Location: Crop Improvement and Protection Research
2011 Annual Report
There are two distinct pathogenic races of V. dahliae causing Verticillium wilt on lettuce. While resistance to race 1 is known, the existence of race 2 isolates in California lettuce production fields is certain. Moreover, it is highly probable that widespread use of race 1 resistant germplasm will select for and increase the economic damage caused by race 2 isolates. Therefore, identification and subsequent development of lettuce cultivars with resistance to race 2 is imperative for sustaining the lettuce industry.
We are currently screening the Western Regional Plant Introduction Station (WRPIS) Lactuca collection for resistance to race 2 isolates of V. dahliae. More specifically, our research is using a working collection of the WRPIS located in Salinas, CA. Our strategy uses greenhouse testing to screen up to eight plants of 160 accessions per year in unreplicated plots to identify candidate sources of resistance. This is followed by replicated greenhouse and field-micro-plot experiments to confirm resistance. To date, we have screened 824 L. sativa and L. serriola accessions using race 2 V. dahliae isolate VdLs17. We have confirmed partial resistance (disease incidence significantly lower than ‘Salinas’) in four accessions (PIs 169511, 171674, 204707, 226641). However, all of these PIs have had at least a few symptomatic plants, and all but PI 171674 have had non-symptomatic plants that are nonetheless colonized by V. dahliae. Complete resistance to race 2 has not yet been found, and we will continue to screen the collection in hopes of finding this trait. This includes new L. serriola accessions collected through the efforts of the USDA, Plant Exchange Office.