PREVALENCE, ETIOLOGY AND MANAGEMENT OF BLACKLEG AND SOFT ROT OF POTATO

Abstract

Potato (Solanum tuberosum L.) is the most important vegetable crop of the world, including Pakistan, ranking number one among all vegetables both in production and consumption. Although bacterial diseases (such as blackleg and soft rot) cause substantial damage to potato crop in Khyber Pukhtunkhwa, yet no significant research has been done so far on any aspect of these diseases. We conducted extensive surveys (2007-2009) to document the status of these diseases in our province and found that disease severity and incidence, at different locations, ranged from 2-40 % and 5-75 %, respectively. The frequency of the three causal organisms was found to be 47.61% for Erwinia carotovora subsp. atroseptica (Eca), 45.23% for E. carotovora subsp. carotovora (Ecc), and 7.14% for E. chrysanthemi (Ech). As there is some debate about the role of tuber-borne, soil-borne, and diseased plant-debris-borne primary inoculum of Eca in causing primary infections, we studied these sources of inocula. The results indicated that the pathogen population in inoculated tubers kept increasing during the entire duration (80 days) of the study whereas that in diseased plant debris declined very slowly. No bacteria could be detected in artificially inoculated soil after two months. Therefore, it was concluded that the infected tubers are the best source of inoculum followed by diseased plant debris. Because studying pathogen population helps in the control of the disease it causes, therefore, we were interested in finding diversity among the isolates of the pathogens causing potato blackleg and soft rot. The intra-specific diversity among the isolates of Ecc, and Ech is well known but that among the isolates of Eca is debatable. For this reason, we used randomnly amplified polymorphic DNA (RAPD) to amplify DNA from 20 Eca isolates using 2 selected random primers. The data (bands) generated were used to perform cluster analysis using the un-weighted pair-group method with arithmetical average (UPGMA) of Genetyx software, version 7.0 and the viiresulting 4 main clusters were presented as dendrogram. We were also interested in knowing if the genotypic differences among the Eca isolates would be correlated to some phenotypic character; so, we used potato stem rot assay to divide the 20 Eca isolates among 4 aggressiveness groups. Our data suggested that although the isolates genetically differed from each other by as much as 40%, these differences were not tightly correlated to the pathogenic differences found among them. Because blackleg and soft rot of potato is a hard-to-control disease, we were interested in an integrated management of the disease. The components of the integrated management that we studied included: (i) use of different plant extracts as seed tuber treatment to kill the contaminating blackleg and soft rot bacteria, (ii) use of balanced nutrition to enhance the competitiveness of the plant host against the pathogen, and (iii) finding resistance, if any, against blackleg and soft rot of potatoes among the popular commercial potato cultivars available to local farmers. Among the different aqueous extracts used, neem extract was found to be the best which decreased the disease incidence and disease severity by 71.18% and 56.86%, respectively as compared to the control. Neem extract also increased yield per plant by 344.44% as compared to that of control. The results of the host fertilization studies concluded that the fertilizer combination of N 3 P 1 K 3 (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. In case of varietal screening, based upon our overall results, no variety was found to be resistant against blackleg and soft rot of potatoes. However, field performance of variety Kuroda was comparatively better than other varieties whereas variety Desiree seemed to be the most susceptible.