DEVELOPMENT AND SIGNIFICANCE OF DICARBOXIMIDE RESISTANCE IN SCLEROTINIA MINOR ISOLATES FROM COMMERCIAL LETTUCE FIELDS IN CALIFORNIA

Three growth stages of each of 20 Sclerotinia minor isolates were test ed for resistance to iprodione. Sclerotia and both vegetative and matu re mycelium of each isolate were plated on potato-dextrose agar (PDA) amended with 0, 1, 5, 10, 25, and 100 mu g of the fungicide per ml, an d radial growth was measured. All wild-type field isolates were initia lly sensitive and did not grow in the presence of iprodione. However, fungicide resistance arose readily in vitro. All 20 isolates produced measurable growth (greater than or equal to 2 mm) on iprodione at 5 mu g/ml after 2 weeks when started from mature mycelium, and 18 of 20 is olates produced measurable growth after 4 weeks when started from vege tative mycelium. In general, fungicide-resistant growth arose more fre quently and mean colony diameters were significantly greater (P less t han or equal to 0.05) with mature mycelium than with vegetative myceli um at all times and concentrations. In sclerotial germination tests, a t least 1% of sclerotia germinated in 18 of 20 isolates after 2 weeks on iprodione at 5 mu g/ml, and in 19 of 20 isolates after 5 weeks on 1 00 mu g/ml. Of growth produced on 79 plates containing iprodione, 73 r emained viable on PDA after 5 months, and 71 retained resistance to 5 mu g/ml. Seventy of these also exhibited cross-resistance to vinclozol in, another dicarboximide fungicide. Pathogenicity of five fungicide-r esistant strains was tested in greenhouse, microplot, and field experi ments, with and without iprodione. Two months after in vitro productio n, one of the five resistant strains was avirulent, but disease incide nce for the other four ranged from 40 to 75%, compared with 40% for th e wild-type isolates. However, the virulence of the fungicide-resistan t strains declined over time. Ten months after their production, two o f the isolates were avirulent and disease incidence for the others ran ged from 3 to 17%, compared with 40 to 90% for the wild-type isolates. The strains that remained virulent 5 and 7 months after in vitro prod uction were not significantly controlled by iprodione sprayed accordin g to labeled rates, although disease was significantly controlled by t he fungicide in treatments inoculated with wild-type field isolates (P > 0.05). In experiments in commercial fields to determine the efficac y of fungicide sprays on the wild-type S. minor population, there was no evidence that fungicide resistance was the cause of lack of lettuce drop control observed in many coastal California fields. Application of fungicides at a less than optimal time may account for some inciden ts of control failure.