Assessment of Temperature-Independent Resistance against Bacterial Wilt Using Major QTL in Cultivated Tomato ( L.).

Bacterial wilt () is a devastating disease of cultivated tomato resulting in severe yield loss. Since chemicals are often ineffective in controlling this soil-borne pathogen, quantitative trait loci (QTL) conferring host resistance have been extensively explored. In this study, we investigated effects of ambient temperature and major QTL on bacterial wilt resistance in a collection of 50 tomato varieties. The five-week-old seedlings were inoculated using the race 1 (biovar 4 and phylotype I) strain of and placed at growth chambers with three different temperatures (24 °C, 28 °C, and 36 °C). Disease severity was evaluated for seven days after inoculation using the 1-5 rating scales. Consistent bacterial wilt resistance was observed in 25 tomato varieties (R group) with the means of 1.16-1.44 for disease severity at all three temperatures. Similarly, 10 susceptible varieties with the means of 4.37-4.73 (S group) were temperature-independent. However, the other 15 varieties (R/S group) showed moderate levels of resistance at both 24 °C (1.84) and 28 °C (2.16), while they were highly susceptible with a mean of 4.20 at 36 °C. The temperature-dependent responses in the R/S group were supported by pairwise estimates of the Pearson correlation coefficients. Genotyping for three major QTL (, and ) found that 92% of varieties in the R group had ≥ two QTL and 40% of varieties in the R/S group had one or two QTL. This suggests that these QTL are important for stability of resistance against bacterial wilt at high ambient temperature. The resulting 25 varieties with temperature-independent resistance will be a useful resource to develop elite cultivars in tomato breeding programs.