Light and scanning electron microscopy (SEM) can distinguish Karnal bunt fungus in wheat from a lookalike fungus that attacks ryegrass—important news for the $5-billion-a-year U.S. wheat export market. Sometimes, tiny amounts of fungus-infected ryegrass seed get harvested along with wheat. Until now, available tests have incorrectly identified the ryegrass fungus as Karnal bunt. As a result, restrictions were placed on the movement of suspect wheat in 1996 and early 1997 from Alabama, Georgia, Florida and Tennessee. ARS scientists analyzed and characterized the teliospores (fungal seeds) of dried and fresh specimens of both the ryegrass and wheat fungus. After examining the teliospores' shape, size, surface characteristics and color, the researchers determined that light and SEM can be used to tell the two fungi apart. Mature teliospores of the wheat fungus (T. indica) appear dark red-brown, often opaque, with fine spines that densely cover the outer seed coat. The teliospores of the ryegrass fungus range in color from pale yellow or golden to dark brown, with thicker, more widely spaced spines covering the outer coat. The ARS technique showed that each of the 70 wheat samples collected from southeastern farms in 1996 was contaminated with the lookalike fungus—not Karnal bunt. As a result, in March 1997, the restrictions on movement of wheat were lifted from the counties where the suspect samples originated. Federal plant quarantine officials now use the technique as a "first cut" to decide if possible quarantine actions are needed. If the test results indicate a sample is Karnal bunt, officials go back and look for bunted wheat seeds.
Systematic Botany and Mycology Laboratory, Beltsville, MD
Lisa A. Castlebury, (301) 504-5280, lisa@nt.ars-grin.gov

A grapevine's resistance to attack by a notorious pest known as phylloxera can now be estimated more quickly and easily. Researchers at the University of California at Davis in collaboration with ARS colleagues developed the lab test. It rates a vine's vulnerability to the root louse in only 8 weeks. That will speed the work of breeders and researchers seeking phylloxera-resistant grapevines for California vineyards devastated by the pest. For the assay, scientists sterilize phylloxera eggs and candidate grapevine plantlets to kill any fungi or bacteria that might otherwise skew the resistance ratings. Then, the scientists place eggs and plantlets—rooted in a nutrient-rich gel—inside small, clear-plastic boxes. Insect and plantlet grow in tandem inside these boxes, kept in a temperature-controlled, walk-in chamber. Key to the procedure: Scientists determined how to sterilize the egg surfaces without killing the louse embryo. This makes the technique an improvement over earlier approaches that relied on eggs that were not surface sterilized. The test has yielded phylloxera-resistance ratings for more than 40 different grape species, most grown from samples from an ARS grape genebank at Davis, CA.
Horticultural Crops Research Laboratory, Fresno, CA
David W. Ramming, 209/453-3061, dramm@qnis.net

Scientists are creating a genetic map of the Hessian fly, Mayetiola destructor, to help wheat breeders develop varieties with more resistance to this costly crop pest. An early dividend may be the discovery of the location of a gene for the fly's eye color. ARS scientists noted that the white-eye color gene is near the one for virulence against H13, a fly-resistance gene carried by wheat. The close position of these two genes in the fly suggests the white-eye trait can be used to advance molecular mapping of the gene for virulence to H13. Each year Hessian flies cause millions of dollars of damage to wheat crops. The female fly lays her eggs in unfurled wheat leaves. When the eggs hatch, larvae crawl down the leaves and feed on plant sap inside the leaf sheath of developing wheat plants or at the wheat head, greatly reducing yields. This new genetic finding is important because the fly has been able to overcome resistance in wheat, forcing breeders to develop new resistant varieties about every seven years.
Crop Production and Pest Control Research, West Lafayette, IN
Richard Shukle, (765) 494-6351, rich_shukle@entm.purdue.edu

A newly found fungal gene holds promise for protecting plants from scab disease, which costs barley and wheat growers millions in losses each year. In the fungus Fusarium sporotrichioides, scientists found a gene called TRI-R, which stands for tricothecene-resistant. This gene codes for an enzyme that protects the fungus from its own deadly tricothecene toxin, T-2, which the fungus uses in its attacks on certain plants. T-2 is not a problem in U.S. crops. But it resembles the scab-fungi toxin, called the DON vomitoxin, that can devastate U.S. wheat and barley. This toxin is made by F. graminearum—a relative of F. sporotrichioides. Information obtained by studying either fungus could lead to strategies for deactivating the fungus that causes scab in U.S. wheat and barley. Scientists placed all the genes from F. sporotrichioides into a collection of yeasts. Yeasts that contained the TRI-R gene continued to thrive in the presence of the T-2 toxin. Researchers believe a gene-produced enzyme coats the toxin, making it safe for the cell. Outside the cell, T-2 regains its potency.
Mycotoxin Research, National Center for Agricultural Utilization Research, Peoria, IL
Nancy Alexander, (309) 681-6295, alexannj@ncaur.mail.usda.gov

The Cereal Disease Laboratory at St. Paul, MN, is stepping up its efforts to find new ways to combat a fungus that damaged wheat and other cereal grains. New funding will enable the lab to purchase equipment and hire two scientists to study the fungus Fusarium graminearum, the culprit behind head scab. The lab specializes in research on fungal diseases that attack cereal grains such as wheat, barley, oats and rye. Since 1991, head scab has caused millions of dollars in lost yields each year in the Great Plains and Midwest. The new research will focus on both short- and long-term solutions to head scab outbreaks so farmers can better cope with it. Scientists will study spores of the fungus from different regions of the United States to determine if they are genetically similar. The scientists will also continue to develop resistant varieties of crops to thwart spread of the disease and evaluate the effectiveness of farming methods such as cultivation and crop rotation to prevent future outbreaks.
Cereal Disease Laboratory, St. Paul, MN
Kurt Leonard, (612) 625-5768, KurtL@puccini.crl.umn.edu

Scientists have identified two different types of Cercospora zea-maydis, a fungus that causes gray leaf spot on corn plant leaves. One type occurs throughout the corn-producing regions of the United States, while the other type appears to be confined to the eastern part of the country. The finding could prove crucial as scientists probe the genetic makeup of the fungus to learn more about its virulence and develop corn varieties resistant to both types. The fungus produces spores called "conidia" that can survive the winter in crop residue left on the soil surface. The next spring, when the spores emerge, they're blown by wind or splashed by rainwater onto crops. A severe infestation can reduce corn yields by 25 percent. Grey leaf spot first appeared in southern Illinois in 1925, but became a serious problem in the mid-1980s as farmers switched to tillage systems that leave crop residue on the soil surface. Since then, the disease has spread across the Cornbelt.
Crop Production and Pest Control Research, West Lafayette, IN
Larry D. Dunkle, (765) 494-6076, dunkle@btny.purdue.edu

Last Updated: November 13, 1998
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