A PROPOSAL FOR A UNIFORM SCREENING PROCEDURE FOR THE REENHOUSE EVALUATION OF VARIABILITY OF XANTHOMONAS AXONOPODIS PV. PHASEOLI AND RESISTANCE ON LEAVES OF PHASEOLUS VULGARIS

摘要

Common bacterial blight of bean caused by Xanthomonas axonopodis pv. phaseoli (Xap) is a limiting factor in many common bean (Phaseohis vulgaris L.) production regions. Consequently, evaluation and screening for resistance have been conducted by different researchers in different countries using different techniques and scoring systems. There is a need for uniform and reproducible inoculation procedures to avoid duplications, and to produce reliable results. Leaves have been inoculated using spraying, multiple needles, razor blades, and pin pricking with flat needles or pipette tips. Results from studies using bacterial strains from Central America and the Caribbean have indicated variability of the pathogen and specificity of the host reaction of some bean genotypes. The methodology employed by Zapata to discriminate between strains of the pathogen and differential host reactions is based on a simple technique developed and employed successfully on leaves with coefficients of variability of 9.0 and 7.1 at 14 and 21 days, after inoculation, respectively. It also discriminates between Xap pathogenic races and host reactions with high level of resistance, moderate susceptibility and a high degree of susceptibility expressed by systemic infection. Insusceptible genotypes, 5% and 42% of systemic infection have been detected at 14 and 21 days after inoculation, respectively. Only one strain is recommended to be inoculated per trifoliolate leaf per plant using three replicates because some genotypes may have high susceptibility and get systemic infection. The following screening and scaling are proposed to utilize uniform methods for the determination of pathogenic variability and the evaluation of resistance of common bean germplasm to Xap under greenhouse conditions. The method is also recommended for P. acutifolius and P. coccineus.