Using more than 5 pounds per 100 square feet can dramatically increase soil pH which can result in both nutritioanl deficiencies and toxicities. Soil problems such as compaction and poor drainage can severely stunt plants. Were any insecticides or fungicides applied. Remember, copper and sulfur fungicides can burn plants if applied in hot weather. Broadleaf weed killers applied to lawns can cause abnormal growth or even kill many garden plants nearby.
Tomatoes are especcially sensitivee.
Beware of using a lawn clipping mulch which may have residual herbicide. Plants that die practically overnight may have been touched by frost. Don't overlook air pollution. Ozone levels may rise as hot, humid weather settles in for long stretches. Look for irregularly shaped spots which may look similar to feeding of mites and certain leafhoppers.