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Soybean borders shield against aphids

Using other crops to reduce damage by pests and disease is not a new idea.
Commonly called 'companion planting' in organic gardening circles, this process
has value in field crops as well.

The theory involves planting one crop beside another that is more attractive
to insects or disease, thereby reducing damage to the crop being protected.
In the case of potatoes, research shows that by planting a 'border' crop around
the potatoes, aphids, vectors of several important potato virus diseases, leave
the infection in the outer crop before moving to potatoes. This is a particularly
valuable form of disease management for producers of potato seed stock.

"You can influence a pest population within a plot or field without using
pesticides simply by using crop borders," says Dr. Ted Radcliffe, an entomologist
with the University of Minnesota. Several years ago, he was consulted on methods
to protect early generation potatoes from disease. "The integrity of the
whole must be maintained," he explains. "If you are growing early
generation Elite seed, you may have several 'postage stamp' plots separated
by fallow. These plots become targets for insects."

Radcliffe's suggestion was to 'hide' the plots with some other plant. His idea
was not initially accepted and, unfortunately, the plots became infected. The
concern was to protect the plots from aphids that can be vectors of potato virus
Y (PVY). In subsequent experiments, plots were set up surrounded by fallow,
soybeans, winter wheat, potatoes and sorghum, and he monitored the flight of
the insects and measured the main crop of potatoes for virus. Tubers harvested
from plots bordered by another crop had 60 to 70 percent less PVY than those
from plots bordered by fallow.

"Growers can reduce the transmission of potato virus Y by 70 percent just
by using crop borders," Radcliffe reports. "In experiments designed
to mimic typical seed production practice, we planted long, narrow plots of
potatoes 12 rows wide bordered on each side, but not at the ends, by either
two rows of potatoes, soybeans or fallow. We got the same results even though
the ends were not controlled."

As a vector for virus, aphids spread PVY by tasting the plant, but once the
insect has probed the plant and deposited the disease, it no longer has the
virus in its system. Therefore, the next plant the insect tastes will not be
infected. The result of planting another crop around potatoes is that the aphids
leave the disease on the border crop before moving to their preferred feeding
plant, the potato.

Radcliffe cautions that border protection is only effective in the case of
potato virus Y. "If growers are dealing with potato leafroll virus (PLRV),
the aphid remains infected for life, so the borders are less likely to help."

The safest choice for a border crop is soybeans, according to the entomologist.
Using a cereal may introduce some challenges over weed control products because
growers need to find one that is registered for both crops. Potatoes can also
be used as a border crop, but growers will want to harvest the border and the
crop separately because the border is more likely to have virus infection. Radcliffe
says seed growers can have the border product tested and if it is free of disease,
it can be sold for seed.

For growers who might want to try canola as a border crop, Radcliffe expresses
caution. "If there is no virus inoculum in the field you are protecting,
canola might work. But, canola is a favoured host of the green peach aphid,
the most efficient vector of aphid-transmitted potato viruses. Unless the field
is free of inoculum, a whole new problem could be introduced."

In the final analysis, soybeans or potatoes are the best choices for border
crops to shield a valuable potato crop from aphids that carry PVY. The goal
is to reduce the chance of disease by giving the aphids something else to chew
on before they reach the main potato crop. Soybeans or potatoes offer the simplest
choice of border crop without concern for introducing other diseases or compatible
herbicide registrations. It is a simple solution that perfected the idea of
'companion gardening' for a field crop situation. -30-