Update on the situation of Tilletia indica in USA
In USA on 8th March 1996, the presence of Tilletia indica (EPPO A1 quarantine pest) was confirmed in Arizona (EPPO RS 96/062) on durum wheat. Bunted wheat seeds were also found in samples remaining in Arizona after a portion of the lots had been planted in Arizona, Texas and New Mexico. Quarantine measures were immediately applied: destruction of fields planted with infected seeds, prohibition to grow wheat for seeds in areas (counties) where infections were found, prohibition to grow wheat for grain in infected fields (surrounded by a buffer zone within a radius of 3 miles), restrictions on the movement of wheat from infected areas. Later, the discovery of Karnal bunt-infected wheat in California extended the quarantine to parts of that State. A national survey was implemented by testing wheat samples from silos (or mills) in each county of wheat-producing states several times a year. Sampling in the field was done in 1996 in some cases, but was stopped in 1997 as it appeared that sampling in silos or mills was more effective (as grains are manipulated, teliospores are released and dispersed). Later in 1997, it was thought that T. indica had been found in south-eastern states (Alabama and Tennessee – EPPO RS 97/007), but the situation was extensively studied and the conclusion was that T. indica was not present in these states. The confusion arose because of the occurrence of a ryegrass bunt whose spores cannot be reliably distinguished from those of T. indica (both morphologically and by PCR). So far, it appears that the ryegrass bunt cannot attack wheat in the field, although artificial inoculation to wheat can be successful in the laboratory. The results of the 1997 survey showed that only one infected wheat sample was found in September from Texas (San Saba county), all other samples either from previously infested or non-infested areas were negative (over a total of 4100 samples tested by APHIS).
Finally, Bonde et al. gives some details on decontamination methods which have been used successfully in the USA. The ‘steam-flake milling’ method is effective in destroying T. indica teliospores. Grain is loaded in special steam cabinet towers and heated 30 min to 109°C, and then passed through rollers that compressed the steamed grain into flakes. Another method is used to decontaminate mill feed (bran etc.) which is then used for cattle feeding (if infected this could present a risk of potential introduction of viable teliospores into a field via animal waste). Mill feed is dry heated inside a ‘Holo-Flite Thermal Processor’ at 84, 101 or 110 °C for 12, 5 or 2 min respectively, and teliospores are efficiently killed.
Sources
Anonymous (1997) 1997 National Karnal Bunt Survey Plan, USDA-APHIS PPQ.
APHIS Web site on the INTERNET http://www.aphis.usda.gov
Bonde, M.R.; Peterson, G.L.; Schaad, N.W.; Smilanick, J. (1997) Karnal bunt of wheat.
Plant Disease, 81(12), 1370-1377.