effective against powdery mildew. make sure you cover every part of the plant - both sides of the leaves - and coat the stems. acts as a preventative, not a cure.
from 'organic gardener - fruit essential guide'
1tsp of bicarb
1/2 tsp sunflower oil
1 drop of diswashing detergent
to 1L of water.
stir well and use immediately.
i've had success using this on mildew on my tomatoes this year. and due to having the detergent included it has kept caterpillers at bay as well. :)
for cucumbers, zucchinis and apparently grapevines, milk spray is more effective.