The growth of some weeds like jointed goatgrass and downy brome was reduced up to 75 percent when researchers tried new, easy-to-use farming practices. They discovered that if they could get winter wheat plants to shade the weeds, the wheat would beat out the weeds. One solution: planting a taller wheat variety like Lamar. Growers traditionally have planted shorter varieties such as Tam 107 and Vona. Seeding wheat at 65 pounds per acre instead of the typical 40 pounds also helped shade out weeds.
Central Great Plains Research Station, Akron, CO
Randy L. Anderson, (970) 345-2259, rlander@lamar.colostate.edu

A new diagnostic test has been developed by ARS and university researchers to identify the mystery insect that spreads "yellow vine." Since 1991, outbreaks of this new disease have caused losses of up to 100 percent in watermelon and other cucurbit crops grown in Oklahoma and central Texas. Melon growers there have had little success fighting the disease, which can cause fruit losses of up to $4,000 per acre. Without knowing the insect carrier's identity, migration or feeding patterns, growers can't effectively use insecticides, repellent mulches or other crop protecting tactics. Initially, researchers conducted lengthy greenhouse studies to determine which cucurbit-loving insects could infect melon plants through feeding. Some of these included squash bugs, leaf hoppers and aphids. But few conclusive leads surfaced. This spring, the researchers will use a different approach. Using their new diagnostic test, they will screen the insects' tissues for genetic "evidence" of the culprit that causes yellow vine: a bacterium-like organism (BLO). Key to their approach is a sensitive molecular technique called polymerase chain reaction (PCR). It copies the organism's unique, DNA "fingerprint" millions of times over so it can be identified. Use of PCR will determine in one to two days whether an insect specimen--or weed it may feed on--harbors the organism.
South Central Agricultural Research Laboratory, Lane, OK
Benny D. Bruton, (505) 889-7395, bbruton@ag.gov

The bacterium that causes fire blight in fruit trees doesn't linger in a tree's older vascular system in sufficient numbers to cause disease. ARS scientists made this discovery using two aseptic, whole tree arborspheres—a kind of plastic growth chamber. Much research has been done on this disease that attacks young fruit trees, especially apple and pear. It causes leaves and fruit to shrivel and blacken as though scorched by fire. But it was not known whether the disease-causing organism, Erwinia amylovora, lived in the limbs of the tree or if it reinfected the tree each season. The arborspheres were constructed in a test orchard, keeping two Rome beauty apple trees under sterile conditions for three months. Test trees were heavily pruned and cankers or damaged bark that might house bacteria were removed. No bacteria were found when the arborspheres were removed. Control trees close by that had not been kept in sterile conditions showed signs of fire blight. The research showed that heavy pruning causes an overabundance of new, tender shoots that are more susceptible to fire blight infection. So, when trees are dormant, growers should remove only the blighted shoots and large cankers caused by bacteria. Proper pruning should ensure adequate light penetration into the tree canopy to maintain good tree growth.
Appalachian Fruit Research Laboratory, Kearneysville, WV
Tom van der Zwet, (304) 725-3451

Gamma ray energy needed for quarantine treatments against fruit flies could be lowered by half or more, ARS studies show. If expanded tests confirm this finding, irradiation would be more practical as an alternative to methyl bromide, a widely used fumigant set to be phased out by 2001. Effective quarantine treatments are essential. They ensure that Mexican fruit flies, Mediterranean fruit flies and related pests don't go along for the ride--and spread--when citrus or other fresh produce arrives at markets. Irradiation leaves no residue in fruit, but disrupts the insects' development so they can't reproduce. Currently, Hawaiian papayas and other tropical fruits are approved for shipping to Chicago. Exposure to 250 Grays (Gy) of gamma rays stops any stowaway oriental or Mediterranean fruit flies in the fruit. To ensure the innermost fruits in the load get 250 Gy, earlier tests showed that a dose level up to 750 Gy would be needed--but this level can damage orange, mango, grape, avocado, olive and other fruits. Preliminary ARS lab tests indicate that only 50 Gy may be needed to halt Mexican fruit flies in grapefruit shipments. Lower irradiation levels would translate to faster, cheaper treatments for more kinds of fruits. Plus, the fruit could arrive sooner at the supermarket--with more of its harvest quality intact.
Crop Quality and Fruit Insects Research, Weslaco, TX
Guy J. Hallman/Donald B. Thomas,(210) 565-2647, hallman@pop.tamu.edu, thomas@rsru2.tamu.edu

A new high-tech defense protects peanut plants from attack by lesser cornstalk borers, cutting the need for chemical pesticides. The key to this new defense is a toxin-making gene that can be "borrowed" from the naturally occurring bacterium Bacillus thuringiensis (Bt). A team of scientists from ARS, the University of Georgia and Mycogen, Inc., used a biotech procedure to insert Bt genes into peanut cells, producing plants that carry the gene to make the Bt toxin. The toxin acts as a natural insecticide in the peanut plant's leaves, stem and roots, where the borer feeds. When borer caterpillars eat enough of the toxin, it kills them or slows down their growth and development into adult moths. In lab experiments, all the corn borer caterpillars died after eating leaves from plants with the Bt gene. But Bt's insecticide is harmless to people, animals and beneficial insects like bees. Field studies begun earlier this summer could help set the stage for developing new, commercial cultivars of Bt-engineered peanuts. Currently, growers apply granular insecticides to soil to kill the borer, a crop threat during hot, dry weather. The borer also can spread Aspergillus flavus, the fungus responsible for producing aflatoxin, a contaminant that can make peanuts unfit for human consumption.
Plant Resistance/Germplasm Enhancement Research, Tifton, GA
Robert Lynch, (912) 387-2375, rlynch@tifton.cpes.peachnet.edu

Last Updated: July 11, 1997
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