Apple scab, Venturia inaequalis overwinter on plant debris. With suitable temperature and moisture it produce ascospores and released into air.

Management:
1. Collect and burn the fallen leaves and twigs to prevent ascospres production.
2. Trimming and pruning of infected young shoots (the diseased shoot have cracked, rather coarse bark) and burning in the winter also removes overwintering spores.
3. Or apply urea (2 Kg/acre at pre-leaf fall stage spring and dolomitic lime (2.5 ton/acre) in autumn over fallen leaves to accelerate decomposition.
4. Systhane (triazole) is very effective as preventive, curative and eradicant.
5. If scab out break is spotted you can use Captan (contact fungicide) because it is not prone to resistance. (Captan blocks the ability of fungus to produce energy)
Captan 50% WP @ 1000 g in 300-400 l of water/acre
Or Captan 75% WP @ 666.8 g/ acre in water required to distribute 10-20 l/tree