In Vitro Fungicide Sensitivity on Pythium Species Associated with Pythium Root Rot.

Pythium root rot is an extremely important and devastating disease of creeping bentgrass putting greens throughout the transition zone of the US. Each year the NC State Turfgrass Disease Diagnostic Lab receives between 100 and 300 creeping bentgrass putting green samples and 18 to 20% are diagnosed with Pythium root rot. Although this disease is serious and widespread, very little is known about Pythium root rot etiology, epidemiology or management. Considering that field efficacy data for this disease in turf is non-existent, an in vitro sensitivity assay using four aggressive Pythium species (P. aphanidermatum, P. ultimum var. ultimum, P. irregulare, and P. arrhenomanes) associated with Pythium root rot was developed.  Fungicides spanning different modes of action with Pythium diseases listed on their labels were examined. Fungicides were added to PDA at six concentrations (0, 0.0001, 0.001, 0.01, 0.1, 1.0 and 10 μg ml^-1). An agar plug collected from 3-day old cultures of each Pythium species was placed mycelia side down in the center of each plate. All plates were incubated in the dark at 23^oC and radial growth was measured with a ruler 24, 48 and 72 hours after plating. All isolates were very sensitive to cyazofamid. Sensitivity to QoI fungicides varied among Pythium species: P. aphanidermatum and P. irregulare were more sensitive to azoxystrobin, pyraclostrobin and fluoxastrobin but less so to trifloxystrobin. P. aphanidermatum and P. ultimum var. ultimum growth was reduced with propamocarb, while fosetyl-Al only decreased P. aphanidermatum growth. No other fungicides showed ability to inhibit growth. While further research is needed on other highly aggressive Pythium species associated with Pythium root rot, this assay offers insight into the chemistries Pythium species are sensitive to. It also provides a basis for appropriate fungicides to test in field trials, and later, for implementation on golf courses.