Disease guide

Symptoms vary depending on the pollutant causing injury and the host. Air pollution damage can reduce yield and affect fruit quality.

Ozone: Sensitivity to ozone among cucurbits varies. Watermelon and squash are most sensitive, pumpkin and cantaloupe are intermediate in sensitivity and cucumber is more tolerant to ozone injury compared to other cucurbits. Damage appears on the upper surface of older leaves, which develop a chlorotic, netted appearance due to loss of chlorophyll between the veins. Later, these chlorotic areas turn brown. Sulfur Dioxide: Plants exposed to chronic, sub-lethal levels of sulfur dioxide may develop chlorosis of the margins and interveinal areas of the leaves. These chlorotic areas generally remain turgid. In cases of acute injury, the margins and interveinal areas become necrotic. New fully-expanded leaves are more sensitive to acute injury than leaves that are not yet fully expanded.

Ozone is produced by the action of sunlight on the products of combustion. Most ozone is generated over large urban areas from automobile exhaust. Ozone damage may occur many miles from the original source of pollution. Ozone is absorbed passively by plants through stomata.

Sulfur dioxide is formed during smelting processes, when sulfuric acid is produced or during combustion of coal or oil. Likelihood of sulfur dioxide damage is greatest when temperature and humidity are high.