Features Agronomy Insect Pests
Green manures for disease control

Interest in the use of green manures to control plant disease is
growing, particularly for controlling soilborne potato diseases.
Growers have been looking to the successes from other areas, including
the Pacific Northwest, and are interested in how to successfully
implement this practice to control soilborne potato diseases on their
farms.

Dr. Mario Tenuta, Canada Research Chair in Applied Soil Ecology at the
University of Manitoba, with Drs. Fouad Daayf and Abdel El Hadrami and
graduate student Oscar Molina, have initiated a project to look at the
potential of green manures in Manitoba and beneficial impact on
controlling early dying of potato and improved soil health. “Our team
has a lot of experience with other types of organic soil amendments and
have expanded our research to include green manures, both in the field
and in the laboratory,” says Tenuta. “Together, this research is
tackling the question of finding alternatives to pesticides in soil to
control plant diseases.”

 One of the main soilborne diseases causing yield losses and economic
impacts for Manitoba potato farmers is verticillium wilt, part of the
complex of early dying disease. “Verticillium wilt is our target, and
is a very difficult disease to control because it is very long lived in
the soil and has multiple alternate hosts,” explains Tenuta. “In
Manitoba, we have very high levels of verticillium wilt in many fields,
and some fields continue to succumb to early dying.”

As part of their research program, the researchers want to determine
the importance of verticillium wilt in terms of yield reduction in
potatoes in Manitoba.

The research project started in 2006 with field trials established at
three locations, one at Canada-Manitoba Crop Diversification Centre
(CMCDC) at Carberry and two other sites in farmers’ fields using
various organic amendments and green manure treatments. “The CMCDC site
is a super site that includes 12 different treatments,” says Tenuta.
The trials followed a three-year rotation, with a common cereal crop
seeded in the first year. The second year included the application of
organic amendments or seeding green manure crops, followed by a potato
crop in 2008. Two treatments included perennial crops seeded in the
first year (alfalfa, sorghum sudan), continued in the second year, and
then plowed down and seeded to potatoes in year three.

Tenuta explains the green manure and soil organic amendment treatments
were designed to address different approaches to soilborne disease
control. Some treatments were designed to be toxic to the pathogens,
which were expected to kill the verticillium wilt pathogen directly.
Other treatments were designed to protect the plants against the
verticillium wilt pathogen. For example, sorghum sudan has been shown
elsewhere to fool the verticillium wilt into germinating, but does not
allow it to infect the plant. The researchers were also interested in
the impact of the various treatments on soil properties and soil
health, such as organic matter, nutrient availability, soil structure,
moisture holding capacity and biological activity. 

The green manure crop treatments included: oriental mustard, white
mustard, Canada milkvetch, fall rye, oat-pea mixture, spring wheat and
a wheat-animal compost mixture. One other treatment included a wheat
crop with a fall Vapam fumigant application. “Vapam is not commonly
used in western Canada, but is regularly used on high value crops
elsewhere,” explains Tenuta. “Growers were interested in finding out if
Vapam was an option here and we wanted to know the consequence of
dramatically lowering verticillium levels and its use on disease in
coming years.

“We added mustard seed meal as a new treatment in 2008 to a plot that
had been seeded to cereals in the first two years,” says Tenuta.
Mustard meal is a result of an oil crushing process, and the remaining
meal product can be used for food processing. “In this case we applied
the mustard meal early in the spring followed by incorporation about
two weeks before the potatoes were seeded.”

The compost treatment also received another application in 2008, prior to seeding potatoes.

The green manure crops were seeded in late spring or early summer, and
then plowed down later in the year. Mustard crops are allowed to grow
to their peak flowering stage, and then either chopped, flailed or
rolled before being plowed under. Soil moisture should also be
adequate; otherwise the green manure will not work as well. The discing
operation is important for incorporating the green manure to a depth of
five to 10 centimetres (two to four inches), and providing a second
extra chopping of the plant material.

The perennial crops can be harvested and then plowed down in late
September. “The Canada milkvetch should be treated the same way,” says
Tenuta. “Canada milkvetch is very experimental and something unique to
the University of Manitoba. We did find it more difficult to get
established, and the slow establishment meant more potential weed
problems, but by the end of July it was doing well.” The researchers
are also experimenting with other potential uses of Canada milkvetch,
such as an extract that may provide an alternative to seed treatment. 

Preliminary results show promise
The potato crop was harvested in late September 2008, but the final
grades and yield information are not available to date. “Our
preliminary results indicate we are definitely getting responses from
some of the treatments, although they are varied,” explains Tenuta. “In
some cases, verticillium wilt pathogens are being killed directly, in
others the pathogen levels are lowered, but this doesn’t necessarily
relate to an increase in yield or quality.”

The question remains whether or not the overall yield is impacted by
the various treatments. “We will be analyzing the results during the
fall and winter, as well as continuing the lab work to refine our
experiments and to determine what is happening in the field,” explains
Tenuta. “Some of the treatments that seem to be the most interesting
are the mustard green manure and seed meal treatments that definitely
knock down the verticillium wilt populations in the soil. The mustards
don’t eradicate verticillium wilt, but they lower the populations.”

Mustard meal also lowers the verticillium wilt populations and more
effectively than the mustard green manure treatments. Some of the other
green manure treatments, such as alfalfa and Canada milkvetch, did not
lower the verticillium wilt pathogen, but the plants were healthier and
yields were higher. The compost treatment also showed similar results,
with preliminary examination indicating the greatest increase in yield.
“Once we have completed our analysis, we expect to have better answers
for growers,” says Tenuta. “We believe there is the potential for the
use of green manures and soil organic amendments in any crop rotation.
With higher value crops, there is potentially more leeway to cover
added labour or other costs by using green manures and soil organic
amendments.”

The key for growers before deciding to use green manures or soil
organic amendments is to know the diseases and soil conditions to
carefully identify what the problem is. “Likely the disease and soil
conditions will design the objectives of a green manure or organic
amendment application and help determine which of the various
approaches and different mechanisms will work the best for that
situation.”