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Soybean viruses – an emerging threat?

Soybean viruses are an emerging threat to soybean production in Ontario. Researchers
and Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) extension
personnel are seeing increased levels of viral infection in Ontario soybeans,
which has prompted increased monitoring and research efforts to prevent viruses
from becoming a major production-limiting problem.

Research in Ontario is focussed on four viruses that infect soybeans: alfalfa
mosaic virus, bean pod mottle virus, soybean mosaic virus and tobacco ringspot
virus. With recent increases in insect vectors such as soybean aphids and bean
leaf beetles, researchers have seen an increase in virus-like symptoms in soybean
fields. These viruses can induce a broad range of symptoms, and cause yield
loss and seed quality deterioration. Deteriorated seed quality due to discolouration
is a concern for food grade soybean growers, as it can cause grade discounts
and lost premiums.

Viral infections not only impact yield and quality, they can also decrease
overall plant health and the ability to handle stress. Unfortunately, these
virus-like symptoms can be confused with other common agronomic problems in
fields such as herbicide injury. Confirmation of viral infection cannot be determined
visually and must be done through a laboratory test. This adds to the difficulty
in diagnosing virus problems in the field.

To address grower concerns that viruses are becoming more common in Ontario,
researchers have conducted field surveys in southern Ontario to measure the
presence and level of soybean viruses in commercial soybean fields and breeder
nurseries.

In 2005, distribution of the four soybean viruses – alfalfa mosaic virus,
bean pod mottle virus, soybean mosaic virus and tobacco ringspot virus –
were monitored in 222 southern Ontario soybean fields by OMAFRA extension agents,
Albert Tenuta and Tracey Baute. Dr. Roberto Michelutti from Agriculture and
Agri-Food Canada's laboratory in Harrow conducted the virus identification using
ELISA laboratory analysis.

Both commercial fields and breeder nurseries were tested for all four viruses.
The predominant virus identified was soybean mosaic virus, which can be transmitted
by the soybean aphid and other sucking insects. In 2005, 28 percent of plants
tested from commercial fields and 83 percent of plants from breeder nurseries
were infected with soybean mosaic virus, significantly higher levels than found
in previous years (see Table 1). Ten percent of commercial fields and 33 percent
of the nurseries tested had alfalfa mosaic virus, which is interesting to note
since it has only been recently identified in Ontario soybean production. Consistently
higher levels of viral infection have been found in breeder nurseries compared
to commercial fields, which currently puzzles researchers.

Researchers believe solutions to managing soybean viruses will come through
incorporating genetic resistance into new soybean varieties. Several major genes
have been identified that convey resistance to viruses. Dr. Aiming Wang at Agriculture
and Agri-Food Canada's London, Ontario, research station is working to evaluate
levels of genetic resistance to soybean mosaic virus in varieties grown in Ontario.
In addition, new genetic sources of resistance are being evaluated for future
use in variety development.

Control options
What can growers do to manage soybean viruses? Control options for viruses are
limited once a field is infected. Currently, there are no direct control measures
for soybean viruses other than the use of resistant varieties (which are limited).
If you have a history of virus problems, check with your seed supplier. Since
viruses can be seedborne, planting clean, virus free seed is important to minimize
the impact of viral disease. Controlling insects, such as soybean aphids and
bean leaf beetles, when they are above threshold levels will reduce the risk
of infection. Visual detection of viral infection is often difficult and can
be confused with herbicide injury. Symptoms on the vegetative plant usually
appear as puckered and bumpy leaves. In the seed, virus infection shows up as
mottling and discolouration of the seed coat.

Genetic resistance is a desirable solution to soybean viruses, but it takes
considerable time and effort to incorporate genes into new varieties and progress
in other important traits may slow down as a result. Preventing the spread of
viruses through planting of virus free seed is likely the most effective control
strategy available to prevent soybean viruses at the current time. -30-

*Crosby Devitt is a research manager with Ontario
Soybean Growers. Research referred to in this story was partially funded by
the Ontario Soybean Growers' license fee investment in research. A complete
list of Ontario Soybean GrowersÕ research investments can be viewed at www.soybean.on.ca