First Report of Early Rot of Cranberry Caused by Phyllosticta vaccinii in Wisconsin

Phyllosticta vaccinii Earle causes early rot of cranberry (Vaccinium macrocarpon Aiton) and previously was reported in fruit and leaves from Massachusetts and New Jersey, but not Wisconsin or Washington (2). This fungus previously was introduced into Wisconsin, apparently on planting stock, but did not persist in the field (2). In the present study, rotted fruit were collected in central Wisconsin in September 1997 from research plots adjacent to a commercial planting that had been started from field cuttings from New Jersey. P. vaccinii was isolated from 12 of 31 symptomatic berries, and its identity was verified by cultural and morphological characteristics (3). P. vaccinii was not isolated from rotted fruit from five other sites in central and northern Wisconsin. In three separate experiments, 10 to 25 cv. Stevens or Searles cranberry fruit were punctured with a needle, inoculated with 2 to 4 × 10⁵ conidia from sporulating cultures of P. vaccinii, and incubated at 28°C in a moist chamber. After 5 to 14 days, soft, watery spots developed at the inoculation point on 8 to 22% of the fruit in different experiments, and P. vaccinii was reisolated from the lesions. Fruit that were punctured but not inoculated neither developed symptoms nor yielded P. vaccinii. Previous attempts at fulfilling Koch's postulates by inoculating mature fruit were unsuccessful (1). P. vaccinii is one of approximately 15 species of fungi involved in the cranberry fruit rot complex in the eastern U.S. where fungicides are applied to greater than 95% of cranberry acreage, usually three times per year, primarily to control preharvest fruit rots. In Wisconsin, however, preharvest fruit rots are insignificant; less than 25% of the acreage is treated with fungicides. The occurrence of early rot in Wisconsin and the threat of introducing pathogens on cranberry cuttings are troublesome in light of the Food Quality Protection Act of 1996, which threatens registration of fungicides used to control cranberry fruit rots. References: (1) D. M. Boone. Pages 35-36 in: Compendium of Blueberry and Cranberry Diseases. F. L. Caruso and D. C. Ramsdell, eds. American Phytopathological Society, 1995. (2) G. J. Weidemann and D. M. Boone. Plant Dis. 67:1090, 1983. (3) G. J. Weidemann et al. Mycologia 74:59, 1982.