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Incidence of Phytophthora Root Rot of Fraser Fir in North Carolina and Sensitivity of Isolates of Phytophthora cinnamomi to Metalaxyl.

A survey of Fraser fir Christmas trees in North Carolina for incidence of Phytophthora root rot was conducted during 1997 and 1998. Field sites (7- to 13-year-old trees) and nursery transplant beds (4- to 5-year-old trees) selected at random were surveyed based on foliar symptoms of Phytophthora root rot. Field sites were surveyed with a random transect method (>3,000 trees/field) or by counting all trees (<3,000 trees/field). Overall, incidence of Phytophthora root rot averaged 9% over the 58 field sites sampled, with a range of 0 to 75%. No relationship was found between number of years Fraser fir had been planted in the field site and disease incidence. Disease incidence did not increase as field sites were rotated through second or third crops of Fraser fir. Phytophthora spp. were recovered from 1.8% of asymptomatic trees sampled from 58 field sites across the state. P. cinnamomi accounted for 91% of the Phytophthora isolates recovered. In nursery transplant beds where a systematic sampling procedure was used, incidence of diseased trees averaged 2%, with a range of 0 to 12% across 16 locations. Recovery of Phytophthora spp. averaged 1.2% from root samples collected from 50 asymptomatic seedlings at each location. Isolates collected from the field and nursery transplant beds were grown on cornmeal agar incorporated with 0, 1, 1.25, 10, or 100 μg a.i. metalaxyl/ml. All 166 isolates of P. cinnamomi tested were sensitive to metalaxyl at 1 or 1.25 μg a.i. metalaxyl/ml. Although incidence of Phytophthora root rot has not increased in the state compared to a survey done in 1976 to 1977, the disease continues to limit production of Fraser fir in North Carolina.