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Healthy Soil Regimen Reduces Need for Chemical Attack

For more than 30 years, potato growers have managed the chronic disease called potato early dying (or potato early die) with a chemical attack that ranges from relatively mild preventatives to complete fumigation.

“Even so, PED (potato early die) continues to be a persistent problem that continues to erode the economics of growing potatoes. Growers need alternatives to fumigation, but the solution is not likely to be in the form of another chemical.”

That is the assessment of Ann MacGuidwin, the nematologist at the University of Wisconsin. She shared the podium at the Great Lakes Fruit, Vegetable and Farm Market EXPO with George Bird, a Michigan State University nematologist.

The two agree that, in the future, the chemical approach will be shared by – possibly totally replaced by – a regimen designed to improve soil quality and reduce PED to inconsequential levels.

When will that future arrive? Growers can start now to implement new practices, especially on fields that don’t have a history of the disease and where prevention would pay the greatest dividends.

“PED is caused by an interaction between the root lesion nematode Pratylenchus penetrans and the fungus Verticillium dahliae,” Bird said.

There is consensus that the fungal pathogen is primarily responsible for the characteristic symptoms of wilting, dieback of foliage and premature senescence of the plant, MacGuidwin said. But the interaction is complicated.

In her studies, she found that in Wisconsin, potatoes will lose 10 percent to 15 percent of their yield when the initial inoculum of the fungus exceeds 10 propagules per gram or 200 root lesion nematodes (RLN) are present per 100 cubic centimeters of soil. But when these pathogens occur together, the disease threshold falls to 3 propagules and 75 RLN.

Most potato soils in Wisconsin exceed these thresholds and some 80 percent of potato ground is fumigated each year with metham sodium, she said. Fumigation is effective because it reduces levels of both pathogens, but it is expensive and reduces the profitability of growing potatoes. About half of Michigan’s potato acreage is “seriously challenged” by PED each year, Bird said, and growers have a long history of using fumigation or chemigation.

New Approaches

The nematologists, not surprisingly, have concentrated their attention on PED control on the nematode side of the equation. They believe a major part of the problem is a messed-up nematode ecology.

As Bird explains it, the root lesion nematode is a plant feeder that attacks potato roots and stolons. Normally, soil contains lots of different types of nematodes, but the coarse, sandy, low-organic-matter soils in which potatoes are grown doesn’t give most of these species much to live on. So when potatoes are planted, only the RLN feasts.

Both his and MacGuidwin’s studies show that soils higher in organic matter are less likely to lead to PED – because there are more nematodes of more kinds and some of them actually feed on RLN.

A number of practices hurt or improve the health of the nematode community:

Tillage is disruptive. Tillage oxidizes and degrades organic matter and clears the slate of critters that live in the top layers of the soil or in the litter on top of it. Bird has shown that potatoes grown with reduced tillage yield better. Practical ways of growing potatoes without tillage have not been developed.

In Bird’s studies, “It appears that risk of PED and RLN is less with chisel tillage than with conventional moldboard plow.”

In a comparison conducted from 2001 to 2004, potatoes grown with chisel tillage outyielded those grown with moldboard plowing by 299 to 236 cwt. per acre, a 27 percent improvement.

Adding organic matter increases the diversity of the nematode community. Organic matter can be added through cover crops such as rye, other cereals or legumes, manure, or by the addition of compost. Bird has shown that compost greatly increases nematode diversity and adds a stable kind of organic matter that has longer soil effects than cover crops.

Resistant varieties play a role. In 1998, Bird said, MSU potato breeder Dave Douches began evaluating varieties for PED resistance. While several more tolerant lines were found, the most resistant strain was heavily infested with scab.

“It is now believed that the severe level of a scab-producing actinomycete associated with the line is responsible for the absence of PED,” Bird said.

What that means is another avenue for research.

Crop rotation helps break the PED cycle.“With few exceptions, the highest tuber yields and least amount of PED were associated with potato crops following two years of alfalfa,” Bird said of research done 15 years ago. “It was also learned that when corn and wheat were removed from the system, risk of crop loss caused by the northern root knot nematode increases significantly.”

Certain cover crops function like fumigants. Bird warns growers to be careful about their choices because some cover crops encourage plant parasitic nematodes rather than suppress them. But research is under way to determine which cover crops and rotation crops work best with potatoes. Brassicas, mustards and radishes, which produce glucosinolates when they degrade, are leading candidates as combination cover crops/biofumigants.

“We’ve conducted large-scale, on-farm experiments to evaluate different cover crops for suppression of PED pathogens, disease symptoms and potato yield,” MacGuidwin said. “The effect of the cover crop treatments on PED pathogens was not always predictive of potato yields the following year. Sorghum-sudangrass, Canadian forage pearl millet and African marigold performed best in terms of yield. Dwarf Essex rape decreased soil inoculum levels of V. dahliae, and rape, marigold and Canadian forager pearl millet reduced population densities of P. penetrans.”

Solarization combined with cover crops had an effect. In one study, soil was covered with polyethylene after cover crops were incorporated, and pathogen levels were reduced and yields improved.

Site-specific management has application. When fields were broken into grids and sampled, it was found that the levels of the two pathogens and their ratio to each other varied widely, but where the two were highest, PED symptoms were worse.

“The patches remained relatively constant over time,” MacGuidwin said. “The data suggest that aiming tactics at only portions of the field is feasible, and that the size of the management unit acceptable to the grower is the most important consideration.”

Soil Quality is Key

“Looking back over the last 30 years, I believe that it was a mistake to separate Michigan potato research into its university disciplines,” said Bird, who’s been at it a long time and plans to retire at the end of 2006. “As all growers are well aware, everything in a potato production system impacts all other parts of the system.”

The type of research that will defeat PED is integrated, he said.

“My goal is to continue to provide the Michigan potato industry with chemical, biological, cultural and genetic PED controls. I believe, however, that PED can be eliminated through enhancement of soil quality and plan to make this a major component of my MSU research activities.”

For the future, he recommends growers take this approach:

Understand the PED risk for each field. That means analyzing records and testing soil and root tissue for nematodes and the fungal pathogen.

Prevention is the best strategy. “Once seed pieces have been planted in a site with a high PED risk, it is almost impossible to control the problem,” Bird said.

Use varieties with a history of yielding well in PED sites.

Nematicides may be needed on sites where nematodes or the fungal pathogen are high.

“The key to elimination of risk to early-die is currently believed to be soil organic matter,” he said. “Both the quality and the quantity of SOM is important.” Soils with high PED risk are almost always degraded or of poor quality.

Nematode community structure (the diversity of types of nematodes) can be used as an indicator of soil quality. Diagnostic services at MSU can be used to assess that structure.

“Understanding the soil as a habitat for V. dahliae and P. penetrans and making it less hospitable to pathogen populations should be an urgent research priority,” MacGuidwin said.

Originally posted Saturday, Apr. 7, 2007

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