When does apple scab occur?

Apple scab occurs when an apple tree falls victim to a fungus called Venturia Inaequalis, a malignant growth which slowly eats away at the leaves and fruit. It is a widespread and serious desease affecting apple trees all over the world, but is more powerful in warm climates. Farmers control the fungus by spraying the trees with sulphur. That is one reason why apples should be washed before eating.