Disease resistance in strawberry, like many other crops, is an ongoing and multifaceted issue. In California, strawberry production is primarily threatened by four soil-borne pathogens: Fusarium oxysporum f.sp. fragariae (Fusarium wilt), Macrophomina phaseolina (Macrophomina or charcoal rot), Phytophthora cactorum (Phytophthora root and crown rot, PhCR), and Verticillium dahliae (Verticillium wilt).
The Strawberry Research Group has discovered that complete resistance to Fusarium wilt can be conferred by any one of several single genes in California strawberry germplasm (Pincot, et al 2017, Pincot, et al 2022). All of the latest varieties released by the program contain at least one of these resistance genes. Ongoing work is focused on understanding the genetic and mechanistic basis of this resistance, using genetic mapping and RNAseq approaches. We are also investigating the efficacy of known resistance sources and wild relatives/heirloom cultivars against a new race of the pathogen that has recently emerged in California.
Partial resistance to Macrophomina and PhCR is conferred by one or a few quantitative resistance loci. The strawberry breeding program is using markers published or developed in-house to increase the frequency and strength of resistance to both diseases – this work has recently been published for PhCR (Jimenez, et al 2022) and is ongoing for Macrophomina.
For Verticillium wilt, no major resistance loci have been discovered in strawberry – however there is genetic variation in resistance, making genomic prediction a promising tool to assess and improve Verticillium wilt resistance in breeding germplasm (Pincot, et al 2020). Preliminary work has validated the potential of this approach, as well as suggested criteria for training populations needed for reinforcing the model used for the prediction.
In the next years, the Strawberry Research Group has planned an investigation into fruit rot diseases, including those caused by Colletotrichum sp. and Neopestalotiopsis sp. These studies, like those of the soil-borne diseases, will take advantage of the field facilities and controlled growth environments available to us at the University of California, Davis; we will first elucidate the genetic basis of resistance before translating that result into biological knowledge and tools for the strawberry breeding program and the strawberry industry.
Photos by Fred Greaves for UC Davis