To control powdery mildew and other fungi, use a mix of powered milk in water. Spray both under and top sides of leaves. So far I have found this to be a very useful solution for fungus on tomato plants, chile plants, cucumber and squash plants! I use about a 50 percent milk solution. I have had no problems with the leaves burning. Leaves already infected generally still die back, but the newer leaves are staying healthy.
To control powdery mildew and other similar fungi, use 1 tsp baking soda in 1 quart of water and spritz the leaves top and bottom. Spray leaves at first sign of the mildew. As the name implies, powdery mildew looks like a fine layer of white or a water spot–white halo spots on leaves. Crepe Myrtle gets powdery mildew in Texas quite often. I spray when the myrtles begin showing leaves in the spring. Snap peas and snow peas also have this problem when the temperature hits about 78. Again, you’ll have the best luck if you spray early.
Because Baking soda isn’t good for the soil, do not overspray. The baking soda changes the pH on the leaves and inhibits the growth of mildews.
I often put a tsp of soap in with the baking soda–the soap works as an insecticide for soft bodied bugs (aphids, some stages of thripes, etc). Use liquid hand soap–not dishwashing soap or detergents.
Tomatoes and other plants can also get a mildew although it is a different type. The leaves turn yellow from the bottom of the plant up. Little brown circles are often on the yellow leaves. The fruit will continue and be unharmed, but a spritz of the milk solution (or baking soda) can help. Note that some bugs such as spider mites will cause yellowing of the leaves from the bottom up.
Update Some recommendations advise using milk one week and baking soda the next. Thus far, I’m having better luck with the milk–it doesn’t burn the plants in 90 degree weather and seems to work better. The baking soda solution works also and has the added benefit of killing bugs when I put soap in the mix–however I have had some leaves burn pretty badly. I read that the milk solution works best against the mildew during sunny days because something in the ultraviolet gets the action started that kills the mildew. A similar milk solution with flour added is rumored to kill spider mites. I’m hoping that the milk alone might kill them also as spider mites are always trying to get a hold on my garden!
Update 2009: Since I’m already using neem oil for spider mites on things like snap peas and tomatoes, I’m going to see how well it does to control the mildew without trying the milk. It’s been a dry winter–that may help all on its own, but at any rate, the neem is supposed to be helpful against mold and mildew so we will see how it goes! UPDATE: Neem doesn’t seem to help much.
Update: 2016: Best solution is the milk—and planting mildew resistant varieties, especially of cucumber (best luck with poinsetta and spacemaster). You can start the milk before you see any powdery mildew too because it doesn’t hurt the plants or the soil.