a simple yet effect spray for mildew on broad leaffed vegies like cucumber and zucchini:
1 part milk to 10 parts water. mix. spray.
from 'organic gardener - fruit essential guide'
use immediately. if you store it in the fridge probably good for another day but would recommend a new mix after that. NOTE: too much milk in the solution can encourage sooty mould, so stick to the recipe.
i had heard adding calcium or milk to the soil around the cucmbers could help ward off mildew as well. i tried that this summer however with the huge amount of rain we had in northern nsw it had no effect. the direct spray worked within a few days.
for tomatos i recommend the bicarb spray. its stops the progression of mildew within a day of spraying. fantastic!