New Weapon against Verticillium

[deck]Researchers are working on developing a viable, commercial test for detecting in the soil both the type and the amount of the fungus that’s known to contribute to Potato Early Dying complex.[/deck]

Potato producers have wrestled with Verticillium wilt for decades. Because some of the Verticillium pathogens can stay present in soil for 10 to 15 years, it’s a long-term problem that can be notoriously difficult to control, never mind test for.

A new tool for helping growers successfully combat the pathogen that plays a role in potato early dying (PED) complex could be available within a couple of years, thanks to the work of Canadian Verticillium researchers.

Mario Tenuta, Canada Research Chair in Applied Soil Ecology at the University of Manitoba, is the lead researcher for a Verticillium wilt study that’s part of a five-year research project into potato diseases being funded by producers, industry partners and the Agriculture and Agri-Food Canada Growing Forward 2 program.

It’s not a fun pathogen to work with, in that it resides in soil for a long time. It’s really a soil issue, and it takes fortitude to work with it because to analyze soil for Verticillium is painstaking and fraught with error.

– Mario Tenuta

Tenuta’s top priority is to develop a viable, commercial test for detecting the Verticillium fungus in soil. While some tests already exist, they are expensive, take a long time to conduct and are largely unreliable, Tenuta says. The main problem with most existing tests, he adds, is they are not specific to the Verticillium species that attacks potatoes — Verticillium dahliae and Verticillium albo-atrum.

Tenuta’s study is using DNA to determine the exact species of Verticillum — and how much of it there is — in each soil sample being studied. The end result, it’s hoped, will be a brand-new tool for helping growers more effectively combat this enigmatic disease.

Unlike other potato diseases, Verticillium wilt is not one that comes with a ready fix or a well-known remedy. The fungus enters through the roots and rises upwards through the vascular system that conducts water and dissolved nutrients in the plant. It can plug up the vascular system, preventing the potato plant from getting the water it needs and causing it wither and die before it normally would.

The exact nature of PED is a real mystery, and one that hasn’t been extensively studied, notes Tenuta. “With PED, it’s not like the potato plant dies extremely early and you have no yield — it dies a bit earlier and growers are often not sure why,” he says. “The disease isn’t decimating the crop per se, but robs it of yield, especially the late tuber bulking phase. If it was easier to control and easy to identify the effect on yield, you’d see more research on it.”

Control Measures

The only known effective control measure for Verticillium, according to research scientist Gefu Wang-Pruski of Dalhousie University in Truro, N.S., is soil fumigation using chemicals like chloropicrin. In parts of the country where the practice is permitted, it can be effective, she says.

Wang-Pruski notes that crop rotation often isn’t an effective option for controlling Verticillium, and can sometimes make things worse depending on the rotation crops used. She’s been studying the problem for several years, and gave a talk at the International Potato Technology Expo in Charlottetown in February where she advised growers on how best to tackle the fungus.

“We don’t know a lot about Verticillium. Based on the DNA analyzes my lab conducted, some fields have symptoms one year and in other years they don’t,” Wang-Pruski says. “We don’t know how to control it — the population in a certain field could be at one level this year and another next year, and we don’t know why. Is it because of the cold winter? Moisture in the soil? Rotation crops? We’re trying to figure out the real factors.”

Wang-Pruski’s lab has surveyed many agricultural lands for two years in three Maritime Provinces. She noted that the field plots that are fumigated have much less the pathogen than those that have not been fumigated for several seasons. Also, the pathogen species and the quantity vary significantly from field to field, based on the species-specific quantitative PCR technology developed in her lab.

That lack of knowledge about Verticillium is a major stumbling block for researchers, Tenuta says. He notes that even exact figures showing the cost of Verticillium wilt to the Canadian potato industry are virtually non-existent.

“It’s not a fun pathogen to work with, in that it resides in soil for a long time. It’s really a soil issue, and it takes fortitude to work with it because to analyze soil for Verticillium is painstaking and fraught with error.”

Thanks to some recent breakthroughs in technology, researchers are now able to accurately detect the fungus in soil. In the past, researchers weren’t able to isolate individual species of Verticillium. The fact they’re now able to do so is a huge step forward, and makes Tenuta’s test possible.

“We now have really precise tools to know what kind of pathogen is in the soil that cause early dying,” Wang-Pruski says. “We have a larger picture and knowledge about its distribution.”

Tenuta’s research may be the breakthrough producers have waited for to determine what kind of species— and exactly how much of each— Verticillium is in their fields. Knowing both of those factors is crucial to battling the problem.

Tenuta’s team including PhD student, Oscar Molina, has been collecting soil samples from both Eastern and Western Canada. They’ve discovered that several different kinds of Verticillium are present in Canadian potato fields — and not all of them are harmful to potatoes.

Early dying is not only caused by Verticillium — it’s a combination of many factors. They have to deal with the problem as a whole.

– Gefu Wang-Pruski

In addition to Verticillum dahliae, the primary causal agent in PED, Molina found Verticillium tricorpus, known to affect tomato in some parts of the world. They’re not sure what, if any, effect it might have on Canadian potatoes. So far, the researchers haven’t been able to get it to cause disease in potato.

In 2014, Verticillium longisporum was found in a Manitoba field. This species attacks canola, and doesn’t appear to have any effect on potatoes.

“Traditional testing methods overestimate the Verticillium that attacks potato, because the test can’t differentiate between different species of Verticillium,” Tenuta says. He’s confident his test will be able to do that.

Once growers know if the Verticillium present in their soil is the kind that attacks potato, Tenuta wants them to be able to know in exactly what amount it is present.

“Simply showing a grower that they have Verticillium in their soil might not mean much. The levels might be very low, in which case they likely won’t need to intervene such as with metam sodium fumigation in Manitoba, and it might not be the kind of Verticillium that harms potatoes,” he says.

Tenuta is in the process of testing New Brunswick soils, and is hopeful a suitable commercial test might be available in two years’ time.

“We have quite a bit more work to do. Growers aren’t just going to willy-nilly start trying something unless it works and is accurate, and we can give them the, ‘So what? What does this mean?” he notes.

“For growers to be willing to invest their money in trying a new testing method, they have to have great confidence in that test.”

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Profile of a Pathogen

According to Agriculture and Agri-Food Canada research, Verticillium wilt can be spread by the movement of infested soil by wind or mechanical means. The disease can be introduced into new fields through the use of infected seed potatoes. Verticillium infects young plants through developing roots. Wounds are not required for infection.

There is an increase in disease incidence and severity when Verticillium and root lesion (Pratylenchus penetrans) nematodes occur in the soil together. The fungus establishes itself in the vascular tissue in plants that conducts water and dissolved nutrients upward from the root, infecting stems, petioles and leaves. Verticillium can infect tubers, but it does not spread easily or cause significant harm in storage.

The disease seems to become more of a problem when crop rotations are short, AAFC research shows. Crop rotations should be at least five years, according to a recent report by the Pesticide Risk Reduction Program at the AFFC Pest Management Centre.

The research shows that rotation crops such as red clover, sunflower and alfalfa should be avoided, while crops such as corn, onions and peas are beneficial for combatting PED. But Gefu Wang-Pruski, a Verticillium researcher at Dalhousie University, cautions there is no one-size-fits-all management strategy for verticillium wilt.

“We have yet to find a more precise way to control the pathogen,” she says. Wang-Pruski recommends growers talk with their agronomists if they think their potatoes are suffering from verticillium wilt-related early dying.

“Early dying is not only caused by Verticillium — it’s a combination of many factors. They have to deal with the problem as a whole,” she adds.

 The Signs and Cost of Early Dying

Potato early dying (PED) syndrome is characterized by a general decline of plants four to six weeks earlier than normal maturity. Although specific, diagnostic symptoms are not associated with the disease, foliage shows various degrees of chlorosis and necrosis, sometimes associated with wilting or dying of individual leaflets or stems.

In the early stages, individual vines may die and remain conspicuously erect in contrast to healthy plants. A light brown vascular discoloration in basal stem tissues is usually present in symptomatic plants. This symptom, however, is not a sure-fire sign of PED since vascular discoloration can result from stress factors unrelated to the syndrome.

As the disease develops within a field, widely scattered individual plants or groups of plants often show early symptoms, resulting in a non-uniform decline of the stand. In severe cases, plants across an entire field will die over a period of several weeks.

When PED develops throughout a field midway through the growing season and then becomes severe during the period of maximum tuber bulking, a reduction in tuber size and total marketable yield can result. In North America, yield reduction in moderately diseased fields can easily be 10 to 15 per cent.

The economic impact of PED across the potato industry is significant because of the direct losses resulting from low yields and because expensive, pre-plant soil fumigation has become a routine disease management practice, particularly in irrigated production areas.

Source: American Phytopathological Society.

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