Crabapple (Malus sylvestris) is commonly used as a source of pollen in apple production. During September and October 2003, a canker and twig dieback disease of ‘Manchurian’ crabapple trees was observed in some commercial apple (Malus × domestica Borkh.) orchards (7- to 10-years-old) in north-central Washington State. A fungus was consistently isolated from 40 to 77% of sampled crabapple trees. During May 2004, the same symptoms and fungal association were observed in an 8-year-old ‘Fuji’ apple orchard in which all crabapple pollenizers and 43% of the Fuji trees were diseased. Canker and dieback appeared to originate from infection of dying or dead fruit spurs or pruning wounds. Cankered areas were slightly sunken, brown and the margin of diseased area often developed cracks in the cortical tissue. Pycnidia were often present in older areas of the lesion. Pycnidia were black, 0.3 to 0.6 mm in diameter, separate to aggregated in small numbers, and partially immersed to nearly superficial in the diseased tissue. To isolate the fungus, outer bark tissues of diseased twigs were scraped and small tissue segments were cut from the canker margin. Tissue segments were surface disinfested for 5 min in 0.5% sodium hypochlorite solution, rinsed three times with sterile water, cut into small pieces, and placed on acidified potato-dextrose agar (APDA, 4.0 ml of a 25% solution of lactic acid per liter of medium). Isolation plates were incubated at 20°C in the dark. Colonies of the fungus first appeared as dense colorless mycelium that later turned light yellow to yellow. The fungus was identified as Sphaeropsis pyriputrescens Xiao & J. D. Rogers (1). To complete Koch's postulates, two isolates (one each from apple and crabapple) were used in pathogenicity tests on ‘Fuji’ apple and ‘Manchurian’ crabapple trees. In the orchard, selected 2-year-old twigs were sprayed with 70% ethanol and allowed to dry. Twigs were wounded to a depth of 1 to 2 mm with a sterile 5-mm-diameter cork borer; a 5-mm mycelial plug from 4-day-old PDA cultures of S. pyriputrescens was placed into each wound. Twigs wounded and treated with sterile APDA plugs were used as controls. Inoculation sites were covered with moist cheesecloth and sealed with Parafilm that was removed 3 weeks after inoculation. Four twigs per isolate on each of four trees were inoculated. The experiment was conducted twice (April and November 2004 for apple; two different locations in March 2004 for crabapple). At 2 and 6 months after inoculation, two apple twigs per treatment were removed from each tree. All crabapple twigs were removed 2 months after inoculation. Canker sizes were measured and reisolation of the fungus was attempted as described above. Both isolates caused cankers on apple and crabapple twigs. Mean canker sizes at 6 months after inoculation were 11 and 32 mm on apple twigs inoculated in April and November 2004, respectively and 7 to 8 mm on crabapple twigs at 2 months after inoculation. No cankers developed on control twigs. S. pyriputrescens was reisolated from all inoculated twigs and was not recovered from noninoculated controls. S. pyriputrescens is the cause of Sphaeropsis rot, a recently reported postharvest fruit rot disease of apple and pear (1,2). To our knowledge, this is the first report of this fungus causing cankers and twig dieback on apple and crabapple trees.
Reference: (1) C. L. Xiao and J. D. Rogers. Plant Dis. 88:114, 2004. (2) C. L. Xiao et al. Plant Dis. 88:223, 2004.
