Resistance to Azoxystrobin in the Gummy Stem Blight Pathogen Documented in Georgia

Abstract

Gummy stem blight, caused by the fungus Didymella bryoniae, is the most destructive disease of watermelon in Georgia and in many other watermelon producing areas of the U.S. The QoI fungicide azoxystrobin has been used for gummy stem blight control in Georgia since 1998. As early as 1999, reduced control of gummy stem blight with azoxystrobin was noted in several research sites and commercial fields of cucumber and watermelon in Georgia. Isolates from several of these fields were later confirmed to be resistant to azoxystrobin. To determine how widespread the resistance problem was in Georgia, extensive surveys of watermelon and muskmelon fields and transplant houses were conducted in 2001 and 2002 to determine the frequency of azoxystrobin-resistant isolates in populations of D. bryoniae. Of the 272 isolates collected in 2001, 247 (91%) were resistant to azoxystrobin. In 2002, 82% of the 170 isolates collected were resistant to azoxystrobin, and of the 40 isolates collected from watermelon transplants, all but one were resistant to azoxystrobin, suggesting that resistant isolates in the field may have originated from seed or transplants. Georgia melon growers are now advised to use alternative fungicides that are chemically unrelated to the QoIs for gummy stem blight control. Accepted for publication 2 November 2004. Published 7 December 2004.