MEASURING INCIDENCE, SEVERITY, AND RESISTANCE
IF YOU’VE NOTICED a higher incidence of northern corn leaf blight (NCLB), or higher incidence in certain hybrid lines, you’re not imagining things — genetic resistance to NCLB is being overcome by the pathogen.
Northern corn leaf blight is the most common and economically-important fungal leaf disease affecting the $2.4 billion/year Ontario corn crop. A large-scale research project looking into incidence and how the existing resistance genes fare against identified races of the disease is now entering its third year in Ontario. The project’s team draws from the Ontario Ministry of Agriculture, Food and Rural Affairs staff (OMAFRA, Ridgetown) as well as Agriculture and Agri-Food Canada (AAFC, Ottawa).
Albert Tenuta, study team member and OMAFRA crop pathologist, says that the project has already identified an NCLB race that overcomes almost all of the current resistance genes — a worrisome finding.
LEVELS OF RESISTANCE
Ninety three hybrids with different resistance genes were evaluated at the AAFC Ottawa Research and Development Centre (ORDC) and at University of Guelph’s Ridgetown campus. The plants were rated for specific resistance (based on lesion types) and general resistance (based on percent area infected) three weeks after silk emergence.
“We found a gene known as Htm1 to be most effective not only at reducing the number of lesions, but with reducing overall disease,” says Tenuta. After Htm1, the other genes found to be most effective (in order of effectiveness) are Htn1, Ht2, Ht3, and Ht1.
Team members also found that hybrids with both parents (male and female) having the same Ht resistance gene fared better than some different Ht gene combinations by about 20 bushels, but hybrids with some Ht gene combinations had better yield than their original hybrids.
Field testing will continue in 2016, after which, inbreds developed by Lana Reid, Xiaoyang Zhu, and Krishan Jindal (AAFC ORDC) will be released to the seed and research industry, along with details on how best to deploy these new genetics to fight NCLB.
In 2014 and 2015, hybrids with both low and high levels of NCLB tolerance were planted and sprayed with various foliar fungicides. The results were analyzed not only to determine how the fungicides and different hybrids controlled NCLB, but also to determine if physiological races of NCLB react different to the most common foliar fungicides used in Ontario. The results suggest that some fungicides were more effective at controlling NCLB than others.
Leaf samples were taken to identify physiological races as well 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 in soybean and wheat for instance,” says Tenuta.
Tenuta stresses that collaboration with American colleagues is very important in achieving progress with NCLB control. “Through the Corn Disease Working Group (subcommittee of the American Seed Trade Association), efficacy ratings against Northern Corn Leaf Blight for many of the common fungicides available to corn producers have been developed and added to a Fungicide Efficacy Table for Corn,” he explains. “Ontario’s contribution to these tables were based on our work and that of Dr. Dave Hooker, University of Guelph-Ridgetown, and this collaboration will continue in 2016.”
This project was funded in part through Growing Forward 2 (GF2), a federal-provincial-territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario. Funding for this project was also provided by Grain Farmers of Ontario.
CORN DISEASE SURVEY 2014 AND 2015
During the 2014 and 2015 corn disease survey, a total of 222 corn fields were surveyed across Ontario. The occurrence and severity (on a scale from 0 to 6) of NCLB was recorded both years.
In 2014, the disease was detected in 83% of fields surveyed. Fifty-three of the 185 fields with NCLB had incidences equal to or greater than 50% and severity ratings of 4 or higher. These most affected fields were found in ten counties: Renfrew, Chatham Kent, Elgin, Essex, Middlesex, Oxford, Dufferin, Perth, Waterloo, and Wellington. This illustrates how widespread NCLB is and why it has become the most economically important foliar disease of corn.
In 2015, 181 corn fields were surveyed and the disease was detected in 97% of them. Ten of the 176 fields with NCLB had incidences greater than or equal to 50% and severity ratings greater than or equal to 4. These most affected fields were found in four counties (Chatham Kent, Dufferin, Elgin and Middlesex), illustrating that 2015 incidence was lower than in 2014 (but was still found across the province in 97% of fields surveyed). However, ten fields in Eastern Ontario had disease severities greater than or equal to 4.
In both years, the disease was found in all fields sampled in Southern and Western Ontario, but increased from 2014 to 2015 in being found in 62% to 93% of fields in Eastern Ontario. Also in both years, mean disease severity and incidence in affected fields was considerably lower in Eastern Ontario and Central Ontario compared to Southern and Western Ontario.
In both years, seed corn fields surveyed in Chatham-Kent and Essex counties had a higher mean disease severity and a higher mean disease incidence than commercial corn fields.
The survey will continue in 2016. •