Potatoes in Canada

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Resistance is better than tolerance

Verticillium wilt encourages the potato plant to die earlier, and researchers are studying how best to prevent the spread of the pathogen. Photo courtesy of Dr. Rick Peters, AAFC.

Savvy growers know that when a potato plant begins to die, harvest is close at hand. However, the presence of verticillium wilt in the field can cause early plant senescence and, with that, the concern that the tubers would not be mature enough to harvest, resulting in low yields and economic losses. Researchers at Lethbridge and Fredericton have been researching how verticillium wilt induces does not kill the plant or damage the tubers, it merely encourages it to die earlier. This can mess with production schedules and allow the verticillium pathogen to enter the soil.

“Early dying can result in a 30 per cent yield reduction, so it is not desired. The plant dies early in response to the pathogens Verticillium dahliae and albo-atrum that cause verticillium wilt,” explains Dr. Helen Tai of the Potato Research Centre in Fredericton. “Verticillium wilt is a soil-borne disease, and when the plant dies the pathogen continues to proliferate on the dead tissue and make its way to the soil.”

The researchers began studying how resistant and tolerant plants responded to the pathogen and how each affected the plant’s maturity and the incidence of verticillium in the soil. What they learned was that resistant and tolerant plants appeared the same with infection. However, resistant plants protected the soil from verticillium whereas tolerant plants were infected at a high level, which results in high levels of the pathogen entering the soil.

“If the plant can tolerate high levels of the pathogen, you can get more verticillium in the soil,” Tai explains. “Plants that resist the pathogen keep the amount low and they won’t respond by dying early.”

“Resistance prevents proliferation of the pathogen, whereas tolerance actually increases the levels of the pathogen,” adds Dr. Lawrence Kawchuk, the lead researcher at Agriculture and Agri-Food Canada in Lethbridge, Alta. Kawchuk discovered the genetic marker that indicates resistance making it possible for plant breeders to breed for resistance more efficiently.

“The resistance is being transferred by various breeding programs and the new varieties should be available in a few years,” Kawchuk continues.

The differences between tolerance and resistance prove the value of using genetic markers to identify resistance, Tai explains. There is also a difference between the genetic makeup of verticillium strains, so it is important to breed resistance to the strain that causes the most problems.

“Verticillium has varying levels of infection geographically and there are genetic variations within verticillium species,” Tai explains. Therefore, researchers are also working to identify the geographic variances and to develop potato plants with resistance to different verticillium strains.

Once resistant varieties are available, growers can ensure the resistance can remain effective longer by following good cultural practices. “Continue traditional management practices, such as recommended rotations and monitoring the crop for verticillium wilt,” Kawchuk suggests.

“Use rotation crops to reduce verticillium in the soil and grow the resistant varieties that are available now, if possible,” Tai adds.

Meanwhile, as breeders work to transfer the gene discovered by Kawchuk into commercial cultivars to give growers better protection from verticillium wilt, growers must continue using cultural practices to minimize the increase of the pathogen in the soil. Kawchuk and his colleagues currently have four clones in which the verticillium resistant gene was identified, but he says it will be a few years before a commercially viable variety will be available for growers to plant.

Another concern raised is the probability of the verticillium pathogen developing tolerance to the potato resistance gene, allowing a stronger strain of the pathogen to become prevalent. Kawchuk suggests breeding resistance into a plant is not the same as control by a pesticide in which the disease can develop tolerance to the product. “Tomato breeders used verticillium resistance for 50 years before a new race emerged that could overcome the resistance,” he says. “This would indicate that verticillium resistance should be effective in potato for many decades.”

Verticillium is such a large problem for potato growers that other means of control, such as soil fumigation, are continuing to be studied. However, the development of resistant, rather than tolerant, potato varieties may prove to be the most effective way to minimize the effects of the pathogen on the plant and in the soil.

November 25, 2014
By Rosalie I. Tennison


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