 degrees Celsius.
• The susceptibility of plant leaves to infection varies depending on their age. Newer leaves are highly resistant, and leaves reach their highest degree of susceptibility to infection when they are 16-23 days old from opening.

Development cycle of powdery mildew on cucumber

The initial inoculum begins with conidial fungus spores, which are spread over long distances by the wind. The conidial spores remain viable for a period of 7-8 days. Although both of the most widespread types of mushrooms have a wide range of plant families other than cucurbits, these plant families do not It is important as an important source of infection, and this is due to the specialization in the pathogenicity of the pathogen. The time between the occurrence of infection and the detection of symptoms ranges from 3 to 7 days, and large numbers of conidial bacteria form during this period.
At the end of the season, when the leaves begin to dry, the fungus sometimes appears in some families in its sexual stage in the form of acacia fruits, which are resistant to weather conditions during the winter.

Losses from the spread of powdery mildew on cucumbers

The disease poses a major threat to production, as the size and number of fruits on plants infected with the disease decreases, the crop harvest period also decreases, and it affects the quality of fruits formed on infected plants.

 Strategy for controlling powdery mildew on cucumbers

Effective control requires precise spraying of the entire area with large amounts of high-pressure water. A common strategy is to perform static or dynamic scheduled spraying operations.

 Preventive measures to prevent the occurrence of powdery mildew on cucumbers

It is easier to defeat pests during the early stages and is more cost effective. The field should also be monitored routinely and looked for the presence of the above signs.

Recommendations for organic and chemical control of powdery mildew on cucumber

1. Plant resistant varieties, if available, or tolerant varieties of cucumber R-C-14.
2. Providing good ventilation and reducing humidity limits disease.
3. Removing diseased plant residues and weeds belonging to the Cucurbit family.
4. Prune infested parts in greenhouses and remove them early in the infestation.
5. Biological control using the fungus Ampelomyces qusqualis, Bacillus subtilis bacteria, and the Trichoderma fungus, as treatment reduces the infection rate by between 55% and 98%. 
6. The use of sulfur, whether by fogging or dissolving. Cantaloupes and cucumbers are crops sensitive to sulfur, especially in hot climates. 
7. Use neem oil and cinnamon as is the case with downy mildew. 
8. An integrated control program can be implemented using tolerant varieties and a fungicide spraying program that depends on the degree of infection. Control begins when one leaf is infected out of 40 old leaves that are collected randomly before starting the spraying process. 
9. Use fungicides such as Carbendazim and Topaz.

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References:

Garden and House Pests Book - Faculty of Agriculture - University of Jordan