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Field management to reduce blackleg risk

Blackleg/basal canker symptoms by harvest time. Photo courtesy Gary Peng, AAFC.

For many years, blackleg disease on the Prairies was managed fairly successfully through the use of disease resistant varieties and an extended rotation of three or four years between canola crops. However, since about 2009, researchers and agronomists have been seeing changes in the pathogen Leptosphaeria maculans that causes blackleg in canola and getting reports of more disease problems.

“We began receiving more reports of higher levels of disease and damage including fields with resistant cultivars/lines in southern Manitoba and northeastern Alberta, with some reports from adjacent areas in southeastern and northwestern Saskatchewan starting in 2009 and 2010,” explains Dr. Gary Peng, research scientist with Agriculture and Agri-Food Canada (AAFC) in Saskatoon, Sask. “We have initiated several research projects since 2011 and are continuing our efforts to try to determine the reasons for some of these spikes, which can be quite complex.”

In 2013, a new, five-year project was initiated across the Prairies to look at the blackleg resistance strategy in relation to management practices in the field. Peng is collaborating with project lead Dr. Dilantha Fernando from the University of Manitoba and Ralph Lange with Alberta Innovates to work with about 25 farms in each province to help understand different management practices and the impact on blackleg pathogens and resistance in canola. This information will help understand what interactions and effects are playing a role in the maintenance/breakdown of resistance.

“With the three of us covering different provinces, we will be able to pool our information and have a bigger picture of blackleg resistance and the Prairie situation,” says Peng. “This project is very much a continuation of the research we have been conducting on pathogen race typing and population structure. The core of this project is to track the population of the pathogen and determine what is changing or what has changed in relation to the different farm operations. We also want to determine the potential linkage of that population change to the disease level we are seeing on those farms.”

In each province, 25 farms with different management practices were selected through collaboration with several retail outlets and field agronomists. For each farm, one quarter or half section fields of canola will be monitored each year of the project. Researchers will be following the canola crop over the time of the project and collect data from each of the fields every year including crop sequence, crop inputs and other practices. Researchers will also be estimating the overall disease severity in the canola crop, either at harvest or shortly after, as well as taking stubble samples from each field and analyzing the pathogen race composition in the lab.

“We are selecting several groups of fields in a five- to 10-kilometre region where the pathogen race composition may not vary that much, but with a range of fields and management practices in terms of shorter and longer rotations,” explains Peng. “Some growers use very short rotations, such as back-to-back or a one-year break, while others may have up to a three- to four-year break in between. Most growers rotate the varieties they use each year to help reduce the risk of resistance breakdown.” The project will help provide information useful to the development of management strategies on a regional basis.

Peng adds that, so far, the severely damaged fields being reported are still fairly isolated cases. Researchers are interested in understanding why those fields are much more severely damaged than neighbouring fields – sometimes the same cultivar is only two or three kilometres away. About 95 per cent of those severely damaged fields have been short rotation fields, either back-to-back canola or only one-year break in between. The short rotation has allowed for accumulation of the inoculum in those fields, which appears to make a difference.

“There is a good opportunity for growers to be proactive in monitoring blackleg and assessing the risk,” says Peng. “During harvest, at the end of the day, pull about 100 stubble stem samples from across the field, clip the stubble sample at the root section and look at the overall blackleg incidence. This is a good way to project the risk for the next season. If growers are seeing an increasing trend of the disease then it is a sign they should make some changes to their management. If the disease incidence is staying below five per cent, then it is probably still a good sign of a healthy crop.”

Peng has also been conducting research over the past four years on fungicides for control of blackleg. Based on the results to date, a widespread use of fungicides, even at the early crop stage, would not be useful in terms of yield benefit. “In most cases our resistant varieties are still standing pretty good and the severe damage seems to happen in fairly isolated cases. Based on the level of damage we are seeing on a regional basis, the overall disease severity is too low for fungicides to be really beneficial in terms of yield improvement. Although there clearly is a reduction in disease severity, the impact on the seed yield is so low that there isn’t a lot of benefit from the investment into a fungicide application.”

Another consideration is that most of the registered fungicides are very similar in modes of action, being the strobilurin type, which are easy for a pathogen to develop resistance to. One older alternative, Tilt, is no longer very effective. “Therefore, if growers routinely use fungicides as a preventative measure or for any other physiological benefit, by the time we really need fungicides we may not have highly effective products due to potential fungicide resistance in the pathogen population,” says Peng. “Therefore, we discourage widespread use of fungicides and they should be considered as the absolute last resort to control the disease. In situations where there are widespread problems with a virulent pathogen race and conditions are expected to get bad for the season, then putting on a fungicide treatment early in the season would be warranted, otherwise it is generally not necessary.”

As the information from this five-year project currently underway is collected and analyzed, researchers expect to have a better understanding of the changes in the pathogen population and the impacts from different farm operations and practices. By the end of the project, researchers hope to be able to demonstrate some of the good practices that may help reduce the risk of blackleg resistance across the Prairies.

September 15, 2014  By Donna Fleury