2019 ASHS Annual Conference

For over 50 years soil fumigation with methyl bromide was a common practice used to suppress soil-borne pathogen populations in strawberry until its complete ban in 2016. The efficacy of alternative chemicals for soil fumigation appears to be significantly lower than methyl bromide, which greatly increases the risk of yield losses caused by soil-borne pathogens. Fusarium oxysporum f. sp. fragariae, the pathogen that causes Fusarium wilt, has emerged as one of the most problematic and widespread soil-borne pathogens in organic and post-methyl bromide production systems. We previously described a dominant gene (Fw1) that confers resistance to Fusarium wilt. The most predictive SNP (AX-166521396) associated with the resistant Fw1 allele, had a frequency of 0.18 in the UC Davis germplasm collection. AX-166521396 accurately predicted Fusarium wilt resistance phenotypes in 97.3% of the germplasm collection. Three heirloom cultivars (Wiltguard, Guardian, and Earliglow) identified in the previous study were resistant to Fusarium wilt but shared the DNA marker haplotype associated with the susceptible allele (fw1). We hypothesized that these cultivars carry novel R-genes. To explore this further, we phenotyped segregating populations developed by self-pollinating Earliglow, Wiltguard, and Guardian. Two of these populations segregated for resistance to Fusarium wilt and were genotyped with a 49,000-SNP genotyping array—the Earliglow population was completely resistant. Here, we show that Wiltguard and Guardian carry novel dominant R-genes. Guardian carries Fw2 which mapped to chromosome 2-4 in close proximity to Fw1. Mapping of Fw3 in Wiltguard is underway. We developed sub-genome specific SNP markers to accelerate the introduction of R-genes from these heirloom cultivars into modern cultivars through marker-assisted selection.