Tomato breeding for resistance to Ralstonia wilt

Ralstonia wilt (RW) caused by bacteria of the Ralstonia solanacearum species complex is a major disease of Solanaceous crops throughout the world. As part of a RW resistance and control research program, a set of World Vegetable Center tomato breeding lines, nine commercial tomato rootstock hybrids from Japanese seed companies and 15 new hybrid tomato lines from Fengshan Tropical Horticultural Experiment Branch (FTHES) were screened for reaction to inoculation with a mixture of eight different R. solanacearum (race 1) isolates collected from various Solanaceous hosts in Taiwan in several different experiments. Hawaii 7996 (H7996), a commonly recognized source of RW resistance was used as resistant/tolerant control, while ‘Cui Hong’ was the susceptible check. The wilt symptoms and wilted plants were scored and recorded weekly with a disease rating scale of 0-5 (0 = no symptoms – 5 = severe symptoms) for four weeks after inoculation. A mean disease index (DI) for each accession was calculated. Daily maximum and minimum temperatures were collected from a local weather station and the maximums were found to be 3-5C lower than the maximums recorded within the plastic house used for the experiments. During the hot summer of 2014 even the H7996 showed no resistance, whereas during the cooler autumn (mean temperature 23.0°C, CWB) H7796 presented a mean DI of 36.6% (moderately resistant) and 11 WorldVeg breeding lines were more resistant than the H7996 with mean DIs ranging from 2.2 to 26.9%. When the nine commercial tomato rootstock hybrids were screened in the late autumn (mean temperature 21.2°C, CWB), five developed DIs lower than ‘H7996’ Additionally, the 15 new hybrid tomato lines showed greater resistance than ‘H7996’ (73.7%) when temperatures were favorable for disease development (average mean temperature 24.6°C, CWB). These results indicate that resistance of tomato lines against R. solanacearum is very temperature dependent, but also that there are some promising new lines and hybrids with greater resistance than the standard check H7996.