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Not all pathogens survive on crop residues

Cropping system affects common root rot of wheat in eastern Saskatchewan.


December 18, 2007
By Donna Fleury

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42Crop diseases can impact a wide range of crops under many different cropping systems.

Common root rot (Cochliobolus sativus, Fusarium spp) is one of the most widespread diseases affecting cereals. Common root rot can occur in most cereal crops, causing small but consistent losses of up to 10 percent per year.

However, researchers have discovered that this disease may be associated with the most common pathogens causing fusarium head blight (FHB) infections in Saskatchewan, and that these fungi causing root and crown rot are affected in a different manner by agronomic practices.

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Seeds planted in Fusarium graminearum infected soil in the greenhouse.  Photos Courtesy Of AAFC Swift Current.

Led by Dr. Myriam Fernandez, cereal pathologist at the Agriculture and Agri-Food Canada (AAFC) Research Centre in Swift Current, researchers initiated a large project to determine the association between agronomic practices and common root rot severity. Another component of the project was to identify and quantify the organisms causing the disease. Despite the trend toward reduced tillage and diversified crop rotations with pulses and oilseeds increasing, common root rot continues to be prevalent.

The main impetus for the research was the concern over the increase of FHB in eastern Saskatchewan in the ‘90s and the search to find ways of controlling the disease in areas it was already established, mainly eastern Saskatchewan along the Manitoba border, and to prevent its further spread westward. Researchers had also discovered that the most important fusarium pathogens causing FHB were also commonly found in roots and crowns of both cereal and non-cereal crops. Researchers had received calls from growers who were facing high rates of seedling blight in wheat crops following pulses in rotation.

“In Manitoba, eastern Canada and many parts of the world, F. graminearum is the most important pathogen causing FHB,” explains Fernandez. “However, in Saskatchewan we found F. avenaceum to be the most common pathogen.” F. avenaceum is not exclusive of cereals, it also affects oilseeds and pulse crops, which are particularly susceptible. “We started making connections and wondering what role infected crowns and roots of plants might have in the development of FHB caused by F. avenaceum.”

Pulses support different root rot pathogens
Fernandez and other researchers initiated this five year Agriculture Development Fund (ADF) project in 1999,
looking at various components, including FHB, root and crown rot, through wheat and barley field surveys and comparison of cropping practices. From 1999 to 2001, researchers sampled more than 400 fields of spring and durum wheat and more than 130 barley fields in eastern Saskatchewan to determine the pathogens causing common root rot. Non cereal crops grown in rotation with cereals were also sampled and the cropping systems compared.

“We found that F. avenaceum was the most common pathogen isolated from residues and roots of alternative crops such as pulses and oilseeds,” says Fernandez. “F. avenaceum was actually more common in the alternative crops than in cereals. We also isolated F. graminearum, which is the most important cereal pathogen causing head blight in North America, from the roots and residues of non-cereal crops. This is not what was expected because F. graminearum is not a pathogen of non-cereal crops.” Surprisingly, rotations with some non-cereal crops increased the levels of F. graminearum in cereal roots and heads. However, this could not be attributed solely to its survival in non-host plant tissues which, overall, occurred at a low rate in this area. Fernandez expects this may be one of the reasons that crop rotations do not seem to be controlling FHB.

Although the research found F. avenaceum to have become an important pathogen in roots of cereals, the fusarium species, in general, were still present at lower levels than Cochliobolus sativus, the main pathogen causing root rot. The drier climate conditions in Saskatchewan in many years favours the development of Cochliobolus sativus, however, in areas where precipitation is higher and soils heavier, infection by fusarium species would be more likely.

Reduced tillage changes pathogen make-up, as well
Researchers have confirmed previous observations made by other researchers that cropping systems using conventional tillage favour the presence of Cochliobolus sativus and decrease the presence of Fusarium spp.
However, under reduced tillage, Cochliobolus sativus decreases, but Fusarium spp increases. More traditional rotations of wheat and fallow in rotation would keep the Fusarium spp in check, but would increase levels of Cochliobolus sativus. Fernandez expects that may be why fusarium species were not as much a problem in the past when conventional tillage and fallow were the main agronomic practices, and there were fewer alternative crops grown in rotation with cereals.

This project identified effects on individual species infecting roots, which are also responsible for FHB, an important emerging disease on the prairies. With the trend to alternative cropping systems and diversified crop rotations, the root rot pathogens are changing. “We’re finding that it is not only reduced tillage that is increasing the levels of F. avenaceum, it’s also the frequency of non-cereal crops in rotation,” explains Fernandez. The highest levels of F. avenaceum tend to be found in wheat or barley followed by two years of crops such as oilseeds and pulses, with the higher levels immediately after pulses.

The same agronomic practices that affected levels of this pathogen in roots also affected head infections, whereas other fusarium pathogens did not respond in the same fashion to cropping systems. “However, the continuous rotations with alternative crops were higher yielding than continuous cereal crop rotations, which would be mostly attributed to their higher N status.” Therefore, the concerns are not really about yield, but about increasing inoculum for FHB.

Fernandez does not want growers to rush out and make changes to their cropping systems, but she encourages them to be more aware of the situation and the potential disease problems. “Growers need to be aware that when
any new agronomic practices become popular, such as reduced tillage or rotations with pulses, those practices are likely going to affect levels of different pathogens. We can now begin to find ways of controlling or decreasing levels of these fungi, now that we have a better understanding of what affects the individual pathogens.” When reduced tillage first became popular, some people were concerned that all pathogens that survive on residues would become a problem. While some have, others have not.

These results convinced researchers they would need a comprehensive approach to address the problem. “Although we know that tillage systems and alternative crops contribute to increases of some fusarium species in roots and crowns, we need to measure the relative contribution of each in order to come up with control measures and recommendations,” says Fernandez. “For example, we may be able to find effective treatments, either chemical or biological, to control the pathogens, or recommend a change in an agronomic practice to reduce pathogen levels.”

Through another ADF research project, Fernandez, in collaboration with other researchers and industry, are comparing various registered seed treatments, and products being trialed but not yet registered, for control of fusarium in several field locations in Saskatchewan. “We haven’t completed the analysis yet, but preliminary observations indicate we haven’t found any successful treatment so far.” More research work needs to be done.

Other related research projects underway include a comparison of pea-wheat rotations to continuous wheat rotations under conventional and reduced tillage and different herbicide treatments. A comparison of alternative cropping systems, including non-organic and organic systems, is underway at the AAFC Scott Research Station.

“We hope through these various projects that we can gain a better understanding of the impact of popular agronomic practices on crop diseases and common root rot pathogens and their possible impact on head infections,” says Fernandez.

For now, growers need to be aware that common root rot is a complex disease that might be associated with important head pathogens. Although Cochliobolus sativus is still the main pathogen causing common root rot in cereals, important fusarium pathogens could potentially become a more important problem under the rightconditions. Reduced tillage practices reduce the presence of Cochliobolus sativus, but increase the presence of common fusarium pathogens, such as Fusarium avenaceum.

Tillage and fallow appear to reduce the presence of fusarium pathogens, but increase the levels of Cochliobolus sativus. “We hope through other research focussing on the contributions, impact on yield and other specific factors, that we can come up with better control measures and recommendations for growers in the future.”