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Geographical Distribution of Physiological Races of Exserohilum turcicum, the Causal Agent of Northern Corn Leaf Blight in Ontario, Canada.

Jindal, K., Tenuta, A., Woldemariam, T., Zhu, X., Reid, L.M. 2017. Geographical Distribution of Physiological Races of Exserohilum turcicum, the Causal Agent of Northern Corn Leaf Blight in Ontario, Canada. 70th Northeast Corn Improvement Conference, Ottawa, ON, Feb 21-22, 2017. Oral presentation.

Abstract

Northern corn leaf blight (NCLB) caused by Exserohilum turcicum (Pass.) Leonard is the most common and economically important fungal leaf disease of the $2.3 Billion Ontario corn crop. Its infection prior to flowering/silking can cause grain yield losses over 50 per cent. Over the past 10 years, NCLB severity and incidence increased many times due to changes in pathogen population (appearance of new races), cultivation practices and climate. To date, 13 races, differentiated on basis of their infection types on four corn inbreds with resistance genes Ht1, Ht2, Ht3 and Htn1, have been identified in various parts of the world but unfortunately little information is available on the occurrence and distribution of E. turcicum races in Ontario/Canada. Therefore the current study was undertaken to determine the spread of NCLB and its pathogen E. turcicum races across Ontario, and virulence of identified races against available resistance genes. The disease was found in 83%, 97% and 82% of the fields visited in 2014, 2015 and 2016, respectively. Mean disease severity and incidence in affected crop was higher in southern and western Ontario as compared to eastern Ontario. A total of 675 single conidial isolates of E. turcicum were obtained from 687 leaf samples with typical NCLB lesions collected during 2012, 2014, 2015, and 2016 provincial corn disease survey, and from 2013 AAFC central experimental farm plots. Six hundred six of these isolates were evaluated on corn differential lines having Ht1, Ht2, Ht3, Htm1, and Htn1 resistance genes under controlled temperature and light conditions (growth chamber). Sixteen physiological races were identified based on the resistance response (R and S) of the differential corn lines. Exserohilum turcicum race 0, 1, 1M, 1N, 1MN and 13MN were most prevalent, comprising 12%, 6%, 12%, 12%, 41%, and 5% of the isolates. Seventy six per cent of the isolates were virulent on more than one Ht resistance genes, with 2% of the isolates virulent on all Ht resistance genes. Race populations were diverse within regions, counties and years. Virulence to the Ht1, Ht2, Ht3, Htm1, and Htn1 resistance genes was present in 82%, 5%, 10%, 66% and 64% of the isolates, respectively. Most of the virulent isolates produced less number of lesions on Htm1, and delayed and smaller small lesions on Htn1.Virulence to Ht resistance genes was fairly distributed across three years i.e. 2012, 2014 and 2015. Temporal variation was observed among numbers of isolates virulent in all years with less number of isolates virulent on Ht1, Ht3, Htm1, and Htn1 in 2016 as compared to 2014 and 2015. Number of isolates virulent to Ht resistance genes also varied among four regions with less number of isolates virulent on all resistance genes in eastern Ontario as compared to southern and western Ontario. Resistance genes Ht2 and Ht3 resisted most of the E. turcicum isolates, and lines with Htm1 produced less number of lesions as compared to other resistance genes. These (Ht2, Ht3 and Htm1) may be incorporated in breeding programs to develop NCLB resistant lines. The information generated in the current study on the distribution of E. turcicum races in Ontario corn crop will help growers to select and grow appropriate cultivars with required resistance genes in their area, and seed companies to deploy resistance genes in corn cultivars with specific resistance for a particular region.

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