Northern leaf blight

MAINTAINING RESISTANCE

NORTHERN LEAF BLIGHT is one of the most common and economically important fungal leaf diseases in Ontario. In fact, for as long as corn has been grown in the province there have been issues with Northern leaf blight. It is not just a problem here, though; producers around the world suffer significant yield losses due to the disease. Although the problem was somewhat rectified about 40 or so years ago with the introduction of new resistance genes (HT genes), in the last 10 to 15 years we have started to see more cases of Northern leaf blight in Canada, particularly in Ontario. Albert Tenuta, Field Crop Plant Pathologist with the Ontario Ministry of Agriculture and Food (OMAF), says that this indicates changes in the pathogen’s populations.

Traditionally, there are four races of Northern leaf blight in Ontario, says Tenuta. “Now there’s talk of maybe 13 or more races in the world where the pathogen has developed.” As a result, Ontario farmers are starting to see more injury in the fields each year.

“In our surveys, over the last couple of years, we’ve seen that up to 90 to 95 percent of the fields we’ve surveyed in the province have Northern leaf blight,” says Tenuta. “Some at low levels; other ones can be quite significant in terms of their overall severity in those fields.” In some cases, where damage occurs prior to silking, he says yield losses can be as high as 50 percent.

“The earlier the disease develops,” says Tenuta, “the greater the potential economic losses can be.”

MORE RESEARCH NEEDED
The existence of these new races indicates that the level of effectiveness of our current resistance genes is likely less effective than they were a decade ago, says Tenuta. Commercial hybrids offer different levels of disease tolerance, some more than others.

“Every hybrid is a little different in the make-up,” he says. “That’s why you’ll often see different reactions in our hybrid tests where we look at different diseases.”

To better understand these new physiological races, Tenuta says more research is needed, particularly when it comes to identification and geographical distribution. Also, research could help to determine whether the resistance genes that are still out there are effective or not.

“It’s important to understand and identify physiological races because they’re closely linked to their ability to cause disease or pathogenicity on the corn plant,” says Tenuta. “Ultimately, the goal is always to provide corn breeders, both public and private, with the necessary information in terms of which physiological races are most prevalent. They can therefore be incorporating genes into breeding programs to ultimately reduce losses to the producer to help maintain our competitiveness globally.”

MANAGEMENT
If a resistant corn plant is affected by Northern leaf blight, it will kill the surrounding tissue at the point where the pathogen is located. This stops the pathogen from moving into the plant.

“These defense systems are both structural, in terms of killing cells, as well as chemical signals or reactions that trigger this cascade of events to set off the defense system,” says Tenuta.

Many foliar diseases will survive on the residue and also on the leaves of plants, says Tenuta. And anything that may delay planting early on in the season could result in more foliar leaf diseases.

“If plants are delayed there is that potential for injury,” he says. “The earlier it starts in the earlier growth stages, the greater potential for injury.”
When it comes to effective disease management, crop rotation still serves as an important tool, no matter what disease you are looking at. Proper crop rotation – that is, moving one to two years out of corn – will help in reducing the overall survivability of the spores. However, Tenuta cautions rotation on its own is not enough. Spores do blow around and will end up on crop residue or in the soil.

Since most foliar diseases, except for rust, will survive on residue, the less residue you have, the less potential there is for inoculant to survive from year to year.

“There’s a fine line between reducing your disease risk with management of residues,” says Tenuta. “You don’t want to go too far beyond the other side and cause soil problems like soil erosion.” In fact, OMAF field crop experts recommend about 30 percent residue to help reduce risk of disease, while maintaining the benefits you get from having that residue to protect the soil from erosion.

Good hybrid selection is also key, says Tenuta. When it comes to resistance genes, there are differences found in corn hybrids, which is why it is still important to look at your foliar leaf disease rating, particularly when it comes to the big three: Northern leaf blight, grey leaf spot, and rust.

FUNGICIDE
Producers may choose to use a well-timed fungicide application to control Northern leaf blight in corn, particularly where disease pressure is high enough to incur yield loss.

Dr. David Hooker, Field Crop Agronomist and Assistant Professor at the University of Guelph’s Ridgetown Campus, says there are three factors to consider when deciding whether or not to use a fungicide. What is the farmer’s rotation system? What are the weather conditions like? And how susceptible is the hybrid to disease?

When it comes to determining susceptibility, Hooker says it can be tricky, since company ratings are not the same across the board. When it comes to hybrid selection, he suggests asking a trusted seed dealer and communicating with other growers.

There is also the question of economics. Would using a fungicide put the crop in a profit loss situation? To avoid that, Hooker recommends calculating the costs by predicting the price of corn at tassling time for the price come harvest.

“The best way to do that is to acquire the forward contract price for fall delivery or whenever you want to market your crop,” he says. “And then that would be the price that you would set your economics at.”

A lot of growers choose to use yield response data, says Hooker. On average, they see an increase of six bushels to the acre when a fungicide is used. “But if there’s a chance of high disease, and if the environment is conducive for the development for that disease, we can get yields of 10 bushels to the acre or more,” he says.

To stay on top of news related to the presence of foliar diseases in the field, both Tenuta and Hooker recommend Peter Johnson’s CropLine as a great source. CropLine is a province-wide phone service that offers weekly crop management tips via podcast. It can be found at: www.omafra.gov.on.ca/english/crops/cropline/. •