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Insensitivity to Metalaxyl Among Isolates of Phytophthora erythroseptica Causing Pink Rot of Potato in New York. S. B. Goodwin, Department of Plant Pathology, 334 Plant Science, Cornell University, Ithaca, NY. 14853 . M. T. McGrath, Long Island Horticultural Research Laboratory, Cornell University, 39 Sound Avenue, Riverhead, NY. 11901. Plant Dis. 79:967. Accepted for publication 10 July 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0967D.

Insensitivity to metalaxyl in Phytophthora erythroseptica Pethybr., the cause of pink rot of potato, was detected first in Maine during 1993 (1). Because pink rot was more common than usual in New York during 1994, we were able to obtain isolates to test the hypothesis that insensi-tivity to metalaxyl also occurs in P. erythroseptica populations in New York. Fungicide sensitivity was determined by plating mycelial plugs (8 mm diameter) onto rye B agar amended with 0, 5, or 100 μg/ml metalaxyl (two replications per treatment), and measuring the growth after 3 days (2). The 20 P. erythroseptica isolates tested for metalaxyl sensitivity (three from a commercial field in Orleans Co., and 17 from storages on three farms in Suffolk Co.) fell into three distinct classes. Sensitive (S) isolates grew from 0 to 21 % of the control at 5 Mg/ml and from 0 to 3% at 100 μg/ml metalaxyl. Moderately insensitive (M) isolates grew from 38 to 51% of the control at 5 μg/ml and from 13 to 26% at 100 μg/ml metalaxyl. Highly insensitive (H) isolates grew from 85 to 101% of the control at 5 and 100 ?g/ml metalaxyl. Among the 20 isolates, three (one from Orleans Co., two from Suffolk Co.) were S, 14 (from Suffolk Co.) were M and three (two from Orleans Co., one from Suffolk Co.) were H. One sensitive isolate from Maine (courtesy D. H. Lambert) was also rated S in our assay. Faster-growing sectors developed from two of the M isolates after 3 to 7 days of growth on plates containing 5 or 100 μg/ml metalaxyl. Isolates subcultured from these sectors had changed from M to H (growth at 5 μg/ml metalaxyl 99% compared with 48 to 50% for the original isolates). On the basis of these field and lab results, we propose the hypothesis that insensitivity to metalaxyl in P. erythroseptica is conditioned by a single incompletely dominant gene, in which S isolates are homozygous for the sensitive allele, M isolates are heterozygous, and H isolates are homozygous for the H allele. If this hypothesis is correct, then the H sectors (homozygotes) from the M isolates (heterozygotes) were most likely generated by mitotic recombination followed by selection. The field in Orleans Co. that had both S and H isolates received three applications of metalaxyl (Ridomil MZ58) during 1994. The two S isolates from Suffolk Co. were from a farm where metalaxyl has never been applied. Surprisingly, pink rot on this farm was rare and was only found in potatoes from a cull bin. In contrast, the other two Suffolk Co. growers have used Ridomil MZ58 since it was registered in 1983 and pink rot was observed both before harvest and in storage. All potatoes in Suffolk Co. were from Maine seed (cv. Norwis) grown in continuous or short rotations. This is the first report of insensitivity to metalaxyl in P. erythroseptica in New York.

References: (1) D. H. Lambert and B. Salas Plant Dis. 78:1010, 1994. (2) J. M. Matuszak et al. Plant Dis. 78:911, 1994.