White heads in spring wheat, durum and winter wheat crops across the Prairies this season have led some farmers to send in samples for testing, suspecting that the cause might be take-all disease.
No positive samples
The cause of white heads in wheat can be a number of things including common root rot, aster yellows, fusarium head blight and simple heat stress. So far, no samples have tested positive for take-all in Manitoba or Saskatchewan labs.
“Our Plant Diagnostic Lab has confirmed that there were no samples of wheat that tested positive for the take-all pathogen this year which isn’t that unusual,” says Holly Derksen, a field crop pathologist with Manitoba Agriculture, Food and Rural Initiatives.
“Generally it’s pretty rare that we get a sample in the lab that is diagnosed as take-all. That isn’t necessarily representative of whether or not there is take-all in the province — perhaps it is being recognized in the field, although it is hard to diagnose accurately.”
Brent Flaten, integrated pest management specialist with the Saskatchewan Ministry of Agriculture in Moose Jaw says he has no information that would indicate that take-all disease is on the increase overall. “Certainly both take-all and common root rot have been more prevalent in the last three years, but some of that due is to the saturated soil conditions that we have had in spring which stresses and weakens plant roots because they don’t like the lack of oxygen,” he says. “It’s then a discussion of which came first? Was the disease there initially or was the plant starting to suffer because of the wet conditions which made it more susceptible to infection?”
Ieuan Evans, a forensic pathologist with Agri-Trend Agrology Ltd., says that he has not seen any incidences of take-all in Western Canadian wheat crops for several years. The last significant outbreak he noticed was in the early 1980s, when he found one wheat field that was 70 per cent infected with take-all. The field had been broken up out of alfalfa, which Evans suspects may have contributed to the occurrence and severity of the disease.
“Quack grass provides a natural reservoir for the take-all fungus and alfalfa fields generally have some quack grass in them,” he says. “It’s likely the incidence of take-all was so high in the subsequent wheat crop as a result of the fungus being present in the quack grass.”
Most Prairie farmers keep quack grass under control, which could be one reason take-all hasn’t been a huge problem, says Evans. He also theorizes that western Canadian rotations of canola and wheat could be reducing the potential for take-all pathogens to build.
The fungal spores of take-all overwinter on infected crop residue. In spring, the fungus grows in the soil, where it comes into contact with the crop roots and causes infection. The fungus may grow from root to root, infecting new plants.
Infection may occur throughout the growing season, but the early infections are the most damaging because they move up into the plant crown. The disease organism can also be spread by transport of infected soil or crop residue from field to field.
Take-all thrives in cool soils (12 C to 20 C) and high soil moisture. The severity of the infection will increase in alkaline, compacted or poorly drained soils, or soils deficient in nitrogen, phosphorus or copper.
A severe infestation can result in considerable losses as the disease kills headed-out plants. In spring wheat, a 62 per cent infection of take-all reduced yields by 50 per cent and in barley a 64 per cent infection of take-all reduced yields by 24 per cent.
Root rot pathogens infect cereals to some extent every year. Symptoms are more severe when infections occur early in the year or during times of excess moisture or extreme lack of moisture, says Derksen. “When there is excess moisture available some root rot pathogens, such as pythium, thrive and will cause more damage,” she says. “When there is an extreme lack of moisture the root system becomes much more important, to access any moisture that is available in the soil. When root rots come in and take out even a small fraction of that root system the end result can be more severe.”
Take-all is most common in wheat, although it can also affect barley, rye and grasses. It’s most severe when wheat follows wheat. Crop rotation is recommended as one of the best ways to reduce the incidence and severity of take-all.
Take-all is usually identified by patches of stunted, empty, white heads that stand out in the field. Roots of these plants are dark brown to shiny black, and so rotted that plants can easily be pulled from the ground.
Take-all disease cannot be completely ruled out in all areas, it’s likely that a combination of diseases and environmental factors may be causing white heads in wheat and affecting yields this year, says Derksen.
“The symptoms of suspected take-all that I heard about this year turned out to be caused by different factors, sometimes more than one,” says Derksen. “The symptoms of take-all include dark discoloration of the lower stem and in severe cases, white heads. However, other root rots such as those caused by fusarium also can cause these types of symptoms. Unless plants exhibiting these symptoms are sent to a lab for testing we cannot be sure what is causing them. If plants are infected by take-all the root system and basal stem will have shiny brown to black fungal growth, possibly even look slimy, which is characteristic of take-all. But I still recommend sending a sample in for testing.” †