FACTORS AFFECTING VIRULENCE AND PIGMENT PRODUCTION OF XANTHOMONAS PHASEOLI VAR. FUSCANS

Isolates of Xanthomonas phaseoli var. fuscans (Burkh.) Starr & Burkh. obtained from naturally infected bean (Phaseolus vulgaris L.) seeds stored at 10 C for 2–4 months were more virulent, as indicated by a stunting effect on Sanilac bean plants, than isolates obtained from seeds stored for 7–24 months. Neither the rate of growth of the organism nor its ability to produce a brown diffusible pigment was correlated with virulence. Growth rate and pigment production of all isolates in nutrient broth were similar and reached a maximum after 36 and 48 h respectively. In a medium containing inorganic salts and yeast extract, the growth rate of the organism was relatively slow and pigment development depended upon amendments added to the medium. Tyrosine enhanced, while glucose retarded, pigment production, but neither affected the growth of the pathogen adversely. The formation and color of the pigment in the media were not affected by pH values ranging from 5.5 to 9.0. The presence of living cells was essential for the continued production of the pigment. The melanoid nature of the pigment was demonstrated.