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Unusual green manures target wilt and nematode

Could a cover crop of mustard be an option to suppress nematodes?

Potato growers who are troubled by nematode or verticillium wilt could
get some help with suppression from unusual sources. Researchers in
Washington State are targetting these cropping challenges with arugula
and mustard with mixed results. But, could these solutions work in
Canada? It is possible the research could be applied in some areas if
the growing season is conducive.

“I have had some interesting results when using arugula to suppress
nematode, but not as good as I would have hoped,” admits Dr. Ekaterini
Riga, a nematode researcher at Washington State University in Prosser,
Washington. “We did use a field that had 500 times the number of potato
plant parasitic nematodes present, which would not be normally used for
potatoes, so in essence, we started with the worst possible scenario to
test our theory.” In another field, the infection was 10 to 50
nematodes per 250mL soil and the results were better.

There is zero tolerance for potato root knot nematodes in Washington,
so growers are faced with a huge challenge. Typically, they are using
Telone II for root knot nematode control. By adding arugula to the mix,
Riga was able to suppress plant parasitic nematodes, thereby reducing
the amount of Telone that was needed for total control. “Normally, a
grower would use 20 gallons per acre, but we were able to use half that
rate plus arugula to get the same control,” she says. “The nematodes
were reduced to zero with this method and the cost of control was
reduced by US$100 per acre.” She explains that the cost to grow the
arugula was factored into the cost comparison, so the savings was still
substantial. She also used Mocap in the same research with similar
results.

“When we started with 500 times the density of nematodes per acre and
then grew arugula plus a reduced application of Telone at $10 per acre
and Mocap at $2.00 per acre, we got complete control,” Riga reports.
“When we used Telone at full rate on its own, we did not get complete
suppression.”

The added value of growing arugula is the cash crop it can provide
prior to potatoes and the addition of green manure to the potato crop
when it is plowed under prior to planting. According to Riga, the roots
of arugula have strong nematicidal compounds, so growers could sell the
top and incorporate the roots. “The advantage of arugula is that you
can plant in early fall and still get a big biomass.”

“We are now looking at using arugula for suppression of other types of
nematodes,” continues Riga. “However, we suspect that arugula is a host
for lesion nematode, so it must be used carefully. Therefore, in this
case, it could not be used as a green manure, but only as a top crop.”
She says, in her research, arugula has proven to be an excellent
control of the Meloidogyne chitwoodi nematode which is not found in
Canada, but its close relative Meloidogyne hapla is, and the
suppression should be similar. She says arugula can stand some freezing
so it could work very well in some areas of Canada. She suggests it
would need to be planted from the beginning to middle of August in
Canada in order to get the full value from the crop as well as the
nematode suppression.

Riga has also worked with a range of mustards for nematode suppression,
but because some mustards can encourage nematodes, it would be
important to know what species of nematode is present in the field
before trying suppression using mustard.

A more promising use of mustard as a green manure is for suppression of
verticillium wilt and work is being undertaken on this front by
researchers from Washington State University Extension, led by Andy
McGuire. “Our target is verticillium wilt in potatoes with an effort to
replace metam sodium as a control,” he says. He adds that in 2002, his
research showed a potential savings of US$45 to US$65 per acre by using
mustard and reducing the use of metam sodium to control wilt. Still, he
says, mustard is not a cheap cover crop but is still less expensive
than a commercial fumigant. As well, the value of mustard as a green
manure is hard to quantify.

McGuire’s initial research was with Russet Norkotah, which is
susceptible to wilt, and in almost all cases, potato yields after the
mustard were equal to those that had been fumigated. Other varieties,
particularly those that have some resistance to wilt were also studied.
McGuire says that varieties with some resistance obviously show more
effective suppression when mustard is used as a green manure.

“We have not documented the exact mechanism that is at work to make the
mustard effective,” explains McGuire. “We also find that oriental
mustard varieties produce the best results.” He says that in almost all
their research cases, mustard suppressed verticillium wilt but there
were a few that were less effective. Researchers are at a loss to
explain why, at this time. In order to get a better idea of how mustard
works, the researchers are developing a soil test that would help them
predict wilt suppression.

While McGuire and Riga work with their respective crops to control
different pests, it is important that growers know what their
particular problem is before attempting to test either researcher’s
results. If the majority of the problem in a field is nematode, then
using mustard will not provide the same level of suppression that is
offered by arugula. The researchers caution growers to be aware of what
their problem is in a field before attempting any new method of
control, particularly if two problems exhibit similar symptoms. McGuire
says mustard, when used as a green manure and wilt suppressant, should
be integrated into a cropping system in order to reap the maximum
benefits. He adds that any use of green manure or bio-fumigants for
pest control cannot be used in isolation, but must be part of a whole
potato production system.

As well, Riga and McGuire state there are benefits from using arugula
and mustard that have not been measured, such as their value as cash
crops or for adding organic matter to the soil prior to a potato crop.
Green manure is not a new technology, but the techniques and varieties
of plants are giving it new life. While the research is being done in
Washington, there may be some aspects of it that will work for Canadian
potato producers in different parts of the country. The challenge is
learning, as the Washington experience shows, what plant variety and
method works best in each potato growing area.