THE RELATIVE IMPORTANCE OF LATENT TUBER AND SOIL INFESTATION BY RALSTONIA SOLANACEARUM ON THE INCIDENCE OF BACTERIAL WILT OF POTATO
Pradhanang P.M.1, Elphinstone J.G.2
1
2 Central Science Laboratory, Sand Hutton, York, YO4 1LZ, UK.
Field experiments were conducted in Ghandruk, Western Nepal (2000 masl) in 1995 and 1996 to determine the relevant importance of latent tuber infection and soil infestation by race 3 of Ralstonia solanacearum on the incidence of bacterial wilt of potato. Two sources of seed potato were compared: (i) Pre-basic seed (PBS) obtained from the national seed programme and (ii) farmer-saved seed (FSS) from R. solanacearum infested fields in Ghandruk. Latent vascular R. solanacearum infection was not detected in the PBS, whereas 15% to 19% of FSS was latently infected. Three soils with different natural inoculum densities were compared; one non-infested soil, and two infested soils where bacterial wilt (BW) affected potatoes had been grown 7 or 18 months prior to planting of the experimental potato crop.
Differences in wilt incidence with seed health status were highly significant (p < 0.0001) and the effect was consistent in both years. Similarly, differences in wilt incidence with different levels of soil infestation were also significant with average wilt incidences of 48.2% (p = 0.009) and 63.2% (p < 0.0001) respectively in 1995 and 1996 in infested plots with 7-month break between successive potato crops irrespective of seed health status. However, mean wilt incidence (32.5%) in non-infested and infested soil with an 18-month break period between potato crops did not significantly differ. Rhizosphere populations of R. solanacearum, detected using semi-selective medium and indirect ELISA, were significantly affected by seed health status (p=0.016) and soil inoculum density (p = 0.051) in 1995 and the effects were consistent in both years. Populations of R. solanacearum in the potato rhizosphere in infested plots often exceeded 105 colony forming units per g dry soil and were readily detected by indirect ELISA.
In 14% to 29% of FSS, sunken skin spot symptoms were observed on the tuber surface. High populations of R. solanacearum were isolated from all potato tuber tissue with these symptoms. The pathogen was also easily detected in tubers with typical brown rot (BR) symptoms when tissue cores from heel ends were tested. However, R. solanacearum was often found to be present in sunken skin spot tissues but absent in vascular tissue in tubers where brown rot symptoms were not evident. The reliability of tuber indexing methods which are based on testing of vascular cores only may therefore require further examination.