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Title: Decline in a Tilletia indica teliospore population in a naturally infested Arizona (USA) wheat field

Author
item Peterson, Gary
item KOSTA, KATHLEEN - California Department Of Food And Agriculture

Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2017
Publication Date: 10/16/2017
Citation: Peterson, G.L., Kosta, K.L. 2017. Decline in a Tilletia indica teliospore population in a naturally infested Arizona (USA) wheat field. American Journal of Plant Sciences. 8, 11.

Interpretive Summary: Karnal bunt is a destructive disease of wheat. This disease was first detected in the U.S. in 1996 in Arizona and subsequently in limited areas in California and Texas. USDA APHIS places fields with wheat testing positive for the presence of the disease under Federal quarantine regulation. USDA ARS conducted studies to determine how long the teliospores of this pathogen remain viable in the soil. Scientists artificially contaminated soil with teliospores inside mesh bags and buried the bags in a University of Arizona field in Tucson Arizona. Bags were removed at different times and teliospores extracted from the soil, counted and tested for viability. Results suggested that the teliospore populations decline to low levels by approximately 91.8% after 3 years. Based on these results, USDA APHIS decided that fields could be removed from quarantine regulation after 5 years. Since this research was conducted under controlled conditions, Researchers from the USDA, ARS Foreign Disease - Weed Science Research Unit, and the California Dept. of Food and Agriculture conducted a joint field research project in Maricopa, AZ to determine if similar results occurred under natural conditions. In this study, Karnal bunt was detected in a naturally infected wheat field in Arizona at the end of the 2004 season. After harvest, samples of soil were taken throughout the field to estimate the level of teliospore contamination in the field. Sampling was repeated several times a year from 2005 thru 2007. Teliospores were extracted from soil samples, quantitated and tested for viability. Results showed that under natural field conditions, the population of teliospores in the soil does decline rapidly after 3 years and supports the results of earlier studies conducted under controlled conditions. This also supports the best management practice of removing contaminated fields from quarantine regulation after 5 years.

Technical Abstract: Following a 2004-outbreak of Karnal bunt in an Arizona wheat field where infection had not been detected in recent years, the population and viability of Tilletia indica teliospores in surface and subsurface layers of soil were monitored over a four-year period. The field was seeded with two parallel 1.8 m X 190 m strips of wheat, 11 m apart and the remainder planted in barley. Thirty-eight 1.22 m2 wheat plots were harvested and seed examined for infection. Several times throughout each season soil samples were taken from each of 25 points throughout the field. Teliospores extracted from subsamples of each soil sample were examined to estimate the number of teliospores per gram soil and their germination percentage. The average (SD) infection observed for each of the four consecutive seasons were 7.05 (4.6), 2.89 (5.5), 0 and 0.004% (0). The average number of teliospores recovered per gram of soil during the first sampling was 36 and 39 for the 0.5 cm deep (surface) and 5.5 cm deep (subsurface), respectively, with an average teliospore germination of 29%. By the last season the average numbers of teliospores recovered were 5 and 2.5 per gram of soil, respectively, with an average germination of 1.0%. Thus, the number of viable teliospores declined by 96.6% over the four- year sampling period.