Physiological races of Northern Corn Leaf Blight: Occurrence, distribution and management in Ontario

Principal Investigator: Albert Tenuta

Scientific Summary:

Northern corn leaf blight (NCLB) caused by Exserohilum turcicum is the most common and economically important fungal leaf disease of the $2.4 billion Ontario corn crop. This disease appeared repeatedly in epidemic form in different parts of the world including Canada causing huge losses until the discovery and incorporation of a single dominant resistance gene (Ht1) in corn cultivars in the 1960s. The resistance conferred by Ht1 did not last long and the disease is on the rise again world-wide because of the emergence of new races of the fungus in the past two decades. The disease was well managed in the past in Canada by planting resistant hybrids, but the disease has now become the most economically important foliar leaf disease in Ontario. The ubiquitous distribution of the disease throughout the province was highlighted in recent surveys where the disease was detected in over 95% of corn fields sampled in 2011 and 85% in 2012.Thirteen physiological races (different strains of the fungus) of NCLB have been identified to date in various parts of the world. However, little information is available on the occurrence and geographical distribution of the races in Ontario.

The objective of this project was to study the occurrence and distribution of physiological races of the fungus and workout strategies for managing this economically important disease. In-field management trials were conducted to determine differences that exist in foliar fungicide efficacy against the various NCLB races identified in Ontario. If so, determine which fungicides control which races, and if these “new” genes provide additional protection for other common corn foliar leaf diseases. In addition, single conidial isolates of NCLB from leaf samples were collected during surveys of corn fields of Ontario. Inoculations in the greenhouse with the isolates collected from leaf samples from previous surveys to identify races and to study responses of 60 Agriculture and Agri-Food Canada (AAFC) experimental hybrids having different resistance genes.

A total of 568 corn fields were visited across the Ontario Province from last week of August to second week of September during 2014 to 2016 and recorded the occurrence and severity of various corn diseases. NCLB was the most common leaf disease found in the corn fields of province and it was detected in more than 80% of the fields sampled every year.  Significantly more NCLB incidence and severity in seed crop compared to commercial crop during 2014 to 2016 survey is a cause of great concern as it shows that most of the genotypes currently used as parent in the hybrid seed production either have no resistance or very week resistance against NCLB.

Another important observation during survey of 568 corn fields was the detection of resistant and susceptible lesions on the same leaf. Likewise, the reactions of some of the corn hybrids to NCLB varied depending on where they are grown in the province. This suggests the presence of different races of E. turcicum in Ontario corn fields. Sixteen physiological races were identified based on the lesion types (Resistant and Susceptible) on the six corn inbreds with different resistance genes. The frequency and occurrence of the 16 races were studied by county and over years. Race population was diverse within and between regions: 15 races were observed in southern and eastern Ontario, 10 in western Ontario and 6 in central Ontario. The information generated on the occurrence and distribution of E. turcicum races in Ontario corn crop in the current study will help both research institutes and seed companies to deploy resistance genes in corn cultivars with specific resistance for a region, and the growers to select and grow appropriate cultivars with required resistance genes.

Foliar fungicides studies were conducted in 2014, 2015 and 2016 at the University of Guelph Ridgetown Campus. Two commercial hybrids with low and high levels of NCLB tolerance were planted and sprayed with various foliar fungicides to determine not only the effectiveness of NCLB control but also to determine if physiological races of NCLB react different to the most common foliar fungicides used in Ontario corn production and how it relates to hybrid performance/interaction. All the fungicides tested were effective in managing the NCLB disease and representative leaf samples were collected, ground and were prepared to identify physiological races as well as to examine if these NCLB races are displaying signs of fungicide resistance. Over the past three years in North America field crop production many incidences of fungicide resistance have been identified.Fungicide field trials in 2016 at Ridgetown were more variable compared to previous years due to very dry conditions. Inoculating both hybrids did result in a 7.68 bu/ac yield difference between the tolerant and susceptible hybrid compared to 37.0 bu/ac difference in 2015 until more typical environmental conditions.

Project Related Publications