Every year the garden suffers quite badly from powdery mildew. It generally starts on the flowers (the marigolds first, then the sweet peas, roses etc) and eventually I get it on all my cucurbits (squash, melons, cucumbers and courgettes). My understanding is that that there are hundreds of versions of PW and they are type specific, but if I see it on the marigolds you can bet general conditions mean it will appear on the rest of the garden soon.
It’s very easy to spot, it looks like it sounds, mouldy looking white powdery spots on the leaves (especially the undersides as it prefers warm, damp shade). It also thrives where there are high nitrogen levels. Good prevention is to keep plants well spaced so that air can move around them, keep them in bright sunlight and water in the mornings so they have the rest of the day to dry off. It looks unsightly, so to be honest on my Hardy Annual plants (ie the marigolds and sweet peas) I just pull them up and destroy the foliage then replace with something new. For my Cucurbits though, I need them to stay in good condition or the crop will suffer, especially the squash that need a long growing season. In these cases you can spray the leaves (as organic prevention) with a baking soda mix once a week:
1 teaspoon baking soda
1 quart water
A few drops of liquid soap
you need to be sure to spray the undersides as well. I keep reading about Mexican farmers using milk and it is reported to be very successful so I am trying this method. You mix milk (any kind, but skimmed will smell less as it has a lower fat level) 1:9 with water. You must not go over 30% milk as that can cause conditions favourable to other fungus. I’ll let you know how it goes.
On a extra note, my Squash are growing a very different rate from each other…some look like this:
and others like this:
Oh dear! I don’t fancy the chances of this producing may squash!!