Sun scald

Sun scald or sun scorch is caused by too strong sunlight, especially on wet leaves or fruit or a poor root system, so the plant can't draw enough water on hot days.

Symptoms
The plant or fruit will grow a paper-like, thin, grayish-white substance. The sunscalded area may also begin growing black mold which will rot the tomato, and render it inedible.

Treatment
Spray plants with water to keep them cool and less stressed. Glass in a greenhouse, on a window sill or in direct sunlight will be stronger, bear this in mind.

Prevention
Over watering can damage root systems, so watering correctly is critical, a little and often is the best advice to keep the peat constantly moist. Roots need air as well as water and if the conditions are too wet for too long, this will suffocate the roots and kill them, too dry and the plant will overheat on hot days.