Occurrence of bitter rot of apple in Illinois orchards

Bitter rot, caused by Colletotrichum spp., is one of the most important diseases of apple in the world, and is the most difficult summer apple disease to manage in Illinois. In the past five years, outbreaks of bitter rot were experienced in most Illinois apple orchards, causing up to 100% yield losses in some apple cultivars. Our surveys in 2019 and 2020 showed that the incidence of bitter rot in apple orchards in Illinois ranged from 0 to 63% in 2019, and 0 to 100% in 2020. Bitter rot was observed in most orchards in central and southern Illinois, while most orchards in the northern part of the state were bitter rot-free. Incidence of fruit with bitter rot in ‘Empire’ and ‘Honeycrisp’ apples was higher than other apple cultivars. Bitter rot incidence was higher in 2020 compared to 2019 in the orchards where mummified fruits and dead wood from previous years were not collected. Infected fruits were collected from apple cultivars Braeburn, Empire, Fuji, Gala, Golden Delicious, Granny Smith, Honeycrisp, Jonagold, Jonathan, and McIntosh in 28 orchards throughout the state. Infected tissues of fruits were cultured on potato dextrose agar (PDA) medium, and the pathogens were isolated. Identification of pathogen species was based on the morphological characteristics and molecular identity of the isolated fungi. Identified species belonged to the Colletotrichum gloeosporioides and C. acutatum species complexes. More than one species of the pathogen was identified in some orchards.