W. M. Jurick II, Food Quality Laboratory, USDA-ARS, Beltsville, MD;
I. Vico, Department of Phytomedicine, University of Belgrade, Serbia;
V. L. Gaskins, Food Quality Laboratory, USDA-ARS, Beltsville, MD;
W. J. Janisiewicz, Appalachian Fruit Research Station, USDA-ARS, Kearneysville, WV; and
K. A. Peter, Pennsylvania State University, Department of Plant Pathology and Environmental Microbiology-Fruit Research and Education Center, Biglerville, PA
Botryosphaeria dothidea (Moug.:Fr.) Ces. De Not. causes perennial cankers on apple trees and causes white rot on apple fruit in the field and during storage (1). Prolonged periods of warm wet weather favor rapid disease outbreaks that result in severe losses, which range from 25 to 50% for the southeastern United States (3). A B. dothidea isolate was obtained from decayed ‘Fuji’ apple fruit exhibiting white rot symptoms from a local farm market in Beltsville, MD, in May 2010. The fruit had characteristic large dark brown lesions with irregular margins and decay expanded unevenly toward the core and the tissue was soft. The pathogen was isolated from symptomatic tissue by spraying the lesion surface with 70% ethanol. The skin with aseptically removed with a scalpel and small pieces of tissue were placed on potato dextrose agar (PDA) and incubated at 20°C. Once fungal growth was evident, the cultures were hyphal-tip transferred to individual PDA plates and incubated at 20°C. The B. dothidea isolate produced black aerial mycelium with a white margin on PDA and had a black reverse. Conidiomata were evident after 10 to 14 days at 20°C only on oatmeal agar. Conidia were hyaline, smooth and straight, fusiform with an subobtuse apex and a truncate base 20 to 26 (24.33) × 4 to 7 (5) μm (n = 50). Genomic DNA was isolated from the fungus and amplified with gene specific primers (ITS 4 and 5) for the ribosomal DNA internal transcribed spacer region ITSI-5.8S-ITS2 as described by White et al. (4). Both forward and reverse strands of the 542-bp amplicon were sequenced and assembled into a contig. The nucleotide sequence (GenBank Accession No. KC473852) indicated 99% identity to B. dothidea isolate CMM3938 (JX513645.1) and to voucher specimens CMW 25686, 25696, and 25222 (FM955381.1, FM955379.1, and FM955377,1). Koch's postulates were conducted using three ‘Golden Delicious’ apple fruit that were wound-inoculated with 50 μl of a mycelial suspension of the fungus, obtained from aseptically scraping a 7-day-old PDA culture, and was also repeated using ‘Fuji’ apple fruit. Large, brown, slightly sunken, soft lesions with undefined edges developed 5 days after inoculation at 20°C and water-only inoculated fruit were symptomless. The fungus was reisolated from infected tissue and was morphologically identical to the original isolate from decayed apple fruit. To determine if the B. dothidea isolate was resistant to postharvest fungicides, the minimum inhibitory concentration (MIC) was conducted using the 96 well plate method with a mycelial suspension of the fungus as described by Pianzzola et al. (2). The MIC for the isolate was >1 ppm for Mertect and Scholar and 50 ppm for Penbotec, which are well below the labeled rates for these postharvest fungicides and the experiment was repeated. To our knowledge, this is the first report of B. dothidea causing white rot on apple fruit in Maryland.
References: (1) A. R. Biggs and S. S. Miller. HortScience 38:400, 2003. (2) M. J. Pianzzola et al. Plant Dis. 88:23, 2004. (3) T. B. Sutton. White rot and black rot. Pages 16-20 in: Compendium of Apple and Pear Diseases, A. L. Jones and H. S. Aldwinckle, eds. The American Phytopathological Society, St Paul, MN, 1991. (4) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Application. M. A. Innis et al., eds. Academic Press, San Diego, CA, 1990.