So far, we have generated 928 insertion mutants of CL13 by transformation with pSMUT in the presence of BamH! as described (see Materials and methods). Each transformant was grown in liquid culture and used to infect 4 plants initially. Strains that showed a Tum- phenotype in the first screen were rescored by infecting another 20 plants each. Only such strains were considered nonpathogenic that induced neither anthocyanin biosynthesis in the infected tissue nor the formation of plant tumors. By these criteria, 13 of the mutant strains tested were nonpathogenic This shows that pathogenicity defects arise with a frequency of 1.4%.