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Fusarium Head Blight continues to be a challenge

Management of FHB requires an integrated approach.  Photos courtesy of the Canadian Grain Commission.

Fusarium Head Blight (FHB) was observed in the 2007 Canada Western Red Winter (CWRW) wheat class, although disease levels were low in spring wheat. The disease returned to higher levels for both Canada Western Red Spring (CWRS) and CWRW in 2008, aided by warm, wet environmental conditions on the Eastern Prairies. With higher levels of fusarium-damaged kernels (FDK) observed in 2008, Mike Grenier, agronomist with the Canadian Wheat Board in Winnipeg, says that farmers are continually being challenged to develop the best approach to managing FHB. “Weather plays a large role in the development of FHB, likely more than 50 percent, but variety selection, crop rotation, fungicide application and cultural practices can help in reducing the effects,” says Grenier. “But another concern that we have is that the level of vomitoxin with the FHB seems to be changing in Western Canada, and that may have implications as well.”

Changing FHB populations may require management changes
Historically, the main pathogen causing FHB has been divided into three chemotypes based on toxin production: DON/15ADON, DON/3ADON and Nivalenol. The 15ADON chemotype was the only significant cause of FHB in North America, but Randy Clear, with the Grain Research Laboratory of the Canadian Grain Commission (CGC) in Winnipeg says that is changing.
Since 1998, Clear has analyzed samples for chemotype and found out that the 3ADON chemotype is becoming more prevalent, moving from a level of about three percent of Manitoba isolates in 1998 to about 68 percent in 2007. Increasing levels of the 3ADON chemotype also were seen in Saskatchewan and Alberta.

Subsequently, Clear had the chemotypes analyzed for toxin-producing ability, and found that in the lab, the 3ADON chemotypes produce almost three times more DON toxin than the traditional 15ADON type. “That means we should care about what chemotype is present. If 3ADON produces more toxin, that’s the reason we should be concerned with this shift,” says Clear. 

In the lab, the 3ADON chemotypes produced more spores, indicating that they may have an impact on reproduction levels as well. However, greenhouse trials by Agriculture and Agri-Food Canada have not found the same degree of difference in toxin formation or increased reproduction. Clear says that more testing will be required to further understand these differences. “But there is a fitness advantage of the 3ADON over the 15ADON, because it is increasing. It has to be doing something better. This may require a change in grading standards if the change in chemotype results in a change in toxin production,” says Clear.

The FHB chemotypes are changing on the Prairies.

DON production seems to be increasing
One of the cornerstones of the CGC grading standards has been based on the ratio of FDK to the level of DON (deoxynivalenol) mycotoxin in the kernels. Based on research, that ratio has been set around 1:1, for every one percent of FDK in a grain sample: it is assumed that there will be about 1 ppm DON. The ratio is based on composite samples. For any single sample of grain, the ratio can vary widely. Only when the grain from several fields is put together that the ratio begins to settle out at about 1:1. Each year the ratio will vary somewhat, but the expectation for composite samples is that it will follow near a 1:1 ratio as a rule of thumb.

Currently, CGC grading standards are 0.25 and 1.0 percent FDK for #1 and #2 CWRS, respectively, while 1.0 and 2.0 percent are standards for #1 and #2 CWRW. Grenier explains that some of the CWB customers are requesting DON levels of less than 1 ppm.
The CWB conducts an annual, on-farm harvest survey of winter wheat to assess FDK levels, collecting samples right off the combine. One of the problems that Grenier saw in the 2008 winter wheat harvest survey is that the 1:1 ratio is not necessarily holding true. In some areas, the ratio was as 1:2 to 1:3 or higher. “I would suggest this is a big problem, and one we should be concerned about,” says Grenier.

For hard red spring wheat, the harvest survey showed the worst FHB damage was in northeast Manitoba.  However, in Manitoba, about 65 percent of the crop was grading #1 and 2, and only a small percentage of the crop was downgraded to feed because of FHB. “We have to pay very close attention to the blending that we do to stay within the 1 ppm DON level to meet CWB sales contract specifications,” says Grenier.

Use management practices to minimize infestation
While weather is a primary factor in FHB development, Grenier says that several management strategies can be used to reduce the risk of FHB. They should be focused on reducing the quantity of inoculum available for dispersion, prevent dispersal of the inoculum, and prevent the infection of the wheat spikelets if the inoculum is present. 

Crop rotation is one of the key ways to reduce disease levels. Because FHB can infest corn, wheat and barley, longer rotations without these crops can help to reduce the inoculum load.
For winter wheat, a very susceptible crop to FHB, Grenier says that early seeding will help to minimize the disease. Early seeding, ideally before Labour Day, will mean the crop heads out and flowers early in June, providing the best chance of escaping the high-risk weather conditions in late June and early July. “I think we need to revisit that as a management practice, since some farmers have started to go later with winter wheat seeding,” Grenier says.

Tillage can have an effect on FHB dispersal but available research data shows only a subtle difference between tillage systems. Generally, no-till crops have the same risk levels as compared to conventional tilled crops while min-till systems tend to rank higher for FHB disease levels.  

Grenier also says that wheat growers should consider selecting varieties with better FHB resistance. Resistance ratings can be found in variety descriptions in provincial seed guides. He cautions that the new wheat midge resistant varieties, AC Goodeve (VP) and AC Unity (P), may be appealing from an insect management perspective, but they are a compromise in FHB resistance. “I’m concerned that some of the popular varieties have poor resistance to FHB,” says Grenier.  He says that farmers in higher risk areas should consider hard red spring wheat varieties like 5602HR and AC Waskada, which have good resistance ratings. “What concerns me is that over one-half the varieties grown in Manitoba in 2008 are rated poor to fair, when there are better choices.”

Grenier also cites research from Fargo, North Dakota, that shows multiple strategies, such as crop rotation, variety selection and fungicide application can help reduce FHB infestation levels. “The message from this research shows that there is value in multiple strategies when the disease pressure is moderate to high, but not overwhelming the crop.”