Combining nonpathogenic strains of Fusarium oxysporum with sodium chloride to suppress fusarium crown rot of asparagus in replanted fields

Nonpathogenic strains of Fusarium oxysporum (NPFO) were tested alone, and in combination with rock salt (NaCl), for their ability to suppress fusarium crown and root rot of asparagus in replanted fields. Five NPFO strains were first screened with and without NaCl in the glasshouse using autoclaved asparagus soil mixed with potting mix. Asparagus roots were treated with NPFO suspensions or water, then inoculated 1 week later with pathogenic F. oxysporum; then after 1 week pots received 100 mL 1% NaCl or water. After 12 weeks the NPFO strain CS-20 increased root weight when compared with controls. Rock salt did not affect root growth. Root lesions were reduced relative to the control by all NPFO strains and by NaCl applications, but there were no interactions between NPFO and NaCl. The NPFO strains CS-20, CWB 314 and CWB 318 were compared with no treatment in two 4-year replanted field studies in Hamden, and Windsor, CT, USA. Crowns were treated with NPFO at planting, and NaCl was broadcast at 280 kg ha(-1), and at 560 kg ha(-1) in later years. CWB 318 reduced disease at both sites and increased marketable yield in Hamden. Treatment with NaCl increased yield by 29% over controls in Windsor, but did not increase yield in Hamden. Recovered root isolates of F. oxysporum categorized by heterokaryon tests revealed that, in the glasshouse, CS-20 colonized roots better than other strains, while CWB 318 was the only strain found on roots from field plots after 4 years. Applications of NaCl did not hinder the suppressive ability of the nonpathogenic strains. The perennial nature of asparagus may require that nonpathogenic strains possess superior survival traits along with biocontrol traits.