Y. K. Kim, Pace International, Wapato, WA;
R. Caiazzo and
P. Sikdar, Washington State University, TFREC, Wenatchee; and
C. L. Xiao, USDA-ARS San Joaquin Valley Agricultural Sciences Center, Parlier, CA
In March 2012, decayed ‘Empire’ apple fruit (Malus × domestica Borkh.) were sampled from apples grown in Albion (Orleans County) in New York State and stored in bins for 6 months under controlled atmosphere at a commercial packinghouse. At the packinghouse following storage prior to be packed, the fruit were completely rotten, spongy to firm, and light brown without pycnidia. All fruit rots originated from either stem-end or calyx-end infections but no wound infections were observed. The incidence of fruit with these symptoms in the total decay was relatively low (0.1%). To isolate the causal agent, small fragments of fruit flesh from 12 decayed fruit were cut and placed on potato dextrose agar (PDA) acidified with 0.1% lactic acid. The plates were incubated at 20°C for 4 days and sub-cultured on PDA to obtain a pure culture. The colonies initially appeared with dense hyaline mycelium and later turned light yellow to yellow, and black pycnidia formed after about 2 weeks of incubation under a 24-h fluorescent light at 20°C. Conidia were light brown to brown, clavate to subglobose to irregular, and 15 × 8 μm on average. The fungus was identified as Sphaeropsis pyriputrescens Xiao & J.D. Rogers based on the morphology of the fungus (3). The identity of a representative isolate was further confirmed by analysis of nucleotide sequences of the internal transcribed spacer (ITS) regions amplified using the primers ITS1/ITS4. A BLAST search in GenBank showed that the sequence had 99% homology to an S. pyriputrescens sequence (Accession No. GQ374241). One representative isolate was tested for pathogenicity on apple fruit. Organic ‘Red Delicious’ apple fruit were surface-disinfected in 0.6% sodium hypochlorite solution for 5 min, rinsed twice with deionized water, and air-dried. Each fruit was wounded with a sterilized finish-nail head (3 mm in depth and 4 mm in diameter) and inoculated by placing a 4-mm-diameter plug from the leading edge of a 4-day-old PDA culture on the wound. Control fruit were treated with sterile PDA plugs. The inoculation site was covered with two layers of moist cheesecloth to avoid dehydration. There were four 10-fruit replicates for each treatment, and fruit were placed in plastic crispers and stored at 4°C for 4 weeks. The experiments were conducted twice. Sphaeropsis rot developed on all inoculated fruit, while no decays appeared on the control fruit. Koch's postulates were fulfilled by reisolating the fungus from the decayed fruit. Sphaeropsis rot is a recently reported postharvest fruit rot disease of apple and pear (1,3). The disease was first observed on ‘d'Anjou’ pears, and later more serious economic losses were observed in apples in Washington State (1). The disease has also since been reported in British Columbia, Canada (2). To the best of our knowledge, this is the first report of the occurrence of Sphaeropsis rot caused by S. pyriputrescens on apple in New York or in any region outside of the Pacific Northwest in North America.
References: (1) Y. K. Kim and C. L. Xiao. Plant Dis. 92:940, 2008. (2) P. L. Sholberg et al. Plant Dis. 93:843, 2009. (3) C. L. Xiao et al. Plant Dis. 88:223, 2004.