Features Agronomy Diseases
Grazing corn expansion poses fusarium risk

In the absence of western Canadian research on the impact of grazing corn on
Fusarium graminearum and fusarium head blight (FHB) infestation and spread,
plant pathologists say that farmers in western Saskatchewan and Alberta should
err on the side of caution when growing corn for silage or extended winter grazing.
Not that it cannot be done, but rather it should be done as part of an overall
FHB management plan. They should also try to minimize the development of other
disease issues specifically affecting corn.

"From a pathologist's point of view, continuous production of the same
crop or short intervals of only one to two years between host crops is not a
good situation," explains Kelly Turkington, a plant pathologist with Agriculture
and Agri-Food Canada at Lacombe, Alberta. "From a producer's point of view,
using grazing corn may seem like a good alternative, but if corn is grown continuously
or is used in a short rotation, it can start to increase the possibility of
disease. The rotation has to be changed up as much as possible to help limit
disease risk."

Corn is a host of F. graminearum although the crop does not suffer as
much yield loss as other small grain cereals. Turkington says that in 2003,
the last year that residue from corn fields were surveyed in Alberta, all of
the 13 surveyed silage corn fields in southern Alberta had F. graminearum
with at least four to 60 percent of the corn stalks infected. The news is better,
though, in central and eastern Alberta, where the bulk of grazing corn is grown.
In the 19 and 31 fields surveyed in 2002 and 2003, respectively, F. graminearum
was not detected in corn – in areas from Innisfail, up to Edmonton, west
to Rimbey and east to Viking.

While F. graminearum appears to be building in southern Alberta, it
is more common in the southeastern corner of Saskatchewan, specifically Crop
Districts 1, 2 and 5. In these crop districts, significant levels of fusarium
damaged kernels (FDK) and increasing levels of F. graminearum have been
found. In Manitoba, F. graminearum, of course, is well established.

Corn can be a host
Turkington says that research and experience in other parts of North America
and Europe shows that the development and expansion of F. graminearum
in areas that primarily grow small grains cereals has been accelerated by including
corn in the rotation. The concern with corn is the amount of infested residue
that can remain in the field, especially with grain corn, which can lead to
greater production of infective spores.

In Manitoba, provincial plant pathologist David Kaminski, with Manitoba Agriculture,
Food and Rural Initiatives (MAFRI) at Carman, says that including corn in the
rotation may extend the risk of infection from F. graminearum. "We
know that with small grain cereals like wheat and barley, when rotating with
field corn, there is a high incidence of infection."

Much research supports the notion that adding grazing corn to the rotation
must be managed carefully. Turkington says information outlined in the Compendium
of Barley Diseases, published by the American Phytopathological Society,
reported that barley production during the first half of the 1900s was more
or less eliminated when corn was grown in rotation with barley in the eastern
and central corn belt of the US because the level of FHB became so severe.

In Ontario, research in 1984 and 1985 found that FHB levels were higher in
wheat when it was sown after corn. Agriculture and Agri-Food Canada researchers
found that wheat after corn had substantially more FHB; 349 bleached heads per
1000 heads compared with 0.3 blighted heads per 1000 heads when grown after
other crops, and deoxynivalenol (DON) levels of 0.657mg DON/g of grain compared
with 0.054mg DON/g, respectively.

More recently, Art Schaafsma, a professor in the Department of Plant Agriculture
at the University of Guelph at Ridgetown College, looked at a survey of grain
from commercial fields in Ontario under a range of agronomic practices. He found
that in two of four years, DON levels in wheat were higher when planted on corn
residue compared to wheat residue or soybean residue. He also recently published
a paper that showed higher levels of DON in some years in wheat after corn,
relative to wheat after non-host crops.

"However, I am not convinced of the biological significance. Biologically,
variety and environment are the overwhelming overriding factors in epidemics,"
says Schaafsma. He notes, though, that his research applies to areas where F.
graminearum is well established.

Schaafsma is also looking at research that shows F. graminearum takes
advantage of its saprophytic nature – the ability to feed off of dead plant
material. In silage and seed corn in Ontario, as the crop dies off in late summer,
the fungus colonizes the stalks from soil infections. He says that because the
stalk and root masses are so large relative to other types of crop residue,
they become excellent reservoirs for the inoculum.

In a Minnesota trial conducted by Dr. Ruth Dill-Macky, levels of FHB and DON
were higher when wheat followed corn, and were lowest when wheat followed soybean,
and intermediate when wheat followed wheat.

And in Manitoba, the first report of significant levels of FHB and mycotoxin
contamination in wheat, caused by F. graminearum, was associated with
two fields that were previously cropped with corn. Indeed, FHB incidence has
increased substantially in the Red River Valley, northwest Minnesota and eastern
North Dakota as corn acreage expanded.

Albert Tenuta, field crop plant pathologist with Ontario Ministry of Agriculture
and Food at Ridgetown, also says the risk of wheat after corn is too high to
contemplate. "Wheat after corn is at a far higher risk of fusarium than
wheat after virtually any other crop. Incredibly, wheat after corn is likely
at higher risk than wheat after wheat."

Residue plays a role in transmission
Kaminski says that fusarium infects the ears and stalks in corn. Ear rot is
often the result of an insect attack or feeding on the ear, which opens up the
cobs for infection by fusarium spores. Stalk rot develops when fusarium spores,
which are produced in fruiting structures present on overwintering crop residues,
are dispersed by wind, rain or insects onto leaves and stalks of growing corn
plants. Yield losses of 10 to 20 percent can occur from poorly filled ears,
or harvest losses due to lodging. The fungus surviving on remaining corn residue,
then, can infect subsequent crops.

"F. graminearum grows very well on grass and dead plant material.
It can help break down residue into organic matter. But the negative aspect
of that is it can also produce more spores for infection," says Kaminski.

Turkington says the big question regarding standing grazing corn's impact on
subsequent crops may depend on the amount of residue that is left in the field
once the cows are done grazing. He says research at AAFC Brandon showed that
F. graminearum did not survive the trip through a cow's digestive system,
so if stalk rot was present in the grazing corn, it may not be a concern if
consumed by the cow. However, he says that if lessons from silage corn are considered,
there will likely still be enough residues to carry the disease forward.

"Silage corn might reduce the amount of residue left in the field, but
it won't eliminate it," explains Turkington. "And in grazing corn,
if the cow grazes the corn down to the stalk, it may reduce the risk but it
also won't eliminate it. Trampling may also increase the amount of residue left
in the field and the risk."

Tenuta agrees that the level of residue left in the field does not have to
be large. "Silage corn residues may be less than commercial hybrid corn,
but there is still plenty of residue remaining for inoculum production,"
he says. "Over the past few years, some of the highest fusarium levels
were on wheat following silage corn."

Silage corn that is left in the field too long for harvesting can also increase
the risk of mycotoxin contamination in the field, a concern for dairy farmers
in areas where F. graminearum is well established. Earlier harvesting,
especially before a frost, can help keep DON levels lower.

Rotate corn with other crops
In southern Alberta, where F. graminearum is becoming more common, Turkington
says a crop rotation away from cereals to non-hosts, such as canola, pulses
and forage legumes should be considered for at least two years. This practice
will allow enough time for infested residue to decompose before the next cereal
crop is seeded. However, he says it is unlikely to completely eliminate the
risk of FHB caused by F. graminearum, considering the potential of the
pathogen to move from field to field via airborne ascospores.

"Irrigation management will also be a critical disease management tool
for irrigated cereal producers," says Turkington. "Too much irrigation
during late June through to early August can substantially increase the risk
of FHB for producers should F. graminearum be commonly found in infested
residues in their fields or adjacent irrigated fields.

"I know that growers can get pushed into tight rotations for silage or
grazing corn because of feed requirements and the need for the field to be close
to the feedlot, but the risk does increase," explains Turkington. "I
would be more concerned with some of the tight rotations I see in central Alberta
if the pathogen was to become more established."

A wheat-canola-wheat rotation is quite common in central Alberta, and would
likely enhance the build-up of F. graminearum if it was already present
or was introduced via infected seed or straw. Moreover, Turkington says that
because F. graminearum is a wind-dispersed pathogen, over the long-term
spores travelling from field to field may eventually spread the pathogen to
areas where it is not commonly found.

In Manitoba, Kaminski says that growers are using a number of tools to try
to minimize the impact of F. graminearum on their crops. Rotation is
one of the tools and a break of at least one, and preferably two, years is advised
between cereal, grass and corn production. As well, more resistant cereal crops
are recommended in areas with F. graminearum infestations – durum
wheat is the most susceptible, followed by hard red spring wheat, barley and
oats.

Staggered planting dates, growing winter cereals that can avoid infection and
using fungicides are additional tools. Kaminski says that for 2006, hard red
spring wheat growers will have another tool, a newly registered variety with
good resistance to FHB.

"I guess in 10 to 15 years, we will be able to look back and see how good
a job we did in trying to hold back fusarium in the rest of Alberta. Five or
six years ago, F. graminearum was rarely found in southern Alberta. Although
not yet present in all fields in that area, in the last two to three years it
has become easier to find fields with F. graminearum on the residue and
grain," says Turkington.

"Factors related to the increasing appearance and impact of F. graminearum
in southern Alberta include tight rotations with highly susceptible cereal types,
excessive irrigation during the summer and perhaps increased corn production.
I hope the message gets out that we have to do everything we can to hold off
FHB as long as we can." -30-