Can anyone help me? I have just grown 6 trays of wheatgrass in compost, in a cold frame and, over the last 5-6 weeks and they all start off ok, but then they go paler green to yellow with spots of a powdery stuff on them, like dirt has splashed up on them. It has been damp here in the UK with dry warm spells. My cold frame lid is covered with that fine meshed plastic stuff used for shading greenhouses. The water gets in but not the bugs. What is happening and how do I deal with it?