A sensitive genetic test developed by ARS scientists will enable state and federal regulators to detect a nasty bacterial disease of citrus—if and when it invades the country. The disease is citrus variegated chlorosis (CVC), caused by a strain of Xylella fastidiosa bacteria. Some 88 million orange trees in the state of Sao Paulo, Brazil—a major source of orange juice for U.S. consumers—are infected with X. fastidiosa. Brazilian experiments show the bacteria are capable of reducing yields 37 to 100 percent, depending on the orange variety. ARS scientists developed a genetic test that distinguishes the CVC pathogen from other strains of X. fastidiosa, including one that infects Brazilian coffee plants. The test uses PCR—short for polymerase chain reaction—to distinguish one pathogen from another by comparing unique segments of DNA, similar to tests used by law enforcement agencies. Telling the CVC pathogen apart from other strains of the bacterium has been one of the chief obstacles to rapid, accurate detection. Florida and other citrus-growing states are home to leafhoppers and other insects that could transmit CVC from tree to tree. But the pathogen also hides in dozens of woody plants and trees. The danger is that it could enter the U.S. undetected in plant species other than citrus. The citrus industry also needs a good diagnostic procedure to understand how CVC spreads and to develop effective control procedures. Two or three other PCR tests have been developed since the ARS test.
Fruit Laboratory, Beltsville, MD
John S. Hartung, (301) 504-6374, jhartung@asrr.arsusda.gov

What if a crop plant could defend itself from an insect pest by damaging the pest's ability to make skin and grow? This defense is now being developed in molecular genetic and biochemical studies by ARS and cooperating scientists. They've genetically engineered tobacco plants to include an insect enzyme called chitinase, which breaks down chitin, a key component in insect "skin" and gut tissue. Chitin fibers play an important role when insects grow and shed their skin. In lab studies, the scientists found that the genetically engineered plants significantly suppressed the growth of feeding insect larvae. ARS and Kansas State University researchers are aiming their approach—based on chitinase from the tobacco hornworm, Manduca sexta—to provide a biopesticide alternative to traditional chemical pesticides such as methyl bromide. Currently, this chemical is sometimes used to protect raw and processed commodities. The ARS and KSU researchers hold a patent on the only known insect chitinase gene used in transgenic plants. While insect chitinase targets chitin, its presence in plants is harmless to humans or animals. The researchers have been collaborating with several agricultural biotechnology companies to transform other plants including corn, rice, wheat and sorghum with the insect chitinase gene.
Grain Marketing and Production Research Center, Manhattan, KS
Karl J. Kramer, (785) 776-2711, kramer@usgmrl.ksu.edu

Peanut crops could benefit from a new strategy that makes life miserable for noxious weeds. ARS and University of Georgia scientists devised the approach to help peanut farmers avoid the "herbicide treadmill." This term describes the costly practice of applying up to six different herbicides to keep pace with multiple weed species that compete with peanut crops—and each other—for space, sunlight and nutrients. In coastal plains states like Georgia, battling weeds with herbicides can cost peanut farmers roughly $75 per acre. Texas panicum, yellow nutsedge and Florida beggarweed are among the peanut crop's most common and aggressive competitors. To even the odds, scientists combined cultural practices that help frustrate the weeds' germination and growth. One tactic employs the "stale seedbed" concept. This means preparing a seedbed of soil in an unplanted peanut field, then later plowing the top 3 inches to control weeds before they become established. A follow-up tactic, in late April to mid-May, calls for sowing peanut seed in narrow rows so the plants grow closer together. This fosters a quicker-developing leaf canopy than standard row patterns, and blocks late-season weeds from sunlight. The scientists' field studies indicate this approach could save peanut farmers an estimated $12 per acre on herbicides. It could also prevent the need to increase peanut seeding rates. At up to $100 per acre, peanut seed tops the farmer's list of production expenses.
Nematode, Weeds and Crops Research Unit, Tifton, GA
Carroll Johnson, (912) 386-3172, cjohnson@tifton.cpes.peachnet.edu

With a helping hand from science, smut could become the next weapon in the war on weeds. Smut—more specifically, loose kernel smut—is a natural fungus ARS scientists are experimenting with to kill Johnson grass, a noxious southern weed. The smut fungus, Sphacelotheca holci, causes a disease that turns the weed's seed structures into a black, crumbly mass. Scientists hope to encourage the microbe's mischief by formulating it into a mycoherbicide spray. They're conducting field tests to evaluate the spray's potential as a natural alternative to conventional chemical herbicides. The chemicals help prevent Johnson grass from invading farm fields and outcompeting crop plants for space, sunlight, nutrients and water. But using herbicides is costly and raises environmental concerns. The fungus is an appealing alternative: It poses no danger of groundwater contamination, is naturally occurring and is very host specific. So, it poses little or no serious danger to grass crops like sugarcane, wheat and most sorghum varieties that succumb to other species of smut. In field trials, scientists observed plant infection rates of 50 to 80 percent after spraying the fungus onto Johnson grass seedlings—good, but not enough to prevent a noticeable drop in cane yield. Scientists now are contemplating ways to boost the fungus' virulence, or ability to cause disease in the weed.
Sugarcane Research Unit, Houma, LA
Rex Millhollon, (504) 868-3168, rmillhol@nola.srrc.usda.gov

A hedge against sedge? That's what farmers could call a new chart that identifies 27 sedge species that are weeds of U.S. crops. Sedges are grasslike plants in the genus Cyperus. Some Cyperus sedges are useful, the most famous being C. papyrus, from which ancient Egyptians made paper. Generally, however, sedges cause problems on farms, in forests and in urban and other rural areas. Worldwide, they cost billions of dollars a year in crop competition and control measures. Botanically, where there's sedge, there's often confusion. To help U.S. farmers accurately identify these weeds, ARS researchers developed a reader-friendly table in cooperation with scientists in Arkansas and Florida. The genus Cyperus contains about 600 sedge species. Many are often misidentified as other weeds in the Cyperaceae family. This family includes sedges and much more: over 4,000 species in 90 to 115 genera. Currently, no single publication in the United States addresses these 27 Cyperus sedge weed species, including 14 native species and 13 exotic ones. One species, purple nutsedge, is considered by some to be the world's worst weed. The chart also contains information on yellow nutsedge, rice flatsedge and small flower umbrella sedge, all among the world's 34 worst weeds. Nomenclature, common names, distribution, ecology, biology and habitat data are presented for each species. Reprints including the genera key and tables on native and nonnative Cyperus are available from the researcher.
Southern Weed Science Laboratory, Stoneville, MS
Charles T. Bryson, (601) 686-5259, cbryson@ag.gov