The battle for supremacy in wheat-fusarium interactions
By Dilantha Fernando
Wheat is an important crop in Canada, representing nine per cent of total farm cash receipts in 2015, and averaging 16 per cent of crop receipts in Canada from 2011 to 2015, according to Statistics Canada. And Fusarium head blight caused by Fusarium graminearum is the most important wheat disease. Fusarium head blight also infects barley and is a problem in malt barley production. With increasing corn acreage in Manitoba, there is a greater incidence of ear rot caused by F. graminearum as well.
The first and worst epidemic in Manitoba was in 1993. Since then, Fusarium has slowly spread to new areas across the Prairies, and by 2008, it was commonly found in the Dark Brown and Black soil zones in all three Prairie provinces.
There has been an emergence of new Fusarium populations and shifts in existing populations since 2000. A possible cause is the accidental introduction of isolates from one area to another, or one country to another.
Fusarium head blight is a concern because of the mycotoxins that can be produced by the pathogens. Fusarium graminearum produces two toxicologically relevant groups of mycotoxins. These mycotoxins have major impacts on swine feeding, resulting in poor feed intake and poor growth. Swine feed intake is reduced 7.5 per cent for every one part per million (ppm) of deoxynivalenol (DON) found in the diet.
The first mycotoxin group is the Trichothercens, which includes DON and the acetylated derivatives such as 15-ADON and 3-ADON. The DON mycotoxin is very stable during storage, milling, processing and cooking and doesn’t degrade at high temperatures. The other mycotoxin group in the Trichothercens is Nivalenol (NIV) caused by F. cerealis. It is not a virulent but is 10 times more toxic than DON. This group could become a concern and we don’t have a good monitoring system for NIV.
The second major mycotoxin group is Zearalenone and its derivatives.
The current issues with Fusarium mycotoxins in the Canadian feed supply is that Fusarium pressure in Canada is widespread and may be increasing because of wet seasons that promote the disease. There is also the additional risk of mycotoxin exposure from new feed ingredients such as distiller’s dried grains with solubles (DDGS) that are corn or wheat based. There is an increased risk in livestock feed with DDGS, since DON concentrates in in DDGS by approximately three times.
There appears to be a shift in the pathogen population with 3-ADON becoming more prevalent. This is a concern since 3-ADON makes significantly more toxin that is also more toxic. The LD50 for swine with 15-ADON is 113 milligrams per kilogram (mg/kg) while it is 49 mg/kg for 3-ADON. Analysis conducted by Ward et al in 2008 found that 3-ADON was found in six per cent of Alberta samples tested, 11 per cent of Saskatchewan samples, and 39 per cent of Manitoba samples.
We have looked at genetic chemotyping of DON isolates. On winter wheat, we found 3-ADON accounted for 82.4 per cent of F. graminearum isolates in Winnipeg and 84.6 per cent in Carman, Man. At Melfort, Sask., 3-ADON accounted for 100 per cent of the DON population. Canadian Grain Commission samples of CWRS wheat in 2015 indicated a shift to 3-ADON in the Black and Dark Brown soils zones.
This shift to a greater prevalence of 3-ADON brings new issues in managing the disease because of the increased virulence of 3-ADON. And because of the higher toxin production, there will be new issues at the elevator, in DDGS feeding and at the trade level because of potential downgrading.
The accidental discovery of NIV producing isolates in winter wheat at Carman by Chami Amarasinghe at the University of Manitoba is also a concern. Five of 132 Fusarium isolates were found to be NIV. In these isolates, 65 per cent were identified as 3-ADON, 31 per cent 15-ADON, and four per cent NIV. The presence of NIV is a concern, since it is 10 times more toxic to livestock than DON.
The identification of NIV is a concern because F. cerealis and F. graminearum are very similar and difficult to distinguish from each other. Until 2012, NIV had only been detected in a few barley samples in Canadian grain. However, testing for NIV in Canada is not routinely conducted at grain mills or elevators.
Amarasinghe also investigated the possibility of masked mycotoxins in our grains. These mycotoxins are masked because their structure has been changed in the plant. This process occurs when plants detoxify DON by converting it to DON-3-Glucosides (D3G). Masked mycotoxins are also known as modified mycotoxins and can’t be detected by conventional chemical analysis. However the danger is that gut microbes in livestock digestive systems may be able to convert D3G back to DON.
Findings from Amarasinghe’s research showed Canadian spring wheat cultivars produced D3G upon Fusarium infection, and there were significant differences among wheat cultivars. The susceptible cultivars showed a lower D3G to DON ratio (less D3G content) compared to the moderately resistant/intermediate cultivars. She found the level of resistance might have an effect on the production of D3G during the infection.
Looking into the future, Canadian wheat production may be at greater risk of Fusarium infections. An increase of 3-ADON, the potential for NIV to establish, and masked mycotoxins in our grain may be food safety issues. Additionally, with climate change, there is a possible threat of an increase in mycotoxins or having new mycotoxins such as the new NX-2 population establish.
Historically, in Canada we have seen shifts in the past. In the Great Lakes area, we saw a shift from ZEN to DON in the mid-70s, similar to the shift from 15-ADON to 3-ADON on the Prairies in the 2000s.
There are now some wheat varieties that have resistance to Fusarium in winter wheat and Canadian Spring wheat. Other classes also have varieties that are moderately resistant to Fusarium as well. These are important and should be considered as management tools.
This article is a summary of the presentation "War of the titans: The battle for supremacy in wheat-fusarium interactions," delivered by Dr. Dilantha Fernando, University of Manitoba, at the Field Crop Disease Summit, Feb. 21-22 in Saskatoon. Click here to download the full presentation.
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