Integrated Strategies for Managing Pythium Root Rot and Fungicide-Insensitive Strains of Pythium aphanidermatum in Poinsetti

Pythium aphanidermatum is the predominant species causing Pythium root rot of commercially grown poinsettia in North Carolina. Pythium root rot is managed primarily through a combination of sanitation practices and preventative fungicide applications, but growers need additional tools, such as host resistance and rotation programs, to implement a more integrated approach to disease management. Commercially available poinsettia cultivars were inoculated with P. aphanidermatum three weeks after transplant and evaluated for resistance to Pythium root rot. Most cultivars were susceptible to Pythium root rot and none were completely resistant. However, several cultivars demonstrated partial resistance to Pythium root rot. Interspecific hybrid cultivars, including 'Luv U Pink', had higher levels of partial resistance when compared to conventional cultivars. These results suggest that partial resistance in poinsettia could be used in combination with fungicide applications to limit disease caused by P. aphanidermatum. Growers prefer to use mefenoxam for Pythium root rot control because of its high efficacy and low cost, but resistance to mefenoxam is widespread in populations of Pythium. Other products that could be used instead of or in combination with mefenoxam are not commonly used to control Pythium root rot on poinsettia. To identify candidate fungicides for Pythium root rot control, the efficacy of 10 fungicides was assessed on seven poinsettia cultivars inoculated with P. aphanidermatum. One experiment examined control with a single application of each fungicide made at transplant and another experiment examined repeat applications of the fungicides made throughout the experiment. Treatments containing etridiazole, mefenoxam, fenamidone, and cyazofamid provided excellent control of Pythium root rot across all cultivars in both experiments. Mefenoxam and fenamidone, a quinone-outisde inhibitor (QoI), were the most efficacious among the fungicides tested against Pythium root rot on poinsettia. Four isolates of P. aphanidermatum previously collected from plants grown in commercial greenhouses also were evaluated for in vitro sensitivity to 10 fungicides. Etridiazole, fosetyl-al, and potassium phosphite completely inhibited mycelial growth of all isolates, whereas sensitivity varied in response to mefenoxam, cyazofamid, propamocarb, fenamidone, azoxystrobin, and pyraclostrobin. Twenty-one additional isolates were evaluated for in vitro sensitivity to three QoI fungicides. Seven isolates were insensitive to all three QoI fungicides and one isolate was insensitive to all QoIs and mefenoxam. It is not known whether insensitivity to QoI fungicides is widespread in populations of Pythium. In-vitro sensitivity or insensitivity to the label rate of mefenoxam (17.6 mul a.i/ml) and fenamidone (488 mul a.i./ml) was tested on 96 isolates of P. aphanidermatum and isolates were assigned to four fungicide resistance groups. Fifty-eight percent of isolates were insensitive to one (MefR, FenS=36%; MefS, FenR=16%) or both fungicides (MefR, FenR = 6%). A single point mutation in the cytochrome-b gene (G143A) was identified in all fenamidone-insensitive isolates. Mycelial growth at three temperatures, oospore production in vitro, and aggressiveness on poinsettia were evaluated to assess relative fitness of mefenoxam and fenamidone sensitive and insensitive isolates. Isolates insensitive to both mefenoxam and fenamidone had less radial mycelial growth at 30°C and produced fewer oospores in vitro than isolates sensitive to one or both fungicides, whereas isolates sensitive to both fungicides produced the most oospores in vitro..;Aggressiveness on poinsettia varied by isolate but fungicide resistance profiles were not always predictive of in vivo aggressiveness. These results suggest that populations of P. aphanidermatum with dual resistance to mefenoxam and fenamidone may be a threat in poinsettia production but may also be less fit than sensitive populations at higher temperatures. Additionally, this is the first report of insensitivity to QoI fungicides and dual insensitivity to QoIs and mefenoxam in isolates of P. aphanidermatum sampled from greenhouse floriculture crops. Since mefenoxam and fenamidone have high risk for resistance development, treatment programs that incorporate products with two or more modes of action were evaluated on cultivars with varying levels of partial resistance. Treatment programs using single applications (tank mixes) at transplant were compared with rotations of two or three fungicide applications. All six tank mix and rotation programs prevented Pythium root rot on all cultivars inoculated with insensitive isolates, even when mefenoxam and fenamidone were included in the rotation. These results provide growers with a choice of fungicide programs for use in their facilities.