INOCULUM SOURCES, DISEASE INCIDENCE AND SEVERITY OF BACTERIAL BLACKLEG AND SOFT ROT OF POTATO

Comprehensive surveys during spring, summer and fall, 2007-2009 were conducted in the major potato-growing areas of KPK to record the incidence, severity and distribution of blackleg and soft rot of potato caused by the different subspecies of Erwinia. Maximum disease severity (40%) and disease incidence (60-75%) was found in the spring and summer potato crop grown in Abbottabad, Swat and Mardan, Pakistan. The frequencies of Erwinia carotovora subsp. atroseptica (Eca), E. c. subsp. carotovora (Ecc) and Erwinia chrysanthemi (Ech), based upon the survey of 42 locations, were found to be 48%, 45%, and 7%, respectively. Regarding soil, diseased plant debris and seed potatoes as sources of inoculum for Eca, healthy plant material, potted un-sterilized soil and healthy seed potatoes were inoculated with Eca, kept in screen-house and monitored for changes in bacterial population over a period of three months. It was found that seed potatoes were the most important source of primary inoculum as the bacterial population considerably increased in numbers throughout the monitoring period. The bacterial population in the inoculated plant debris did decrease but the rate of decline was quite slow and therefore, diseased plant debris could be considered as a good source of primary inoculum for the appearance of black leg disease in the next season.