Update on Karnal Bunt and Ryegrass Bunt July, 1999 Karnal bunt has not generated the attention that occurred when it was first found in Arizona in 1996 and supposedly found in the Southeast. However, since then much more information is available about where the disease is and more importantly for us in the Southeast, where it is not. The smut fungus thought to be Tilletia indica, cause of Karnal bunt, is now know to be Tilletia walkeri, a newly described species. I have found T. walkeri to be widely distributed in the Southeast at very low levels on ryegrass in wheat fields. Bunted ryegrass harvested with wheat was the source of spores on wheat seed mistakenly called Karnal bunt. One collection I made from a field in Morgan County (south of Madison), GA is the single largest collection of T. walkeri from the field in the U.S. Specimens from this collection are now in the USDA national mycological herbarium and the Washington State University herbarium, which has one of the largest collections of smut fungi in the world. All of my collections of T. walkeri have been confirmed by Lisa Castlebury, USDA mycologist who is one scientists who described the species. I have just published a paper with Dr. Castlebury that describes the collections and distribution of the fungus in the Southeast (Plant Disease 83:685-689). Several other lines of research have made the picture clearer about these fungi. Several laboratories have developed definitive molecular tests that differentiate T. indica from T. walkeri. With the availability of more collections, accurate descriptions of the morphological differences between the fungi are now known. Recently Lori Carris, at Washington State, using my Morgan County isolate, successfully infected ryegrass with T. walkeri in the greenhouse. Likewise, Blair Goates, USDA pathologist at Aberdeen, ID has infected wheat with T. walkeri in the greenhouse. In both case, the teliospores produced look the same as those from ryegrass in the field. This is important because an earlier report stated that when ryegrass bunt was inoculated onto wheat, typical Karnal bunt spores were produced. The wheat seed infections with T. walkeri were very limited, and no one has found infected wheat seed in the field. These studies help to differentiate the pathogens accurately. The national survey by USDA continues but has been reduced considerably. It is now referred to as a monitoring program. In 1999 only seven samples were collected in Georgia. This is in contrast to 165 samples collected just two years ago. All Georgia samples were found to be negative. Therefore Georgia wheat continues to be free of Karnal bunt. A new project I expect to be participating in will monitor survival of Tilletia indica teliospores in soil. Teliospores will be enclosed in mesh bags in columns of soil within PVC pipes also covered with mesh screens that prevent movement of the spores but allow water to enter. Over a period of several years, the PVC columns will be removed and shipped to containment facilities at Ft. Detrick, MD for examination. The system has been field-tested without problems in Arizona and Montana. Additional test sites will be in Maryland, Kansas, and Idaho. This study will give accurate information about long-term survival of T. indica under several environments. This project still needs approval by the Georgia Department of Agriculture and USDA-APHIS. Barry Cunfer
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Georgia Invasive Species Task Force |
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