Alfalfa mosaic virus added to the list.
By Ralph Pearce
In terms of diseases or viruses afflicting soybeans in Ontario, recognizing
alfalfa mosaic virus has not been as significant as the diagnoses of soybean
cyst nematode or phytophthora root rot. Yet its arrival coincides with the detection
of other viruses in soybeans, giving growers one more reason to be on guard.
A relatively new development in soybeans in the province, alfalfa mosaic virus
has garnered some attention from researchers and extension personnel because
it is vectored through the seed and by the soybean aphid, another recent development
for Ontario growers.
"It has been reported and is a disease that has never been truly documented
as a pest of soybeans in Ontario," says Albert Tenuta, field crop pathologist
with the Ontario Ministry of Agriculture and Food at Ridgetown College. Tenuta
acknowledges that alfalfa mosaic virus may not have the damage potential of
SCN. "But when you start adding alfalfa mosaic virus with bean pod mottle
virus, soybean mosaic virus and a soybean dwarf virus that Wisconsin is finding
now, the trend is that we're seeing more viruses in soybeans and it's another
indication that the vectors, besides seed, are out there."
Unless it has an impact on the soybean's marketability, there is not much concern
now among growers. And on assessing the impact of a single virus, Tenuta agrees,
but he also concedes that concern may be warranted, given a cumulative impact
with other viruses. As more viruses infect the same plant, the potential for
stress, yield loss and lower seed quality increases. "On its own, the fact
that we found it is important, but from an overall amount, it's still lower
than some of the other soybean diseases and viruses," says Tenuta, adding
that alfalfa mosaic virus is now part of his virus testing program. "It's
now lumped in with the testing groups, with the other viruses."
Aphids a factor
The other significant point is that viruses are not mobile; unlike insects,
viruses require transportation either from humans planting infected seed or
from plant to plant via an insect vector. That is where the soybean aphid plays
a role. Aphids are the vector for alfalfa mosaic virus, says Tracey Baute, field
crop entomologist for OMAF at Ridgetown College. They also vector soybean mosaic
virus and cucumber mosaic virus. The problem has been one of identification.
Like SCN in the past five years, damage from viral conditions can be overlooked
Viruses are also difficult to identify in a field setting; while SCN or white
mould are easy to distinguish, the same is not true for a virus. "We may
have had some of it in the past but we didn't have extensive monitoring in place
to diagnose it," explains Baute. As with any disease or pest that is relatively
new, there is a familiarity that must be built, and that can take time, identifying
vectors as well as the different virus hosts. "And a lot of weeds carry
these viruses, so if an insect happens to feed on that weed and carry it to
the crop plant, that's an easy way for it to develop."
The virus itself does not survive for more than a few hours on the aphid. Within
that time period, the aphid needs to feed on an infected plant and move to a
healthy plant to transmit it. The more infected plants there are in a field,
the more likely the aphid will come across the virus to vector it.
What helps the situation with alfalfa mosaic virus is that soybean aphids are
more specific to soybeans and edible beans. "So we don't see much vectoring
in other crops," says Baute.
Resistance being built
According to Tenuta, the seed companies are showing interest in developing resistance
to a number of these 'new' viruses but progress is limited. Alfalfa mosaic virus
has a number of genes available for the development of resistance, as does bean
pod mottle virus and soybean mosaic virus. But just as these viruses are increasing
in groups, so the drive for resistance is being grouped into an 'all in one'
approach. "The breeders are looking at all of these available genes and
viruses, and what they'll be doing is looking at overall resistance genes for
most of the viruses," says Tenuta.