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Where have these funguys gone?

Herbicides and “new” crops may play a role in controlling disease

Back in the 1970s and ’80s when I worked for Alberta Agriculture, take-all of wheat and barley and snow mould of winter wheat were hot topics on the Canadian prairies. Yield losses from these two diseases were considerable. Wet summers were bad for take-all, while deep snow winters often led to snow mould.

I remember seeing wheat with up to 80 per cent yield loss from take-all and whole fields of winter wheat killed off after snow melt in late April. What has happened to change this? I know it isn’t “climate change” or plant breeding of better wheat cultivars.

So how could the very destructive take-all disease go from being a major root rot disease of wheat in particular to becoming a rarity on today’s prairie cropland? The answer is glyphosate and other grass killing herbicides.

The take-all fungus does not form resting spores in cropland soil. The take-all fungus survived on cropland by overwintering (between cereal crops) on the roots of weed grasses such as quack and brome grasses. These grass herbicides in general and glyphosate in particular virtually eliminated these formerly persistent cropland grass weeds. By eliminating these grass weeds that were hosts and reservoirs for the take-all fungus we inadvertently got rid of the take-all problem. It has been reported in Australia that canola residue itself is toxic to the take-all fungus. So with shorter canola/cereal rotations, we’re seeing lots of canola residue on fields.

If you want to find the take-all fungus infection on wheat these days you have to look along the field headlands where we still have persistent wild grasses that harbour the fungus infecting wheat on the edge of the field. One word of caution: never plant wheat on old broken pasture or hay fields since these grasses are perfect carriers of the take-all fungus. This fact was well established but perhaps forgotten now on the North American prairies — anything but wheat after pasture. I’ve seen 60 to 70 per cent take-all kill of wheat when producers did not heed this warning.

What about snow mould?

Remember when snow mould of winter wheat and rye was a good reason not to plant these cereals? What changed? We plant winter wheat now and worry about the green bridge when it comes to wheat streak virus and about stripe rust in the spring. No more vast expanses of dead winter wheat crowns in April and no signs of snow mould that were common sights some 50 years ago.

A likely answer is the canola crop. Canola, a member of the mustard family, produces a toxic hot-tasting substance called glucosinolate, like its mustard relatives. Canola seed, unlike hot mustard seed, has been bred to be free of this substance. Nevertheless, the rest of the canola plant — stems and leaves — contains glucosinolates. These break down in soil to form ethylbisdithiocyanate a toxic chemical. This chemical is closely related to methylbisdithiocyanate, which is sold as a soil fumigant known as Vapam. Idaho researchers have shown that an 80-bushel crop of rapeseed or mustard could produce the equivalent of two times the recommended fumigation rate of Vapam when worked into the soil at full bloom.

This soil-fumigating effect of rapeseed and mustard was used extensively in France and New Zealand to fumigate soil to reduce the level of aphanomyces root rot of peas. The present “rotation” of wheat/canola may have a fumigation effect on cropland that has reduced or eliminated the many snow mould fungi. In addition, based on past research, canola and its crop residues may reduce aphanomyces damage to subsequent pea and lentil crops.

This information is speculative but I hope that it may spur future investigation by crop research personnel into investigating the effect of canola residue on the very destructive aphanomyces root rot of peas and lentils.

I may be sticking my neck out with my theories, but I never was one to “stay in the box” and pay lip service to the edicts of the past.

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