Poster Session: Chemical Control

213-P

Azoxystrobin (QoI) resistance monitoring of Rhizoctonia solani isolates causing rice sheath blight in Louisiana.
G. OLAYA (1), L. Sarmiento (1), K. Edlebeck (1), C. Buitrago (2), H. Sierotzki (2), J. Zaunbrecher (3), A. Tally (3)
(1) Syngenta, Vero Beach, FL, U.S.A.; (2) Syngenta, Stein, Switzerland; (3) Syngenta, Greensboro, NC, U.S.A.

Rhizoctonia solani AG-1 IA isolates resistant to QoI fungicides were detected in 2011 in rice fields located in Acadia Parish, Louisiana. The sequencing of the cytochrome b gene revealed that QoI resistant isolates had phenylalanine to leucine substitutions at codon 129 in the cytochrome b gene (F129L mutation). Resistance monitoring programs in rice and soybeans fields have been conducted in 2011 and 2012 for the QoI fungicide azoxystrobin. A total of 457 isolates were collected in 2011 from 23 fields located near the problem area and theirs sensitivity was determined using a Perennial ryegrass (Lolium perenne) bioassay. In 8 fields no resistant isolates were detected. In 15 fields the frequency of resistant isolates varied from 7 to 100% indicating that there is still an azoxystrobin dose response. In 2012, 237 isolates were collected from 12 fields. Isolates from only one field were all sensitive to azoxystrobin. Resistant isolates were detected in the remaining 11 fields and the frequency of resistant isolates ranged from 39 to 100%. In 2011 and 2012, resistant isolates were detected in fields located less than 40 km away from the first detection field. In both years, isolates collected from fields located more than 41 km away from the first detection site were all sensitive to azoxystrobin. Results of these resistance monitoring studies are helping in the recommendation of the best managing practices to control of R. solani on rice in Louisiana.