This year summer is really wet, and I'm already seeing black scab spots on my apple tree leaves. How do I treat it if I'm trying to avoid chemicals? I've heard about copper sulfate and urea, but I don't know when and how to apply them correctly.
The main thing in a rainy summer is to prune branches that are crowding the canopy for better air circulation and remove infected leaves, otherwise no amount of spraying will help. Copper sulfate (Bordeaux mixture) and urea work, but you need a system: apply copper in early spring when buds are swelling and after flowering, and urea in autumn to treat fallen leaves (it kills spores).
Between treatments, spray your apple trees with biofungicides like trichoderma or phytosporin every two weeks during wet periods - they're safe for you but suppress the fungus. And make sure to collect and burn fallen leaves, where scab overwinters, otherwise every summer will be the same.
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