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Brown root rot lurking in alfalfa fields

A little-known root disease hiding under the Prairie snowpack this past winter could give alfalfa growers a surprise in spring.

Weather conditions were ideal for the development of brown root rot, a potentially damaging disease of forage legumes, including alfalfa.

Brown root rot is a soil-borne fungal pathogen with the official name Phoma sclerotioides. It is often described as a “snow mould” because it grows most rapidly when the ground is covered by snow.

That was certainly true during the winter of 2013. A thick snowpack and cold temperatures were general throughout much of the Prairies, unlike the previous winter, which saw mild weather and little snow.

The return of more traditional winter conditions has researchers suggesting brown root rot could be common in alfalfa fields this spring, after lying low for several years.

“This year could be a really good year for snow mould,” says Bruce Gossen, an Agriculture and Agri-Food Canada plant pathologist in Saskatoon. “It increases the opportunities for brown root rot a lot.”

Why? Unlike many soil-borne pathogens, snow mould organisms, such as the brown root rot pathogen, become active when the ground cools down in fall. If an early snowfall occurs before the ground is frozen, the resulting insulation gives the disease a longer period to remain active. Its development stops during the depths of winter when temperatures are at their lowest. But activity picks up again as the approaching spring brings warmer conditions.

“Brown root rot of alfalfa can be a serious problem in regions with severe winters,” says Michael Wunsch, a North Dakota State University plant pathologist at Carrington, N.D. “It is most likely to be a problem when snow cover begins early in the fall before a deep freeze and snow cover is continuous throughout the winter. Anecdotally, the disease is likely to be the worst in places where snow accumulates due to drifting, such as along tree rows and fences.”

Actually, brown root rot isn’t the most damaging snow mould, at least in Saskatchewan. Gossen says that distinction belongs to cottony snow mould, which can cause extensive damage to forage legumes and winter cereals because it is able to grow at even lower temperatures.

But scientists still recommend producers check their alfalfa crops this spring for signs of brown root rot, which can cause substantial stand loss if the disease is able to develop.

The problem with brown root rot is that it’s a bit of a mystery disease which flies below the radar and can be mistaken for something else.

Gossen says typical above-ground symptoms in alfalfa consist of plants that are either slow to initiate growth in spring or die during the winter. Often, such losses are assumed to be the result of winterkill. That’s when plants die from exposure to severe winter conditions, including cold soil, ice or lack of snow cover.

Because the end result appears similar, producers tend to attribute alfalfa stand losses to winterkill, when the real cause is a pathogen working below the soil surface.

The only effective way to verify brown root rot is to dig up the roots of affected plants, wash them and check for telltale symptoms.

The classic signs of brown root rot are darkened lesions, often with a dark-brown border, on the roots. These lesions weaken the plants, delay their emergence, lower plant vigour and reduce yields. When lesions on the upper taproot expand enough to girdle the root, the plant dies.

But because producers don’t usually dig up alfalfa roots to check for lesions, a proper diagnosis of brown root rot is often lacking. The disease works silently without displaying many symptoms, other than reduced performance. By the time growers realize there’s a problem, it’s usually too late.

“The best sign that you may have a problem with this disease is if you had a great-looking alfalfa stand in the fall, a lot of mortality in the spring and quite a few plants showing reduced vigour emerging out of spring dormancy,” Wunsch says.

Even then, establishing the presence of brown root rot is not conclusive without a laboratory test (or assay) to detect the pathogen in plant and soil samples. Gossen says such a commercial test is not available in Western Canada, although researchers at the University of Minnesota are reported to have developed an assay for use in a number of U.S. states.

Without a definitive analysis, alfalfa stand losses are often attributed to insects, harsh weather, drown-out or other causes.

Brown root rot is a native disease in North America and was probably here before the settlers arrived. In 1933, a University of Alberta plant pathologist named G.B. Sanford found the pathogen in virgin prairie soil near Edmonton.

Brown root rot is also known to occur in a number of northern U.S. states, although damage from the disease is less severe in milder climates. Wunsch describes brown root rot as a “yield nibbler” in the U.S. but one which can cause significant economic losses in Canada.

Brown root rot is a frustrating disease to deal with. It occurs sporadically, depending on weather conditions, so predicting its occurrence and severity can be difficult. There is no effective treatment for it while the crop is in the field.

Although primarily a forage disease, brown root rot is also known to affect winter cereals which break dormancy while the pathogen is still active in the soil. Because it is relatively dormant in summer, spring crops are not affected.

The only effective management tools against brown root rot are rotating crops and selecting varieties of alfalfa or other perennial forage legumes that are less susceptible to the disease.

Wunsch suggests producers grow mixed forages which contain grasses as well as legumes. Grasses, which have a fibrous root system, often sustain little damage from the brown root rot pathogen. This can reduce yield losses because a single perennial forage crop with a large taproot no longer dominates the field.

Gossen recommends breaking up an alfalfa stand containing severe brown root rot and replacing it with annual crops for three years to reduce the level of pathogens in the soil. If producers grow only forages, they could plant greenfeed crops for a few years and then return to a forage legume.

Gossen warns that snow mould diseases such as brown root rot build up over time. A susceptible crop becomes more vulnerable with age. A 20-year-old alfalfa stand is a prime candidate for a root disease.

Fortunately, brown root rot is not the worst pest risk to alfalfa crops in Western Canada. Gossen says a far bigger threat is the alfalfa weevil, which is spreading rapidly in Saskatchewan and has been in Manitoba since at least 2001.

But because brown root rot is a sneaky disease, it bears close watching, says Gossen.

“Producers should walk their fields in early spring and keep an eye on the general health of their crop. That way they will know if there are going to be problems during the spring,” he says.

Gossen says if you find plants that are doing poorly, pull them up. If they pull out easily, it could mean the roots are rotting underneath. In that case, get a shovel and dig some out.

“You might have a surprise. And you might learn something that will change your plans for that field for the next couple of years.”

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