Late blight (LB), caused by the oomycete Phytophthora infestans, is one of the most devastating diseases of the cultivated tomato (Solanum lycopersicum) worldwide. Most commercial cultivars of tomato are susceptible to LB. Previously, three major LB resistance genes (Ph-1, Ph-2, Ph-3) were identified and incorporated into a few commercial cultivars of tomato. Reduced effectiveness and potential breakdown of the resistance genes has necessitated identification, characterization and utilization of new sources of resistance. We evaluated the response of 67 accessions of the wild tomato species, S. pimpinellifolium to LB, under multiple field and greenhouse (GH) conditions and compared them with six control genotypes. Sixteen accessions were identified with strong LB resistance in both field and GH experiments. However, 12 accessions exhibited resistance similar to a control line which was homozygous for Ph-2 + Ph-3. Genotyping accessions with molecular markers for Ph-2 and Ph-3 were not conclusive, indicating that resistance in these accessions could be due to these or other resistance genes. Strong correlations were observed between field and GH disease response and between foliar and stem infection.

Agricultural production is hampered by disease, pests, and environmental stresses. To minimize yield loss, it is important to develop crop cultivars with resistance or tolerance to their respective biotic and abiotic constraints. Transformation techniques are not optimized for many species and desirable cultivars may not be amenable to genetic transformation, necessitating inferior cultivar usage and time-consuming introgression through backcrossing to the preferred variety. Overcoming these limitations will greatly facilitate the development of disease, insect, and abiotic stress tolerant crops. One such avenue for rapid crop improvement is the development of viral systems to deliver CRISPR/Cas-based genome editing technology to plants to generate targeted beneficial mutations. Viral delivery of genomic editing constructs can theoretically be applied to span the entire host range of the virus utilized, circumventing the challenges associated with traditional transformation and breeding techniques. Here we explore the types of viruses that have been optimized for CRISPR/Cas9 delivery, the phenotypic outcomes achieved in recent studies, and discuss the future potential of this rapidly advancing technology.

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Core Ideas Genetic characterization of a new source of late blight resistance in tomato Identification of new SNP markers using GBS approach Development of a new genetic map of tomato Late blight (LB), caused by the oomycete Phytophthora infestans (Mont.) de Bary, is one of the most devastating diseases of tomato (Solanum lycopersicum L.) and potato (S. tuberosum L.) worldwide. The importance of LB on tomato has increased due to the occurrence of aggressive and fungicide‐resistant clonal lineages of P. infestans. Consequently, identification and characterization of new sources of genetic resistance to LB has become a priority in tomato breeding. Previously, we reported accession PI 163245 as a promising source of highly heritable LB resistance for tomato breeding. The purpose of this study was to identify and map quantitative trait loci (QTLs) associated with LB resistance in this accession using a trait‐based marker analysis (a.k.a. selective genotyping). An F2 mapping population (n = 560) derived from a cross between a LB‐susceptible tomato breeding line (Fla. 8059) and PI 163245 was screened for LB resistance, and the most resistant (n = 39) and susceptible (n = 35) individuals were selected for genotyping. Sequencing and comparison of the reduced representation libraries (RRLs) derived from genomic DNA of the two parents resulted in the identification of 33,541 putative single nucleotide polymorphism (SNP) markers, of which, 233 genome‐wide markers were used to genotype the 74 selected F2 individuals. The marker analysis resulted in the identification of four LB resistance QTLs conferred by PI 163245, located on chromosomes 2, 3, 10, and 11. Research is underway to develop near‐isogenic lines (NILs) for fine mapping the QTLs and develop tomato breeding lines with LB resistance introduced from PI 163245.