Relative susceptibility of selected apple cultivars to fruit rot caused by Botryosphaeria obtusa

Twenty-three apple (Malus xdomestica) cultivars were tested in the field and laboratory for their relative susceptibility to the black rot pathogen, Botryosphaeria obtusa. Wounded fruit were inoculated in the field at 2 to 3 weeks preharvest with mycelium from 14- to 21-day-old cultures. In the laboratory, detached fruit were inoculated similarly. Fruit were rated for relative susceptibility to the fungus by determining disease severity of attached fruit in the field based on lesion growth (mm/degree-day) and detached fruit in laboratory inoculations of wounded fruit (mean lesion diameter after 4 days). Based on the laboratory and field data from two growing seasons, cultivars were classified into three relative susceptibility groups-most susceptible: 'Orin', 'Pristine', and Sunrise'; moderately susceptible: 'Suncrisp','Ginger Gold','Senshu','Honeycrisp','PioneerMac', 'Fortune', NY75414,'Arlet','Golden Supreme','Shizuka','Cameo','Sansa', and 'Yataka'; and least susceptible: 'Creston', 'Golden Delicious', 'Enterprise', 'Gala Supreme', 'Brae-burn', 'GoldRush', and 'Fuji'. Compared to previous cultivar rankings, the results of the present study indicate that no new apple cultivars from the first NE-183 planting show greater resistance to Botryosphaeria obtusa than current standard cultivars.