Incidence of diseases on foliage of nine apple genotypes as influenced by six fungicide treatments

Three commercially important apple cultivars ('Delicious,' 'Golden Delicious' and 'York'), three scab-immune cultivars ('Liberty,' 'Redfree,' and 'Freedom'), and three scab-immune numbered selections from New York were subjected to six different fungicide/timing treatments for three years to evaluate their effects on resulting foliar disease symptoms. Apple scab (Venturia inaequalis) infections occurred only on 'Delicious,' 'Golden Delicious' and 'York' and were prevented by early-season fungicide sprays. Leaf spot symptoms were not identified by cause and could include frogeye leaf spot (Botryosphaeria obtusa), early symptoms of Alternaria blotch (Alternaria mali) or early phases of rust infections on resistant genotypes. Leaf spot symptoms were generally most severe on 'Redfree' and NY 74840-1 and least severe on 'Golden Delicious' and NY 74828-12. Necrotic leaf blotch was observed on 'Golden Delicious' each year. The percentage of leaves that had abscissed by late Aug. was greatest for 'Golden Delicious' (probably due to necrotic leaf blotch), 'Redfree,' and NY 73334-35 (possibly due to cedar apple rust or Alternaria blotch). These results indicate that foliage of scab-immune genotypes may possess varying susceptibility to other diseases.