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First Report of Brown Rot Caused by Monilinia fructicola in Sweet Cherry in Maryland

Monilinia fructicola (G. Wint.) Honey is the most important causal agent of brown rot of stone fruits in North America. In July 2010, 20 sweet cherry fruit (Prunus avium) of unknown variety with symptoms resembling brown rot were collected from one commercial orchard in Maryland. Each cherry fruit came from a different tree. Symptoms included necrotic areas up to 10 mm in diameter with brown conidia and conidiophores developing from the infection center. Spores from nine symptomatic fruit collected each from different trees of a single orchard were suspended in sterile water, spread onto the surface of 1% agar plates, and incubated at 22°C. After 12 h, single, germinated spores were transferred onto 9-cm petri dishes with potato dextrose agar (PDA). Nine fungal colonies, each from a different fruit, were investigated in three replicates for cultural characteristics on separate petri dishes containing PDA. They were very similar in morphology and grew 12.4 mm per day on average at 22°C, forming branched, monilioid chains of grayish colonies with concentric rings and little sporulation. Rich sporulation was observed on tomato sauce medium (250 ml tomato sauce and 20 g agar in 750 ml water). The lemon-shaped spores had an average size of 15 × 10 μm, which is consistent with M. fructicola. Two colonies were randomly selected to identify the pathogen to the species level using a PCR technique based on cytochrome b sequence amplifications (2). Resulting gel electrophoresis patterns were consistent with M. fructicola. Koch's postulates were fulfilled by inoculating 15 mature sweet cherry fruits of cv. Bing with a conidial suspension (10⁵ spores/ml) of one of the single-spore isolates from cherry. Fruit were stab-inoculated at a point to a depth of 2 mm using a sterile needle. A 10-μl droplet was placed on each wound; control fruit received sterile water without conidia. After 3 days of incubation at room temperature in airtight plastic bags, the inoculated fruit developed typical brown rot symptoms with lesions that were 20.6 mm in diameter. The developing spores on inoculated fruit were confirmed to be M. fructicola. All control fruit remained healthy. The entire detached fruit experiment was repeated 1 week later. M. fructicola is assumed to be the main causal agent of brown rot of sweet cherry in the northeastern United States, but recent studies show that M. laxa is also causing the disease on sweet cherry in many northeastern states (1). For this reason, it is important to delineate species for accurate disease assessments. This study confirms assumptions that M. fructicola is a causal agent of sweet cherry in Maryland.

References: (1) K. D. Cox et al. Plant Dis. 12:1584. 2011. (2) J.-M. Hily et al. Pest Manag. Sci. 67:385, 2011.