Evidence for Resistance to Metalaxyl in Isolates of Peronospora hyoscyami. Robert I. Bruck, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Guy V. Gooding, Jr., and C. E. Main, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Plant Dis. 66:44-45. Accepted for publication 28 April 1981. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-44.

Isolates of the tobacco blue mold fungus (Peronospora hyoscyami) were obtained between 23 July and 8 August 1980 from areas of western North Carolina that grow Burley tobacco. Only fields treated with metalaxyl (4.7 L/ha, preplant soil incorporation) were sampled. Mass conidial suspensions from nine plants per field were obtained by swirling exised lesions in sterile distilled water. Potted tobacco plants in the four-leaf stage were treated with metalaxyl at 0–200 µg (a.i.)/ml and were inoculated 24 hr later with 2 ml of a suspension of 2,000 conidia per milliliter. Three of the 14 isolates screened were able to form lesions and sporulate on plants treated with ≤100 µg/ml of metalaxyl; the conidia produced on these plants were used successfully to infect other metalaxyl-treated tobacco plants. No symptoms or signs of infection were visible on plants treated with as little as 25 µg (a.i.)/ml and inoculated with the other 11 isolates. These data suggest that metalaxyl-resistant isolates of P. hyoscyami may exist widely in nature and that continued use of the fungicide should be carefully evaluated and monitored.

Keyword(s):