First Report of Black Rot on Apple Fruit Caused by Diplodia seriata in California

Abstract

A fruit rot was observed on apple (Malus domestica Borkh.) cv. Fuji in a commercial orchard during 2015 and 2016 in San Joaquin Valley County, California. Early symptoms of fruit rot included light to dark-brown lesions (3-4 cm in diameter) with defined margins and firm texture. In more advanced stages of the disease, the fruit became completely rotten and soft. Additionally, the affected apple trees developed fungal fruiting bodies on shoots initially blighted by Erwinia amylovora, the causal agent of fire blight. A fungal species was consistently isolated by cutting small pieces of symptomatic fruit and blighted shoots and placing them on acidified potato dextrose agar (APDA). The plates were incubated at 22°C for 7 days. To obtain pure fungal cultures, hyphal tips were transferred to PDA. Upon isolation, 75% of the colonies formed on PDA developed dense white aerial mycelia that became dark gray after 7 days. The reverse of the colony ranged from olive green to black. The fungus formed pycnidia on pist...