I know I posted this a while back, but I just read an article about Downy Mildew becoming a big problem in Jersey, so I thought I'd repost it.
Last season, my squash plants were covered with this white powdery stuff and the leaves were turning a very sad yellow, so I called my mom, the only person I know that can eat any fruit or vegetable, stick its seed or pit in the ground and have it magically grow.
My Mom's Organic Fungicide
2 TBS baking soda
1 TBS oil (she uses vegetable oil)
1 tsp liquid dish soap (later restated it as a squirt or two)
32 Ounces of water
1. Mix together, then spray all the tops and undersides of the infected leaves.
(I did it early evening, I remember reading somewhere that baking soda can burn leaves if put on leaves during the day, but my Mom didn't know about that either way.)
2. She also said to cut off all the yellow and dying leaves and the ones touching the ground. And to bag those leaves and throw them away. Do not compost them.
3. Make sure you wash whatever you used to cut the limbs off.
4. She said some other things, but now I don't remember...her explains can be long...if it comes back to me, I'll fill you all in.
Here's a tip from me: wear gardening gloves and long sleeves, and whatever you do, don't try to break off the leaves off of a squash plant with your unprotected hand. You'll wind up with a bunch of little, teeny, tiny, painful splinters. And make sure you spray both sides of the leaves!
The leaves that weren't badly infected cleared up right away, but the ones that were worse for the wear but not so diseased I thought they needed to be removed, took a few more days of spraying.