Weeks of heavy rain and overcast weather can mean only one think for your cucumbers: fungus. Specifically, powdery mildew. Powdery mildew looks like a white or grey powder that accumulates on the foliage of plants. It commonly attacks cucumbers, delphiniums, lilacs, roses, melons, and squash. Although the mildew can affect the yields of your fruits and vegetable crops, many plants do not die from the fungus. Cucumbers, however, are especially vulnerable to crashing in the conditions where fungus thrives.
Mildew is a challenge to control once it has infested your greenhouse. The best you can do is to control it by a combination of physical removal (meaning remove the affected leaves before they release spores) and treating with a sulphur-based fungicide.
Mildew usually cannot be “cured” in a greenhouse, but you can plant mildew-resistant or mildew-tolerant varieties and try to catch infestations early. Good air circulation is also essential — we have fans running 24/7 in our greenhouses. Avoid watering plants from above and getting water on the leaves.