Apple and Pear Scab
The two most common fungal diseases on apples and pears in the South West BC are Apple Scab and Pear Scab. These are two different but very closely related diseases. Both diseases overwinter on leaves under the trees (and maybe fruit). In the spring the fungi develop structures that look like peashooters with 8 spores each in last years leaves. Spore maturity is timed to coincide with bud swell of the apple and pear trees respectively. Once mature all it takes is a light rain for the spores to be shot into the air. These are very tiny spores and they can travel significant distances on windy days. If a spore lands on a leaf of the right kind of tree (apple or pear) they are unable to infect without water. The required water is free water, which means from rain or irrigation. Without water the spores will die in a few hours. But, of course, the same rainfall that caused the spores to be shot into the air provides enough water for spore germination. a couple of weeks after an infection small olive green to brown spots form on the leaves and sometimes fruit (the first two pictures above). Once the initial infection is established, a second type of spore is produced on the leaves that will infect other leaves and the fruit. A new infection will occur every time the leaves or fruit are wet and warm enough for long enough see the chart for timing.
There are resistant trees