Since its appearance in 1995, Verticillium wilt of lettuce has spread through the Salinas River Valley, where nearly 60% of California's lettuce acreage is located. A replicated field trial was conducted to assess various modern and heirloom lettuce (Lactuca sativa) cultivars, plant introductions, and L. virosa lines for resistance to Verticillium wilt. Based on horticultural type, lettuce plants were destructively sampled at harvest maturity and assessed for the incidence of Verticillium wilt. Of the L. sativa cultivars, only the iceberg type displayed pronounced foliar symptoms of stunting and wilting. Disease incidence based on root symptoms ranged from 0% to 100%, with continuous variation found across and within lettuce types. Most cos, crisphead, and leaf cultivars exhibited 20% or greater disease incidence. Butter cultivars exhibited the lowest disease incidence among the major lettuce types examined, and Latin and Batavia type cultivars exhibited the lowest disease incidence overall. Disease progression was further monitored for 10 select lettuce cultivars for 2 weeks past harvest maturity. Disease intensity increased over the 2-week period for some cultivars, demonstrating the need to assess plants for Verticillium wilt past harvest maturity to avoid misclassifying plants. The L. sativa plant introduction lines tested, predominantly stem and oil-seed horticultural types, were quite susceptible and exhibited distinct symptoms of wilt and defoliation, possibly due to their elongated growth habit. The variation in disease incidence among the L. virosa lines tested was discontinuous, with discrete differences in susceptibility. Overall, the results reflected trends found in previous greenhouse and field trials.
Fusarium wilt of tomato (Solanum lycopersicum), caused by fungal pathogen Fusarium oxysporum f. sp. lycopersici (Fol), is one of the most important diseases in tomato production. Three races of the pathogen are described, and race-specific resistance genes have been applied in commercial tomato cultivars for controlling the disease. Race 3 (Fol3) threatens tomato production in many regions around the world, and novel resistance resources could expand the diversity and durability of Fol resistance. The wild tomato species, Solanum pennellii, is reported to harbor broad resistance to Fol and was the source of two known Fol3 resistance genes. In this study, we evaluated 42 S. pennellii accessions for resistance to each fusarium wilt race. F1 plants, developed from crossing each accession with the Fol3 susceptible line ‘Suncoast’, were evaluated for Fol3 resistance, and BC1F1 plants were screened to determine the likelihood that Fol3 resistance was based on a novel locus (loci). Nearly all accessions showed resistance to Fol3, and many accessions were resistant to all races. Evaluation of F1 plants indicated a dominant resistance effect to Fol3 from most accessions. Genetic analysis indicated 24 accessions are expected to contain one or more novel Fol3 resistance loci other than an allele near the I-3 locus. To investigate genetic structure of the S. pennellii accessions used in this study, we genotyped all 42 accessions using genotyping by sequencing. Approximately 20% of the single nucleotide polymorphism (SNP) loci were heterozygous across accessions, likely due to the outcrossing nature of the species. Genetic structure analysis at 49,120 unique SNP loci across accessions identified small but obvious genetic differentiations.