What causes small blisters or bumps on leaves?
A physiological condition caused when plants take up water faster than it can be used and transpired. Excess soil moisture combined with cool temperatures, overcast weather, and poor air circulation can cause a buildup of water pressure within the leaves. Sometimes it develops during periods of warm, humid days with rainfall, and cool nights. The water pressure causes cells on the lower leaf surface to rupture, forming brown, corky, scab-like blisters. These spots are also visible from the upper leaf surface. Edema stops developing when the weather conditions change, but existing spots remain on the leaves. Prevent edema by allowing the top 2 inches of soil to dry between watering and space plants to allow for sufficient air circulation. Edema is most often seen in cabbage growing in waterlogged soil and in container tomatoes during wet spring weather.