ARS researchers are building a multiple gene complex that controls different types of rust resistance for incorporation into new wheat varieties. The genes come from two ancestors of modern wheat: Aegilops tauschii (also known as goatgrass) found from Afghanistan to Syria, and Triticum timopheevii from Iran, Iraq, and Turkey. Wheat breeding programs have released disease-resistant varieties in the past. However, many of these varieties—but not all—began to lose their effectiveness in as little as 2 to 3 years because they relied on resistance conferred by one gene. With the gene complex, varieties with more durable resistance can be developed to help farmers fight leaf rust throughout the Great Plains. Leaf rust is caused by the fungal pathogen Puccinia triticina. In the last decade, growers in the hard winter wheat-growing area of the Great Plains have averaged annual losses of 50 million bushels. With wheat prices running about $3 a bushel, leaf rust has cost Great Plains farmers about $150 million a year. Besides reducing yields, the disease also seriously affects the milling and baking qualities of wheat flour.

Plant Science and Entomology, Manhattan, KS
Gina Brown-Guedira, (785) 532-7260, gbg@ksu.edu

A multinational investigation into the microbe that causes Pierce's disease in grapevines may yield new ways to outwit this destructive microorganism. ARS and Brazilian researchers are collaborating in a joint project to determine the makeup, or sequence, of all of the genes in the bacterium, Xylella fastidiosa, that's responsible for this costly disease. In Northern California, Pierce's disease has chronically attacked vineyards, costing growers an estimated $33 million from 1995 to 1997 alone. In California's Temecula Valley, south of Los Angeles, the disease has led to approximately $6 million in damage to vineyards since 1997. A half-inch-long insect known as the glassy-winged sharpshooter can harbor Xylella in its gut, then move the pathogen into plants when it punctures grapevine stems to feed. Once inside a grapevine, X. fastidiosa bacteria multiply, blocking the flow of water and nutrients. Severely infected vines die. Pierce's disease affects wine, table, and raisin grapes. Neither the insect carrier nor the disease harms humans, however. Brazilian scientists have already sequenced the genome of a related X. fastidiosa strain that causes a disease known as citrus variegated chlorosis. In addition to ARS, sponsors of the new research venture are the American Vineyard Foundation, the California Department of Food and Agriculture, and the State of São Paulo Research Foundation. Scientists expect to finish sequencing the genome of the Pierce's disease strain in less than a year.

ARS National Programs for Crop Production, Product Value, and Safety, Beltsville, MD
Kevin J. Hackett, (301) 504-4680, kjh@ars.usda.gov

ARS scientists found more than 100 new species of the pathogenic plant fungus Fusarium when they recently compared genetic material called DNA from more than 3,000 strains collected worldwide. Some Fusarium species cause stalk rot and ear rot in corn and head blight or scab in wheat and barley. The newly discovered species have left the scientists with questions. Could these fungi survive in fields of U.S. grain? Should plant breeding programs and quarantine programs take into account genetic information about these fungi? In greenhouse tests, eight of the new species, mostly of exotic origin, produced scab disease in wheat. The scientists say three of the scab-causing fungi may be native to Africa, three to South and Central America, and one to Asia. The remaining one, like the common cold, is widespread and may be native to the Northern Hemisphere.

National Center for Agricultural Utilization Research, Peoria, IL
Kerry L. O'Donnell, (309) 681-6383, kodonnell@sunca.ncaur.usda.gov

Cereal Rust Laboratory, St. Paul, MN
H. Corby Kistler, (612) 625-9774, hckist@puccini.crl.umn.edu
U. Liane Rosewich (612) 625-9774, ulrose@cdl.umn.edu

Growing wheat as a rotation crop when replacing apple orchards may help prevent replant disease, ARS scientists found. The technique could also serve as an alternative to fumigating the soil with methyl bromide, typically used to sterilize old orchards before planting new ones. When nothing is done between taking out an old orchard and putting in a new one, the young trees are often stunted and have small, decayed root systems. In the Pacific Northwest, replant disease seems to be caused by buildup of four types of soilborne fungi. Soil where apple trees grow supports these detrimental fungi. Some wheat soils, on the other hand, foster growth of a beneficial bacterium, Pseudomonas putida. This bacterium appears to protect young apple roots. ARS has patented use of a strain of this bacterium to prevent replant disease. The next step is to determine how long wheat would have to be grown before orchard replanting in order to protect new apple trees.

Tree Fruit Research Laboratory, Wenatchee, WA
Mark Mazzola, (509) 664-2280, mazzola@tfrl.ars.usda.gov

Boosting organic matter in soil may help create ideal soil conditions for weed-suppressing microbes, called deleterious rhizobacteria (DRB). Living on, or within a few millimeters of, weed roots, these microbes produce toxins and excessive concentrations of plant growth hormones that put weed seedlings into overdrive, although they normally do not interfere with crop plant growth. This overdrive weakens weeds as the fast-growing root cells rupture and leak. The DRB thrive by feeding on the substances that ooze from the roots. The weakened weeds are less able to compete with other plants for soil nutrients, moisture, and sunlight and are vulnerable to other control measures. In laboratory and field experiments at Columbia, MO, ARS scientists researched cultures of DRB associated with the most dominant species of weeds in six different cropping systems. In general, the highest number of weed-supressing DRB came from fields where crops were rotated, chemical applications and tillage were minimal, and organic materials such as compost were added. DRB fared best in a corn-soybean-wheat-cover crop rotation. An organic strawberry system with compost was a close second.

Cropping Systems and Water Quality Research, Columbia, MO
Robert J. Kremer, (573) 882-6408, KremerR@missouri.edu

A new compound for luring Mediterranean fruit flies into traps stays potent three to four times longer than today's most widely used attractant, ARS scientists have found. Medfly, or Ceratitis capitata, is one of the world's worst insect pests of agriculture. It can attack more than 250 different kinds of fruits, vegetables, and nuts. Currently nicknamed "minus-ceralure" by its ARS developers in Beltsville, MD, and Hilo, HI, the new compound could dramatically cut the number of times that traps need restocking with a male medfly lure. Today, state and federal agencies in the United States use about 150,000 traps a year, equipped with an older, ARS-developed compound known as trimedlure. A procedure developed by an ARS chemist in Beltsville yielded enough minus-ceralure for the ARS outdoor tests in Hawaii, where medfly is already established. The outdoor studies indicated that minus-ceralure is about four to nine times more attractive to medfly males than trimedlure. Minus-ceralure is one of 16 components, known as isomers, that make up a parent compound called ceralure. Other ARS scientists working at the Maryland and Hawaii labs patented ceralure in 1988. ARS is now seeking a patent for minus-ceralure (U.S. Provisional Patent Application No. 60/176,192).

U.S. Pacific Basin Agricultural Research Center, Hilo, HI
Eric B. Jang, (808) 959-4340, ejang@pbarc.ars.usda.gov

State-of-the-art technology has been used to identify and pinpoint, for the first time, the fungi responsible for powdery mildew on tomatoes. In 1999, powdery mildew devastated much of the $1.8 million U.S. tomato crop—particularly that part grown in greenhouses. The organism causing powdery mildew is a fungus that has been difficult to identify. Only one similar organism was known on tomatoes, and it was reported from Australia. ARS and a team of international mycologists examined all available specimens of Oidium fungi-24 powdery mildew isolates and 29 herbarium specimens gathered from all the continents on which tomatoes are grown. Using scanning electron microscopy, classical morphology, and molecular fingerprinting, they were able to determine the molecular fingerprint of the powdery mildew fungus and identify two species that attack tomatoes. The study showed that this emerging disease is caused by a previously unknown fungus which is distinct from the one from Australia. The scientists identified and renamed the fungus O. neolycopersici. Naming and describing the fungus gives plant pathologists and others studying the disease a more precise way to communicate about and treat this disease.

Systematic Botany and Mycology Laboratory, Beltsville, MD
Amy Y. Rossman, (301) 504-5364, amy@nt.ars-grin.gov

ARS researchers and a colleague in Scotland have adapted a strategy they developed for citrus tristeza virus to more accurately sample large areas for plum pox virus (PPV). This method, called hierarchical sampling (HS), will be used in the U.S. Department of Agriculture's $1.4 million national PPV surveillance program. Stone fruits, such as peaches and plums, are highly susceptible to PPV. The value of U.S. stone fruit production was $1.3 billion last year. Previous sampling methods used for spotting diseases and their causative agents on citrus and other crops are based on the number of infested soil samples, disease lesions on a leaf, proportion of diseased fruit, or number of insects found on each plant. Unfortunately, the methods don't quantify the amount of disease present in any given tree. HS relies on the theory that it's possible to predict disease at one scale by sampling at another. By sampling only 6.25 percent of the trees in a given orchard in groups of four trees—the location of the trees in the orchard is critical—and using unique statistical methods, scientists are able to accurately predict the incidence of infection in the whole orchard. After performing thousands of simulations, researchers have shown HS to be much more accurate at detecting plum pox virus infestation than other sampling methods.

U.S. Horticultural Research Laboratory, Ft. Pierce, FL
Tim R. Gottwald (407) 897-7347, tgottwald@ushrl.ars.usda.gov