Screening of virulence-attenuated strains for Fusarium oxysporum f. sp. cucumerinum after treatment with parsley fresh root extract

Previous studies have shown that parsley extracts from fresh roots, rotten roots, rhizospheric soils, rhizospheric rotten soils, seed or volatiles have allelopathic effects on Fusarium oxysporium f. sp. cucumerinum (Foc). Different parsley extracts had different allelopathic effects on Foc and were related to extract concentration and action time. In terms of time-invariance, allelopathic effects increased with increasing concentration of different extracts. However, in terms of concentration, allelopathic effects weakened with prolonged duration of action of different extracts. Other studies have noted that different extracts (ethanol, acetone and distilled water) of fresh parsley roots inhibit mycelia growth of Foc, reduce the number of pathogenic spores and limit spore germination rate. Did treatment of Foc with successive generations of different parsley fresh root extracts always resulted in allelopathic inhibition of pathogens? Did any such allelopathic inhibition influence Foc virulence? Based on the above issues, the effect of fresh parsley root extract on Foc growth and pathogenicity were studied in laboratory antifungal experiments and artificially inoculated potted cucumbers in greenhouse conditions. In the first part of the study, Fusarium colonies were treated with 50 mg·mL^-1 of fresh parsley root extracts respectively in acetone, ethanol, distilled water and the corresponding control, and colony diameters assessed for 5 generations. Compared with the control, both acetone and ethanol extracts had lower colony diameter after continuous assessment for 5 generations. Aqueous extracts had lower colony diameters than the control only in the 3^rd generation. Both inhibition and promotion of allelopathy on Foc were expressed in the other generations. In the second part of the study, each generation of treated Foc strains was used to inoculate cucumber in order to determine their pathogenicity. After 7 days, disease index was assessed on the seedlings. The results showed that fresh parsley root acetone, ethanol and distilled water extract treatments had lower disease index than the control after 5 continuous generations of inoculation. It also had attenuation effect on Foc virulence. Foc virulence gradually decreased with increasing subculture generation. By the time when the 5^th generation strain was inoculated, disease index of acetone extract dropped to 0. It also had the greatest attenuation of virulence and colonial morphology change. Colony color changed from white to lavender and colony density dropped from dense to sparse. The study screened virulence-attenuated Foc strain through fresh parsley root extracts under continuous processing.