Answer: Based on your description, it sounds like the plants have powdery mildew. Powdery mildew is unique among plant diseases in that it doesn't require a wet leaf
surface to spread. It can thus thrive during hot, dry weather, as well as in high humidity.
The first line of defense is to grow resistant varieties, and secondly, keep a close eye out
for early infection, and use preventive measures to keep it from spreading. Spacing the plants for good air circulation is important, and pruning overly dense tomatoes can help somewhat, and removing leaves at the very first sign of infection can help limit the spread.
The general advise to inhibit the spread of fungal diseases is to avoid wetting leaf surfaces.
In the case of powdery mildew, you can actually inhibit infection with periodic strong
sprays of water to dislodge mildew spores (not so strong as to damage the plant.) Some
people have had good luck with pesticides containing neem, a plant extract. You can also
try using a homemade baking soda spray--mix 1 teaspoon baking soda per quart of
water; add 1/4 tsp mild soap to help it stick to the leaves. Apply this once a week, and
hose down plants between sprayings. Be sure than any fungicide you use is labeled for
powdery mildew, since some are ineffective against this disease.
Finally, remove and destroy any fallen foliage and do a very thorough garden clean up this fall, since the spores can overwinter on the old plants. Do not compost them unless your pile reaches a very high temperature.
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