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Poster: Biology & Disease Mgmt: Integrated Pest Mgmt


Practical resistance to mefenoxam in Pythium aphanidermatum and its impact on managing Pythium root rot in poinsettia cultivars
E. Lookabaugh (1), B. Shew (1) (1) NC State University, U.S.A.

Mefenoxam (mef) resistance is widespread among populations of Pythium in NC greenhouses, but resistant (MR) and sensitive (MS) isolates often are found concurrently. The combined utility of host resistance and mef to manage root rot in plants exposed to mixed populations of P. aphanidermatum was tested on seven poinsettia cultivars (cvs). One MR and one MS isolate were used, giving four inoculum/fungicide combinations: MR+mef, MR-mef, MS+mef, MS-mef. Plants were treated twice with the labeled rate of Subdue Maxx. Plants were scored for root rot (RR, 1 = 0% rot; 5 = 100%) and aboveground disease severity (DS, 1 = healthy; 5 = dead plant) 10 wk after inoculation. MS-mef treated plants had the highest RR (mean = 4.0) and DS (mean = 3.9) in all cvs. Mef controlled RR in all cvs inoculated with the MS isolate (mean = 1.6) but not in 5 of 7 cvs inoculated with the MR isolate (mean = 3.4), demonstrating practical resistance to mef in P. aphanidermatum. MR-mef treated plants had lower DS severity than MS-mef treated plants (3.3 vs. 3.9), suggesting that the MS isolate was more aggressive than the MR isolate. Treatment with mef lowered DS in MR inoculated plants only slightly (mean = 2.6). If confirmed with other MS and MR isolates, this may explain why MS isolates persist in facilities that regularly use mef. Resistant poinsettia cvs could be used to offset loss of fungicide efficacy and reduce losses to Pythium root rot in facilities with mixed Pythium populations.