CHARACTERIZATION OF THE CAUSAL ORGANISM OF BLACKLEG AND SOFT ROT OF POTATO, AND MANAGEMENT OF THE DISEASE WITH BALANCED FERTILIZATION

Based upon colony morphology, physio-biochemical tests and polymerase chain reaction (using species or subspecies-specific primers) studies, 20 isolates (out of a total of 42) were found to be Erwinia carotovora subspecies atroseptica (Eca), 19 were identified as Erwinia carotovora subspecies carotovora (Ecc), and 3 as Erwinia chrysanthemi (Ech). Results of the subspecies-differentiating biochemical tests indicated that majority of the candidate Ecc isolates did not produce acid from alpha-methyle glucoside (as expected) but their reaction to the production of reducing substances from sucrose was variable. Likewise, some of our Eca and Ecc strains (unexpectedly) were sensitive to erythromycin. Also, most of our Eca strains unexpectedly grew at 36 degrees C. Our strains slightly deviate from the standard description in some of their minor characteristics but they still remain the valid members of the Eca, Ecc or Ech group as similar variations in minor characteristics have been found by other workers. The occurrence of intermediate forms of Eca and Ecc (sharing some of the characteristics of both the groups) indicates variability happening among these strains. This variability indicates the potential ability of the pathogen to break the resistance of the host. The results of the effect of balanced nutrition in controlling blackleg and soft rot of potatoes indicated that the fertilizer combination of N3P1K3 (262/252/262 kg.ha(-1)) which is slightly higher than the normally practiced dose (247/247/247 kg.ha(-1)) was the best in bringing the disease to a minimum and subsequently increasing the yield.