I thought all was well, but I am the type to learn by doing (and so I soon learned). In February, a few dusty spots appeared on them, which spread rapidly. Soon, many of the formerly healthy leaves were blighted by the dusty stuff.
I looked it up in my trusty gardening book (Xanthe White's Organic Vegetable Gardening). "Powdery mildew" which is best prevented rather than treating. If I had kept the plants consistently well watered, and sprayed the leaves all along with water with a bit of dish soap the mold would have had a harder time staying. (And knowing how much it spreads, I would have reacted far more quickly at the first spots.)
As it was, the mildew on the leaves got so bad I had to spray them all over with copper spray, both sides (Bordeux mixture would have been better, see recipe below). It was a lot of work, and the leaves still didn't look great, so I ended up destroying much of the plants trying to remove it. We did get a few large pumpkins out of it, but I had alot of sadness destroying the infected plants, along with the many flowers that all would have become pumpkins.
Boo hooo. Next year I will know to keep the leaves clear by watering the plants very well so they stay strong, and spraying with water and a bit of dish soap all along (or even Bordeaux mixture fortnightly) - but mostly by staying observant.
(I am also going to plant them in a devoted pumpkin monster plant area. Those things spread like crazy, tangling up all the other plants, they get damaged as you try to free them.)
Excerpt from Xanthe White's Organic Vegetable Gardening book:
"Cucumbers, courgettes, pumpkins and beans are particularly susceptible to this common garden affliction. Powdery mildew thrives in the hot dry of late summer so water regularly and consistently. If you are hit, strike back with a spray of old dishwashing water or detergent mixed with water. If this fails, try baking soda dissolved in water at a ratio of half a teaspoon to one litre. Treat more serious cases with Bordeaux mixture or flowers of sulphur. Act swiftly and remove as much infected material as possible before the fungus spreads." -Xanthe White
Recipe (from her book as well):
"Bordeaux mixture can be bought pre-mixed or made at home quite easily. It is the organic gardener's best defence against fungal disease, effective against everything from potato blight to powdery mildew, but should, as with all chemicals, natural or otherwise, be used with caution. It can damage soft or new growth so using the appropriate dilution is important.
"100 g copper sulphate dissolved in 1 litre of hot water
"150 g hydrated lime dissolved in 1 litre of cold water
"Add the copper sulphate solution slowly to the lime solution, stirring continuously. Dilute this mixture with a further 10 litres of cold water and use while fresh. Dilute to 10 parts further if you are using on tender young growth. If used as a preventative spray fortnightly; otherwise us as infection requires. Do not store in a metal container."