COMPARISON OF THE VIRULENCE OF ISOLATES OF TILLETIA-INDICA, CAUSAL AGENT OF KARNAL BUNT OF WHEAT, FROM INDIA, PAKISTAN, AND MEXICO

Four Tilletia indica teliospore field populations, two from Mexico and one each from India and Pakistan, were tested for virulence on five K arnal bunt-resistant cultivars, one moderately susceptible, and two Ka rnal bunt highly susceptible wheat cultivars. The five resistant culti vars represented the most Karnal bunt-resistant germ plasm in the bree ding programs at the International Maize and Wheat Improvement Center (Centro Internacional de Mejoramiento de Maiz y Trigo [CIMMYT]), Mexic o, and the Department of Plant Breeding, Punjab Agricultural Universit y, Ludhiana, India. Rants at the boot stage were inoculated by injecti ng into the boot 1 ml of a water suspension containing 10,000 allantoi d sporidia per ml, incubated in a mist chamber for 3 days, then mainta ined until maturity in a greenhouse. All inoculated and control wheat spikes were harvested individually, and percentages of T. indica-infec ted seeds were determined. In addition, infected seeds from 10 randoml y selected infected spikes per treatment were examined to estimate the proportion of each infected seed converted to a sorus. On the most re sistant wheat cultivar (HD-29), percentage of seeds infected varied fr om 10 to 30%, depending on pathogen aggressiveness. On the most suscep tible cultivar (Bacanora), infection varied from 55 to 84%. Although t here were differences in pathogen aggressiveness, there was no evidenc e of the existence of races among the field populations. Wheat cultiva rs resistant to the Mexican fungal populations also were resistant to those from Asia, and vice versa; there was a significant correlation ( P less than or equal to 0.05) between percentage of seeds infected and extent of fungal colonization of infected seeds with all but one path ogen population when comparing resistant versus other wheat cultivars.