Large Patch (Rhizoctonia solani) Control Affected By Fungicide and Target Site of Application.

Large patch (Rhizoctonia solani) affects the sheaths and stems of Japanese lawngrass (JLG) (Zoysia japonica) in the transition zone of the United States. Large patch is difficult to control using traditional fungicide sprays because most of the fungicide solution remains on the leaf blade and away from the site of pathogen infection which occurs lower in the canopy. Our objective was to determine the amount of protection provided by fungicides deposited on the leaf, sheath, or stem parts of the plant. Greenhouse experiments were conducted in 2015 in Knoxville, TN, to evaluate large patch control using fungicides deposited on three target sites of JLG (leaf, sheath, and stem). Azoxystrobin (0.61 kg ai ha^-1), flutolanil (4.69 kg ai ha^-1), chlorothalonil (8.05 kg ai ha^-1), and tebuconazole (0.83 kg ai ha^-1) were applied using a pipette as 2.5µl droplets that were dispensed singly on the leaf, sheath, or stem. Plants were inoculated with R. solani and kept in a growth chamber under high humidity. Measurements of visual disease severity (0-100%) and plant stress (Fv/Fm) were collected every 7 days.  In both experimental runs, JLG treated with fungicides applied on the sheath or stem exhibited significantly lower large patch severity and higher Fv/Fm values compared to JLG receiving leaf applications on most rating dates. Large patch control of azoxystrobin treated JLG was most affected by the site of application. At 28 days after treatment, leaf applications of azoxystrobin exhibited 74% disease severity, whereas the sheath and stem applications both exhibited <5% disease severity. Chlorothalonil, a contact fungicide, was least affected by the site of target application on most rating dates. Future research is needed to improve lower canopy fungicide deposition under field conditions. Fungicide sprays that incorporate surfactants may help redistribute the fungicide from the leaf blade to the lower plant canopy.