Occurrence and management of apple bitter rot in Illinois

During 2016-2019, bitter rot of apple occurred in more than 60% of commercial orchards in Illinois. Incidence of fruits with bitter rot ranged from 0.3 to 63% in affected commercial orchards. Percent infected fruits in ‘Honey Crisp’ and ‘Empires’ was higher (1 - 63%) than other cultivars. The incidence of fruits with bitter rot ranged from 0.3 to 36% in ‘Gala’, ‘Golden Delicious’ ‘Jonathan’, and ’McIntosh’ apples. The fungus Colletotrichum acutatum has been identified in most of the orchards. Studies to identify additional causal species of Colletotrichum are underway. In 2019, a trial was conducted in a ‘Honey Crisp’ orchard that had more than 50% of fruits with bitter rot in 2018. Prior to the bloom, all mummified apples left from 2018 and prunes and dead wood in the orchard were collected and burned. The trees were sprayed with mancozeb plus Inspire Super alternated with mancozeb plus Fontelis at 7-day intervals, from the green-tip growth stage to one week before the petal fall stage, to control spring diseases (powdery mildew, rusts, scab). Several fungicides, including Aprovia, captan, Merivon, potassium phosphides, and Topsin-M, were tested for their effectiveness for managing summer diseases of apples (sooty blotch/flyspeck and fruit rots). Spray-applications of the fungicides began at petal fall stage and ended 21 days prior to harvest, at 7- and 10-, and 14-day intervals. Only bitter rot developed on fruits. Aprovia, captan, and Merivon were effective fungicides in preventing development of bitter rot on fruits.