If you're growing the cucurbits family - cucumbers, pumpkins, squash or zuchinnis, you'll probably be on the lookout for powdery mildew.  If you haven't seen it before, it's a fungus that grows on the leaves of those plants and looks like someone has sprinkled talcum powder on the leaves.  If you ignore it, the flowers and fruit will fall off your vines and you won't get any of those delicious cucumbers or pumpkins you're after.

By far the best way to treat powdery mildew is to treat your plants before it appears.  Get yourself one of those small spray squirter bottles and pour in 1 part milk to 9 parts tap water.  Shake it up and spray it on your cucurbits one a week.  Generally it will keep powdery mildew away. 

Just make sure the cat is inside when you do it.  My old cat used to follow me around the garden and licked off the milk as I sprayed.