RESISTANCE OF COMMON BEAN TO HALO BLIGHT AND PROGRESS OF THE DISEASE IN THE VALLEY OF MEXICO

Abstract

Halo blight caused by Pseudomonas syringae pv. phaseolicola (P. syringae) frequently affects rainfed bean (Phaseolus vulgaris) in the summer and decreases seed yield. Furthermore, inoculum is diseminated by rain and seed. The disease is controlled by cultural practices, use of seed free of the disease, agrochemicals and resistant cultivars. The aims of this research were: a) to identify sources of resistant to P. syringae in 32 elite bean genotypes and b) to evaluate the progress of the disease throughout the crop growth cycle. In 1997 two experiments were carried out to: 1) characterize 32 bean genotypes sown on April 24, May 8 and June 12, under rainfeed conditions during the summer. The disease was evaluated at the vegetative and reproductive stages using a scale from one (resistant) to nine (susceptible), and 2) evaluate the progress of the disease in 10 genotypes sown on April 24 and May 8, by checking ten plants four times when the initial symptoms were observed. In the first trial Pinto Villa, Amarillo de Calpan and Puebla 152 were immune and other eight were resistant, most of them of the Jalisco race, while MDRK and Cacahuate 72 from the Nueva Granada race and of determinate growth habit were susceptible. In the second trial, Mexico 332 and G 13746 were immune and Flor de Mayo Bajío and Negro Tacana showed a delayed infection. Among the genotypes, the progress of the disease and the area under the curve displayed different latent periods and severity grades, suggesting the presence of different interactions and possibly several races of P. syringae in the Valley of Mexico, acting alone or in concert, resistant.