UC Davis

California produces over 97% of the processing tomatoes in the United States. An emerging threat to processing tomato production is Fusarium falciforme, a soilborne fungal pathogen that causes severe stem rot and premature vine decline. Early season vine decline predisposes fruit to sunburn and subsequent fruit rot, reducing fruit quality and causing severe yield losses. This pathogen has been reported in the top tomato-producing counties in California. At the time this study was initiated, no known management options were available for this disease. Some economically important soilborne diseases have been successfully managed by deploying tolerant and resistant cultivars. In order to develop a cultivar-based management strategy, commercial processing tomato cultivars were screened for resistance to F. falciforme. Complete resistance was not observed in this study; however, commercial cultivars varied in susceptibility to F. falciforme. Tolerant cultivars — N6428, HM4909, H8504, HM58841, and H1776 — wherein yield was less affected by F. falciforme, were identified. Several of these cultivars had extended field holding traits (that enables fruit to retain quality in the field an extended time) which might contribute to improved performance. Conversely, susceptible cultivars — HM3887, H5608, N6416, H9663, and H1310 — that had statistically significant yield reductions and economically significant impacts in fruit quality were identified and should be avoided when F. falciforme is known to occur in a field. Disease phenotypes that corresponded to susceptibility were high incidence of early season vine decline (5 weeks pre-harvest) and vine decline at harvest. Foot / stem rot incidence and severity did not correspond to yield performance, as 80-100% of plants developed rot across all cultivars. In addition, we evaluated the consistency of cultivar performance across years and sites having different inoculum pressure to determine how variable environmental, and other factors impact disease severity. Of the nine cultivars under study, we observed that only N6428 consistently performed well across sites and years, emphasizing the importance of assessing the stability of field tolerance over growing seasons and locations. To test the hypothesis that resistance to the Fusarium wilt pathogen, Fusarium oxysporum f. sp. lycopersici race 3 (I3 resistance gene), and the Fusarium crown and root rot pathogen, Fusarium oxysporum f. sp. radicis-lycopersici (Frl resistance gene), contributes to F. falciforme tolerance, we included cultivars with the I3 gene (F3 cultivars) and the Frl gene (Fr cultivars) in our variety screenings. F3 cultivars ranged in susceptibility to F. falciforme, indicating that the F3 resistance gene did not contribute to tolerance. Fusarium falciforme symptoms, including foliar chlorosis, necrotic flecking and sometimes leaf deformity, affect the entire plant, which might mean that this is a systemically colonizing pathogen. To test the hypothesis that F. falciforme can systemically colonize tomato plants, we evaluated seeds from artificially infected plants for colonization. We did not observe F. falciforme colonization at a detection threshold of 0.0002%, indicating that F. falciforme is not seedborne. As these and parallel studies indicate that F. falciforme may produce phytotoxic secondary metabolites, we assessed the impact F. falciforme infection had on seed viability (germination rates) across cultivars. We observed that the highly susceptible cultivar, HM3887, had reduced seed viability; however, there was a wide range in seed response to F. falciforme. Together, these results indicate that F. falciforme-infected tomato seeds do not provide a means of pathogen dispersal, although there may be impacts of the pathogen on tomato seed viability. In conclusion, F. falciforme is a pathogen of significant impacts to both yield and seed viability; however, this pathogen can be managed using tolerant cultivars.