Ozone damage is a key indication of air pollution with excessive ozone levels. The damage can cause bleached appearance on cucurbit leaves as seen on watermelon here.
The underside of the cucurbit leaves do not normally show beached appearance. However, it may show areas with marginal to high levels of necrosis.
Plants normally recover back from ozone damage. However it can affect the ability of the plant for optimum production. In some cases the symptoms appear as bleached spots.
A leaf with bleached midrib and veins and edges of leaf blades. Leaves may become brittle if damage is severe and can desiccate rapidly. Typically many leaves and plants are affected at a single time.