Multiplication of Xanthomonas campestris pv. Phaseoli and Intercellular Enzyme Activities in Resistant and Susceptible Beans

In order to assess resistance to common bean blight, populations of two isolates of Xanthomonas campestris pv. phaseoli were monitored in leaves of two Phaseolus vulgaris breeding lines, BLT87-2 (susceptible) and OAC88-1 (partially resistant) and a resistant tepary bean accession, P. acutifolius P. I. 440795. The breeding line OAC88-1 possesses resistance to common bacterial blight which was incorporated from P. acutifolius by an interspecific cross. In susceptible, leaves, bacterial populations increased to 10⁸ CFU/g leaf at 3 wk after inoculation whereas, in resistant leaves, bacterial populations declined to 10¹ - 10³ CFU/g leaf. In partially resistant leaves the population first declined similar to that in resistant P. acutifolius but later increased, and typical bacterial blight symptoms appeared. Cellulase, protease and amylase activities were monitored in culture and intercellular leaf spaces. Only cellulase activity was, clearly related to bacterial growth in the susceptible host; other enzyme activities were variable in their relationship to host resistance and bacterial growth. Differences between strains in cellulase activity inside partially resistant leaves corresponded to their ability to secrete cellulase in culture. Measuring cellulase activity in intercellular wash fluids may be a simple and sensitive method for determining X. c. phaseoli populations in leaves.