Check out the list of diseases that you need to watch out for in forage alfalfa and you’ll find some very familiar names. Phytophthora root rot, Pythium damping off, anthrac-nose, common leaf spot and Verticillium wilt are all well known — too well known — by corn and soybean croppers.
Still, alfalfa brings some additional diseases into the picture, including brown root rot and Aphanomyces root rot, two relative newcomers to forage fields in Ontario.
As with most diseases, these two promise to get worse before they get better.
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Brown root rot
Confirmed in Ontario in 2007, brown root rot is often mentioned as a companion to winterkill. Infected plants are slow to emerge from winter dormancy and they lose yield as a result.
Brown root rot is thought to be widespread across most of the province and even into Quebec, and it is also common across the northeastern border states.
Brown root rot isn’t brand new to Canada or North America. It was found in Western Canada in the 1930s and since 2003 has been confirmed in western states including Minnesota and Wisconsin.
In 2007, Gary Bergstrom, a plant pathologist at Cornell University and graduate research assistant Michael Wunsch carried out a survey that included 10 fields in the Woodstock-Niagara region and around Ottawa. The results were somewhat startling, even on such a small scale. Seven of the 10 field samples tested positive for Phoma sclerotioides¸ the pathogen of brown root rot. The incidence in individual fields ranged from very low — five per cent — to moderate high, with 30 per cent of the roots or crowns being infected.
These results were judged to be preliminary but researchers did suggest that based on the U.S. experience, brown root rot is likely widespread, yet has gone undiagnosed.
“It’s in quite a few fields,” confirms Albert Tenuta, field crops pathologist with the Ontario Agriculture Ministry. “It’s not decreasing… there are pockets throughout the province.”
One reason why brown root rot has gone undiagnosed in so many fields is because there is little visible damage to the above-ground plant. Instead, the disease affects the plant below ground, causing sunken brown (sometimes, almost black) lesions on the tap roots, the lateral roots and on the crown. In severe infections, the tap root is completely rotted away.
The fungus that causes the disease actually thrives in soil temperatures of 15 C or less, so it would normally only be active in spring and fall. However, this past winter has been warmer than normal, and the cool, wet soils that are linked to winterkill have been the rule, not the exception.
The other concern with brown root rot is its slow growth, which contributes to its slow identification. In some fields, the infection goes unnoticed until the second or even the third year, when the plants become stunted or die off altogether.
“The warm, wet fall we had is a concern for me,” says Joel Bagg, forage specialist for the Ontario ministry. “Alfalfa likes cool, dry weather in the late fall in order to winter harden properly. When it’s warm and the soils are saturated, then it doesn’t winter harden to the same level, so it’s more susceptible to all of those winter issues.”
In addition to the weather, Bagg believes many growers did a fair amount of fall cutting in September during the critical harvest period.
“Sometimes, we get away with that, sometimes we don’t,” Bagg says, recalling a similar situation in the fall of 2008, resulting in an increase in winterkill in 2009. “We have to be aware of the risk, and producers should get out there early in the spring, at green-up with a shovel and scout their fields. You don’t want to wait and find that you only have half a crop.”
Much of the reason for concern over brown root rot in Ontario is the lack of resistant varie-ties for growers, meaning management is limited to avoiding late or excessive fall cutting in order to reduce stress on the plant going into that winter-hardening stage.
Maintaining proper soil fertility and rotating out of alfalfa for three years can also help reduce yield losses and improve a stand’s longevity.
Aphanomyces root rot
Aphanomyces root rot has attracted little research effort in Ontario. Its distribution and economic impact have not been measured, and the consensus is that the disease is likely being underestimated, especially given the discovery of new Aphanomyces races in the U.S.
Since the major defence against Aphanomyces is the development of resistant varieties, the sooner the status of the disease in Canada is confirmed, the sooner breeders can decide whether to target this root rot in their programs.
What is known about Aphanomyces root rot is that, like brown root rot, it is most damaging in saturated soils. But where brown root rot affects the roots while leaving the above-ground growth appearing normal, Aphanomyces root rot attacks alfalfa seedings and can also affect mature alfalfa plants, reducing yield and vigour in established fields.
Aphanomyces leaves infected seedlings stunted yet upright, with yellow leaflets and cotyledons. The stems and roots can also turn grey and appear waterlogged or water-soaked. In severe cases, the seedlings can also appear light to dark brown. The finer, fibrous roots that are common with alfalfa plants will be missing with Aphanomyces root rot. Even the lateral roots can be rotted and absent.
Overall, the alfalfa stands can be thin, yellowed and weedy. On closer inspection, the seedlings may show reduced rhizobia nodulation. Often, the physical manifestations can be mistaken for a nitrogen deficiency, with poor vigour slowing regrowth. And since the root systems are compromised, infected stands will struggle in droughty weather.
Another factor that hinders the management issue is the survivability of the Aphanomyces euteiches pathogen. It can survive on infected plants, but also on plant debris. And the disease development is favourable, not just with saturated soils, but with moderate to high temperatures (16 to 30 C) and with humid or wet conditions. The disease is also prone to strike those fields that are poorly drained or compacted.
On the research front, Tenuta points to the efforts of Andre Levesque of Agriculture and Agri-Food Canada in Ottawa, who is working with the Ontario Forage Council to develop an effective soil test for the fungi that cause Aphanomyces and brown root rot. The work is being funded through the Farm Innovation Program and the Ontario Research and Development Program and is being administered by the Agricultural Adaptation Council.
“We’re looking at developing new molecular techniques, not only for these but for other alfalfa disease pathogens as well,” says Tenuta.
This new molecular identification tool will allow for quicker sampling of grower soil and plant samples, and help breeders identify lines with potential resistance.
The other good news is that breeding work to combat the disease is similar to that for breeding resistance to Phytophthora root rot in alfalfa. Researchers and breeders have identified Race 1 and Race 2 isolates of Aphanomyces, with resistance to Race 1 being the more common. Of the two, Race 2 is the more virulent with the least amount of resistance.
“What we’re seeing is that the forage varieties are progressing every year,” says Tenuta. “We’re finding more and more varieties out there, so I would think growers are tending to go with new varieties, and every time we go with newer varieties, there are always some new advantages that come along with those.” CG
