Last week the question was: What is the white stuff on these cucumber leaves?
This is powdery mildew. It loves warm, humid weather and can spread quickly when these conditions occur. A greenhouse is a great place for powdery mildew to develop since they normally stay warm and humid. Once noticed, a fungicide should be applied to keep it in check if the environmental conditions are to persist. Take a look at the fungicides labeled for powdery mildew on cucurbits in the SE Vegetable Crop Handbook starting on page 180. Be sure to read and follow labels closely, especially in greenhouses.
Here is this week’s question: What is this on an oat leaf? (magnified 35x)