Karnal Bunt Lookalike Is Unmasked, Helping SE Wheat Growers

By Hank Becker
September 25, 1998

This year, southeastern wheat growers should face less risk of a quarantine on their crop due to Karnal bunt disease. That's because Agricultural Research Service scientists discovered that light and scanning electron microscopy (SEM) can distinguish Karnal bunt from a comparatively harmless lookalike fungus on ryegrass.

Sometimes, tiny amounts of ryegrass infected with the lookalike are inadvertently harvested along with the wheat. Until now, available tests have incorrectly identified this fungus as Karnal bunt. As a result, restrictions were placed on the movement of wheat in 1996 and early 1997 from Alabama, Georgia, Florida and Tennessee because it was suspected of being infected with Karnal bunt.

But the ARS technique quickly showed 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, these restrictions were lifted from the counties where the suspect samples originated. Federal plant quarantine officials now use the ARS technique as a "first cut" to decide if possible quarantine actions are needed. If the test results indicate a sample is Karnal bunt, they go back and look for bunted wheat seeds.

ARS mycologist Lisa A. Castlebury in Beltsville, Md., developed the technique. It uses light and SEMs to characterize teliospores (fungal seeds) of dried and fresh specimens of both fungi.

After examining the two fungi's shape, size, surface characteristics and color, Castlebury determined that these characteristics can be used to tell these two fungi apart. She validated the technique by rigorously comparing her test's results with known differences between the two fungi. Mature teliospores of Karnal bunt (T. indica) on wheat are dark red-brown, often opaque. Fine spines densely cover the outer seed coat. Teliospores of the unnamed Tilletia species on ryegrass range from pale yellow or golden to dark brown; spines are thicker and more widely spaced.

In 1996 and 1997, U.S. wheat export markets had a total average value of about $5 billion a year. Most of this market was threatened by the ryegrass fungus.

ARS is the chief research agency of the U.S. Department of Agriculture.

Scientific contact: Lisa A. Castlebury, Systematic Botany and Mycology Laboratory, Beltsville, Md., phone (301) 504-5364, fax (301) 504-5810, lisa@nt.ars-grin.gov