Abstract
Potato bacterial wilt caused by Ralstonia solanacearum is one of the most destructive diseases in potato production areas worldwide. One-hundred-six colonies were obtained from a single typical colony recovered from a potato tuber showing internal brown rot symptoms. Colonies' identity was confirmed as R. solanacearum phylotype II sequevar 1 via PCR. Based on the morphological characterization and the average attenuation index (AI), colonies grouped into typical, intermediate, and atypical forms. However, the small variations in AI make it very difficult to make sure identification and this index cannot be a definitive identification system for R. solanacearum forms. The atypical colonies did not revert to typical or intermediate forms, whereas the both latter ones changed to the three forms with different percentages. For forthcoming experiments, two subcultures of each morphological form were selected. The pathogenicity experiments showed that these subcultures were also grouped into three forms: virulent, intermediate and avirulent. The source of peptone in the agar medium caused significant effects on R. solanacearum colonial morphological characteristics and its pathogenesis. Consequently, it is recommended to use one source of peptone during subculturing and maintaining R. solanacearum forms, as well as studying host-pathogen interaction. The subcultures of R. solanacearum produced a brown diffusible pigment from L-tyrosine on TZC media devoid of ascorbic acid but 1 mM ascorbic acid affected pigmentation. It is suggested that the efficacy of ascorbic acid as a plant-resistance inducer in controlling bacterial wilt disease should be carefully evaluated as it apparently favors R. solanacearum inside the plant tissues.