In fall 1996, cankers were observed on branches of 7- to 9-year-old apple trees (Malus domestica cvs. Empire and McIntosh, strains Marshall and Redmax) in a research orchard in Sturgeon Bay, WI. By summer 1997, cankers had developed on scaffold limbs or the central leaders of 48 of 48 Marshall McIntosh trees and 26 of 40 Redmax McIntosh trees, but they rarely were found in an adjacent block of 40 Empire trees. In 1998, new cankers were not observed, but existing cankers expanded. By 1999, the Marshall McIntosh block was so severely affected it was removed; 50% of the Redmax trees and 13% of the Empire trees also were removed. Initially, cankers appeared as orange discolored areas on bark and frequently, but not always, were associated with pruning wounds. Canker expansion was greater during spring and fall than during summer. At later stages of canker development, bark cracked and peeled, and leaves distal to cankers died. These features were consistent with descriptions of Leucostoma canker of apple (2,3), a disease previously reported only in Michigan, Germany, and Iran (1--3). All of 10 cankers sampled in 1997 and several more cankers sampled in 1998 yielded fungal colonies on potato-dextrose agar resembling Leucocytospora spp., which are anamorphs of Leucostoma spp. (2). Colonies initially were white to buff and later turned brown. Conidium morphology was consistent with descriptions of Leucocytospora spp. (2). Alternaria spp. also were isolated frequently; other fungi, which were not identified, were isolated rarely. The presence of abundant pycnidia of Leuco-cytospora, but not perithecia of Leucostoma, on cankers was different from the signs described for Leucostoma canker caused by Leucostoma cincta in Michigan (2,3). However, simple matching coefficient analysis of randomly amplified polymorphic DNA data indicated 74% similarity between a typical isolate from apple in Wisconsin (97-82) and two isolates of L. cincta from apple in Michigan (3; ATCC 64878 and 64879) but only 49 to 54% similarity to L. cincta and L. persoonii isolated from stone fruit trees (Prunus spp.). Inoculation of wounded branches of mature apple trees (cvs. Golden Delicious and Cortland) with isolate 97-82 in July 1998 and 1999 resulted in cankers that resembled young cankers observed on trees in the research orchard. Eight weeks after inoculation, the pathogen was reisolated from these cankers, which fulfilled Koch's Postulates. Therefore, we conclude that Leucostoma canker was responsible for the serious damage to apple trees of different cultivars at the research station, with strains of McIntosh affected most severely. We speculate that low-temperature injury during the unusually cold winter of 1995-1996 might have created infection sites and predisposed trees to disease development. This is the first report of Leucostoma canker of apple in Wisconsin. It is rare that this disease causes such significant losses (2).
References: (1) M. Ashkan. Iranian J. Plant Pathol. 30:33, 1994. (2) A. L. Jones and H. S. Aldwinckle, eds. 1991. Pages 40--41 in: Compendium of Apple and Pear Diseases. American Phytopathological Society, St. Paul, MN. (3) T. J. Proffer and A. L. Jones. Plant Dis. 73:508, 1989.
