Occurrence and Distribution of Physiological Races of Exserohilum turcicum in Ontario, Canada

摘要

Northern corn leaf blight (NCLB) caused by Exserohilum turcicum is the most common and economically significant fungal leaf disease of corn in Ontario, Canada. During the past 10 years in Ontario, severity and incidence of NCLB have increased, possibly owing to the appearance of new races. Several races have been identified in various parts of the world; however, information on occurrence and distribution of races in Ontario is lacking. In the current study, 677 single conidial isolates of E. turcicum were isolated from 687 symptomatic leaf samples collected between 2012 and 2016. These isolates were evaluated for pathogenicity on six corn differential inbreds (A619, A69Htl, A69Ht2, A69Ht3, A632Htnl, and H02Html) under controlled environmental conditions and then grouped into 17 physiological races (0, 1, 2, 3, M, N, 12, 1M, IN, 3M, 13M, 12N, 13N, 1MN, 12MN, 13MN, 123MN) based on the reaction of the inbreds to infection (resistant or susceptible). Four races (0, 1M, IN, and 1MN) were most frequent, with an isolation frequency of 13, 10, 12, and 41%, respectively. Seventy-six percent of the isolates were virulent on more than one Hi resistance gene, with 2.4% (16 isolates) virulent on all five Hi resistance genes used in this study. Further analysis of the distribution of races in four regions over the years revealed that the occurrence and distribution of the races changed with timein Ontario. Overall, the frequency of virulence of the 677 isolates screened on the differentials with resistance genes Htl, Ht2, Ht3, Html, and Htnl varied from 6 to 81% (Htl 81%, Ht2 6%, Ht3 12%, Html 64%, and Htnl 64%). Virulent isolates produced fewer lesions on the Html differential, and smaller lesions that were slower and having less sporulation on the Htn 1 differential, compared with infection of the differentials with Htl, Ht2, and Ht3 resistance genes. Virulence frequency also changed withinthe four geographical regions of Ontario, with fewer isolates virulent on all resistance genes in eastern Ontario compared with southern and western Ontario. Isolates from southern Ontario had greater virulence frequency against Htl and Html, whereas isolates from western Ontario were more frequently virulent on Htl and Htnl. The information generated in this study on the distribution of E. turcicum races in Ontario corn will help growers to select appropriate hybrids with required resistance genes andwill assist seed companies in deploying resistance genes in corn hybrids across the province or within a particular region. .