California produces over 95% of the processing tomatoes grown in the United States. High salinity of soils plague many of the top producing counties in CA, particularly Fresno county which accounts for 30% of production. In these areas, salt accumulation in soil leads to detrimental alterations of soil chemistry and elevated soil electrical conductivity (EC). Although tomatoes are moderately tolerant to soil salinity, they decline steeply in yield at soil EC levels ≥ 3 dS/m. Another threat to processing tomato yields is Fusarium wilt of tomato, caused by the soilborne fungal pathogen Fusarium oxysporum f. sp. lycopersici race 3 (Fol race 3). Fol race 3 was limited to the Sutter basin for decades, but over the past ten years the pathogen has spread throughout all major tomato growing regions in the state. Fusarium wilt is managed by planting tolerant (F2) or resistant (F3) cultivars (cvs); however, Fol race 3 has been positively diagnosed in symptomatic F3 cvs and in both F2 and F3 cultivars there are cases of rapid within-field spread in areas where salt stress may be playing a role (such as Fresno county). In order to establish a relationship between soil salinity and Fusarium wilt losses, soil surveys were conducted in 2018 and 2019 in tomato fields that had high Fusarium wilt pressure. In two fields that had especially bad losses due to rapid spread of Fusarium wilt, the soil EC was elevated (~3.6 mS/cm), supporting the hypothesis that soil can influence Fusarium wilt. To test the hypothtesis that salt stress may be compromising the effectiveness of tolerant F2 and resistant F3 cvs.
Field trials were conducted in 2018 and 2019 to look at the effects of NaCl and CaCl2 on the incidence and severity of Fusarium wilt in a tolerant F2 and and three F3 cvs. At the time of harvest, disease incidence was 29% higher under the salt treatment in the tolerant cv HM 3887, and in 2018, disease only developed under salt. In F3 cultivars, Fusarium wilt only developed under the salt treatment in H 1310 (3.7% of plants) in 2018 and in HM 58801 (1.7% of plants) in 2019 compared with 0% in the no salt treatment. N 6428 showed symptoms in both salt and no salt treatments in both years of the trial with slightly higher incidence under salt; 6.7% of plants developed symptoms under the salt treatment as compared with 3% of plants developing symptoms in the no salt treatment.
To test the hypothesis that disease enhancement under salt could be attributed in part to enhancing colonization of the host, controlled trials with a tolerant F2 cv were conducted. In the presence of salt (NaCl + CaCl2),incidence of systemic (shoot) colonization was 158% to 207% higher at medium and low levels of inoculum, respectively. Further, in the first trial, root colonization only occurred under salt at medium inoculum loads. These results indicate NaCl salt-mediated increases in root and stem colonization may contribute to disease enhancement seen in field studies.
To test the hypothesis that disease enhancement could be attributed in part to direct effects on the pathogen Fol race 3, we initiated controlled studies to examine the effects on both survival of Fol race 3 in soil and effects of various salts on pathogen sporulation. In soil survival trials, under soil salt treatment with NaCl and CaCl2, Fol race 3 colony forming units (CFUs) were 66% lower in the salt treatment than the no salt controls at 120 days post infestation (DPInf) (P = 0.029). In vitro sporulation trials revealed that spore loads had increased 3 in all salt treatments(NaCl, CaCl2, and Na2SO4) as well as no-salt controls by 90 DPInf, with no differences between treatments. At this same timepoint, the osmotic control, polyethylene glycol (PEG 1500), decreased CFU viability by ~80% as compared with no salt controls indicate that the effects of salt on Fol race 3 viability were due to specific ion effects and not osmotic stress. Together, these results indicate that salinity-mediated Fusarium wilt enhancement is not due to direct effects on the pathogen. To the author's knowledge, this research is the first multi-year study to document the effects of salinity in enhancing Fusarium wilt of processing tomato and the first to document that salt compromises I3 resistance gene efficacy; colonization and survival studies provide important insights into the mechanistic basis for salt-mediated disease enhancement. Although these studies were limited in scope, this research has broad implications for moving towards a more customized, integrated approach to Fusarium wilt and soil salinity co-management in processing tomato.
|Advisor:||Swett, Cassandra L|
|Commitee:||Bostock, Richard M, Rizzo, David M|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||MAI 82/2(E), Masters Abstracts International|
|Subjects:||Plant Pathology, Agronomy|
|Keywords:||Fusarium wilt, Salt stress, Tomato|
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