Take That!
Natural compounds on offensive against
pests and pathogens.
American consumers
are becoming increasingly aware of chemicals used in producing the food
they eat. Yet most wouldn't buy a fungus-spotted green bean or bug-eaten
cabbage on a bet.
 Producers want to meet consumer demand
for healthy, attractive vegetables and the use of chemicals approved for
agricultural applications has been the most cost-effective way to do that.
As part of UT's vegetable initiative, Experiment Station scientists are
investigating some ground-breaking "natural" alternatives and
will pursue those ideas to the point of cost effectiveness and fitting them
into a practical production schedule.
Biological Warfare-On Bugs
Imagine a naturally occurring fungus that gives an aphid
or whitefly a terminal case of dermatitis. Or a tiny nematode (type of worm)
that invades a pesky insect's body and sets up housekeeping there, eventually
killing it.
These scenarios aren't science fiction, says Dr. Roberto
Pereira, an insect pathologist with the UT Agricultural Experiment Station.
They are two of the four types of commercially available biopesticides.
As part of the vegetable initiative, his entomology laboratory will be testing
biopesticides for application by Tennessee vegetable producers. These include
viruses, bacteria, fungi, and nematodes. Such pathogens are harmful only
to the insects they target, he emphasized, and are not a danger to helpful
insects or plants.
Good Bacteria
Naturally occurring bacteria may work to protect tomatoes
from root diseases, says Dr. Bonnie Ownley, an assistant professor researching
plant pathogens for the UT Agricultural Experiment Station.
Biocontrol means using living organisms to treat plant
parts to protect them against pathogens, she explained. Preliminary work
in her laboratory seeks to control Rhizoctonia, a fungus commonly found
in the soil. It causes damping-off, foot rot, root rot, stem cankers, and
even fruit rot if a tomato touches soil, she explained.
"There are plenty of bacteria that live in concert
with tomato plants causing no diseases whatsoever," Ownley said. "We
suspect these have good potential as beneficial organisms."
She and her colleagues have isolated and tested 200 types
of soil bacteria. Of those, 18 show promise in working against harmful fungi.
The helpful bacteria are painted onto tomato seeds.
"As the seed germinates and roots begin to grow, the
bacteria will grow along with it and help protect against any pathogens
that may be in the soil. We know that the effective isolates produce antibiotics
that kill Rhizoctonia."
Timing Is Everything
Other "natural" methods of controlling Rhizoctonia
may be as simple as delaying planting by a week or so.
Ownley has also been studying Rhizoctonia as it affects
no-till tomatoes. When Roundup is used to "burn down" a cover
crop prior to planting, Rhizoctonia attacks the dying plants with a vengeance.
"One or two days later, we put that transplant into
a bed full of Rhizoctonia," she said. "Every single plant was
affected. They had stem cankers from the soil line up. They were stunted.
We had significantly less fruit from those plants."
Her research will determine the optimum time for planting
no-till tomatoes after applying Roundup-after the Rhizoctonia danger has
passed.
Soil Fumigation via Brassica
Yet another interesting aspect of "natural" control
is as a replacement for methyl bromide in fumigating soil.
Farmers know methyl bromide as a reliable tool for eliminating
many potential problems before they even get started, including soilborne
diseases, pathogens and weeds. But beginning in the year 2001, new government
regulations will ban methyl bromide for good, unless an extension is granted.
Researchers are scrambling to find a replacement.
Dr. Carl Sams, vegetable physiologist with the UT Agricultural
Experiment Station, is investigating some advice from mom. Eat your vegetables,
she admonished. And researchers have found that certain vegetables can lower
the risk of certain types of cancer. What if, Sams theorized, the same compounds
that kill cancer cells in humans would get rid of pathogens in soil?
He is looking at wild varieties of brassica plants, cousins
to broccoli, cauliflower and Brussels sprouts, that produce extra high levels
of glucosynolates-the sulphur-like smell you get when cooking cabbage. The
idea is to use these wild plants as a cover crop, then plow them under and
cover the area with plastic. As the brassica plants rot, they release glucosynolates-which
further degrade to isothicocynates-in a gaseous form that may work to fumigate
the soil. In early lab studies, a variety of Indian mustard seems to be
very effective.
"We could take the crop that has the potential for
producing these compounds and plant it in the fall as a cover for, say,
spring tomato production. These brassica plants are cool season crops, so
many of them will overwinter in our climate which would provide soil erosion
protection. Then in the spring we could plow it under and immediately cover
it with black plastic. That would trap the gases that break down from the
crop. In addition to that, we could even add a layer of clear plastic over
the black for a week or two. The sunlight through the clear plastic would
warm the soil even further, and the intensive heating effect might enhance
the work of the gas as a soil fumigant."
It's too early to say if this will be the practical solution
to replacing methyl bromide, Sams said, but his research will focus on finding
the most effective of these brassica plants and working to fit it into a
production schedule for crops currently using plasticulture-like tomatoes
or strawberries.
- by Lisa Byerley Gary
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