Yuefeng Ruan, Ph.D., Research Scientist, Agriculture and Agri-Food Canada;
Ron DePauw, Ph.D., Science Advisor, SeCan;
Ron Knox, Ph.D., Research Scientist, Agriculture and Agri-Food Canada; and
Mitchell Japp, MSc., Provincial Specialist, Cereal Crops, Saskatchewan Ministry of Agriculture
Fusarium head blight (FHB) has become the number one enemy of durum wheat producers in Canada. By providing information on durum wheat variety response to FHB, this article complements the Saskatchewan Ministry of Agriculture factsheet.
FHB is a very complex disease caused by several species of Fusarium. Some Fusarium species infect all cereal grain crops, including durum, and some forage grasses, and can survive on both living plants and plant debris.
Fusarium is favoured by moist conditions during flowering. Moist conditions lead to increased production of spores from debris of a previous host crop, and higher rates of infection of heads in durum and other cereals. Premature white heads or spikelets are symptoms of the disease and resulting infected kernels (fusarium damaged kernels or FDK) are often shrivelled and light weight leading to loss of grain yield.
Infected kernels usually contain mycotoxins that are harmful to humans and animals. The primary mycotoxin is deoxynivalenol (DON, also known as vomitoxin) primarily produced by Fusarium graminearum. Although environmentally dependent, generally, the higher the FDK the higher the level of mycotoxin, which contributes to FDK being a grading factor. Grades 1 and 2 of Canada Western Amber Durum permit only up to 0.5% FDK, while Grades 3 and 4 up to 2% and Grade 5 up to 4%.
Management of FHB requires an integrated strategy of multiple practices that when used together reduce losses caused by FHB and include:
- Use of the most resistant cultivars available;
- Cultural practices that break or interfere with the disease cycle; and
- Chemical control in conjunction with FHB forecasting.
Growers will experience the greatest benefits when multiple practices are used together instead of alone and should never rely on a single management practice to control FHB. A decision to apply a fungicide must be made prior to any symptoms being observed. Therefore, FHB forecasting is very important in the decision making process.
Symptoms of FHB are classed as incidence, severity, FDK and DON.
- Incidence is the number out of 100 heads in a field displaying infection of at least one floret.
- Severity estimates the average percentage of the head displaying infections in 100 heads.
- From a harvested sample, threshed using a low wind speed, a representative sub-sample is taken. The kernels are examined for evidence of fusarium damage. The weight of FDK is expressed as a percentage of the subsample weight.
- DON is chemically measured on a representative grain sample and expressed as parts per million (ppm).
Although FDK and DON are problematic to farmers because of down grading, they are less important to the control of the disease than the proportion of the crop showing symptoms of the disease.
In general, disease propagation comes from sporulation associated with infected tissue. Lower severity and incidence of disease means less infected tissue, therefore, less area contributing to sporulation. Less sporulation in general means less opportunity for infection and less disease. By growing cultivars with the greatest resistance to FHB incidence and severity, the number of infecting spores can be reduced in a subsequent crop cycle, thereby reducing disease levels over time.
Genetic variation for resistance to FHB is limited in tetraploid wheat, which includes durum wheat, compared to the variation in hexaploid wheat. The limited variation makes it more challenging for breeders to increase the resistance in durum. Fortunately, some ancient genotypes have been identified to have resistance and efforts are in progress to incorporate these genes into modern durum. Studies have detected genetic effects on virtually every chromosome. The effects are generally small and often additive or cumulative. There are multiple mechanisms of resistance including resistance to initial infection of a floret, resistance to Fusarium spread within the head, detoxification of mycotoxin (DON), and escape, such as flowering within the boot.
Growing varieties that exhibit lower incidence and severity of FHB is a long term approach to control, through the reduction in plant debris that produces spores to infect the next vulnerable crop. Although the best level of resistance to FHB in durum wheat varieties is less than the best CWRS spring wheat varieties, durum varieties range from susceptible (S) to moderately susceptible (MS) with some of the MS varieties expressing better resistance. The Varieties of Grain Crops in Saskatchewan 2017, provides recommendations on durum varieties to use as part of a long-term management approach to FHB control.