Last summer, (2006) came in with wind and rain. That didn't suit our tomato crop at all. A wet late June and July left our plants covered with a white filmy substance that dried out the leaves and turned them brown and brittle.
Again we hit the Internet to find out what was eating our plants. We plugged search terms "tomato diseases" into Google and found several sites with pictures and explanations of every tomato killer imaginable. Our babies had powdery mildew.
To find a solution we went off to hardware/garden outpost Saiffee on Seventh Street and 1st Avenue in Manhattan (we've found the store a great, inexpensive place to buy tomato accessories like bamboo tomato stakes.) There we found Copper Soap Fungicide. Well, again we wanted to farm without chemicals, but we didn't want to lose our plants, so we sprayed. The affected areas remained brittle and dry, but new growth came in green and healthy.
The mildew stunted our crop, but the plants still produced enough brandywines, cherokee purples, patio tomatoes and cherries to share with friends. And they still tasted great.
In reading the fungicide label this year, I note that it can be used to prevent mildew by spraying before a period of protracted rain (watch the Weather Channel for predictions.)