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 Karnal bunt-resistant cultivars, one moderately susceptible, and two Karnal bunt highly susceptible wheat cultivars. The five resistant cultivars represented the most Karnal bunt-resistant germ plasm in the breeding programs at the International Maize and Wheat Improvement Center (Centro Internacional de Mejoramiento de Maiz y Trigo [CIMMYT]), Mexico, and the Department of Plant Breeding, Punjab Agricultural University, Ludhiana, India. Rants at the boot stage were inoculated by injecting into the boot 1 ml of a water suspension containing 10,000 allantoid sporidia per ml, incubated in a mist chamber for 3 days, then maintained until maturity in a greenhouse. All inoculated and control wheat spikes were harvested individually, and percentages of T. indica-infected seeds were determined. In addition, infected seeds from 10 randomly selected infected spikes per treatment were examined to estimate the proportion of each infected seed converted to a sorus. On the most resistant wheat cultivar (HD-29), percentage of seeds infected varied from 10 to 30%, depending on pathogen aggressiveness. On the most susceptible cultivar (Bacanora), infection varied from 55 to 84%. Although there were differences in pathogen aggressiveness, there was no evidence of the existence of races among the field populations. Wheat cultivars 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 pathogen population when comparing resistant versus other wheat cultivars.