Evaluation of bacterial wilt resistance sources for presence of major resistance QTLs Bwr-12 and Bwr-6, and as potential sources of new resistance QTLs

Bacterial wilt (BW) (caused by three Ralstonia species) is one of the most destructive tomato diseases in the tropics and subtropics. If available, resistant cultivars would be the easiest means to manage the disease although few such cultivars are available. Resistance sources, notably those from Hawaii, the Philippines, and North Carolina, have been identified and exploited. However, expression of resistance is strongly influenced by environmental conditions such as soil temperature, pH, and moisture, and is linked to undesirable horticultural traits such as small fruit size and cracking. Two major genomic regions conditioning BW resistance have been identified: Bwr-12 and Bwr-6, associated with resistance to phylotype I (Asia phylotype, now classified as R. pseudosolanacearum), and phylotype II (America phylotype remains as R. solanacearum) isolates, respectively. Considering the vast diversity of this pathogen, there is great need to identify and exploit additional BW resistance sources. A set of BW resistance sources collected over the years by WorldVeg Tomato breeding was characterized for presence of Bwr-12 and Bwr-6, and screened in a WorldVeg greenhouse by drench inoculation for reaction to one isolate each of R. pseudosolanacearum (Pss4) and R. solanacearum (Pss1632) in 2017 and 2018. Based on the percent of healthy plants four weeks after inoculation, tomato entries H7996 and other entries homozygous for Bwr-12 and all or parts of Bwr-6 showed relatively high resistance levels. A few entries that were heterogeneous for or lacking Bwr-12 and/or Bwr-6 were identified that might harbor new BW resistance.