Synergistic Interactions between Plant-parasitic Nematodes ( Meloidogyne hapla and Paratylenchus projectus ) and Verticillium dahliae

Synergistic interactions between soil-borne fungi and plant-parasitic nematodes can cause significantly more damage to a crop than either pathogen alone. For peppermint, synergistic interactions between Verticillium dahliae and plant-parasitic nematodes have only been described for the migratory endoparasite Pratylenchus, whereas there is no information for such interactions for ectoparasitic or sedentary endoparasitic nematodes. To determine whether species of these nematode groups co-occur with V. dahliae on peppermint, and to evaluate whether their interactions with V. dahliae on peppermint are synergistic, we conducted a field survey and three greenhouse experiments. In the field survey, we confirmed the presence of V. dahliae in infected plants and detected eight plant-parasitic nematode species in the soil associated with peppermint. Among the detected taxa were the sedentary endoparasite Meloidogyne hapla and the ectoparasite Paratylenchus projectus, with the latter found at particularly high densities. Both such species were selected for greenhouse experiments. The synergistic effect was assessed primarily on the basis of symptom severity and a measure of plant development. Furthermore, we employed isolation and qPCR to detect and quantify V. dahliae in peppermint stem base, stem and leaf samples. Results suggested potential synergistic interactions between V. dahliae and both nematodes, which was associated with increased wilt symptoms and reduced plant growth. The qPCR and isolation approaches confirmed the presence of V. dahliae and suggested that both nematodes influence V. dahliae distribution within a plant, as differences in Ct values between stem base and stem varied among treatments. In V. dahliae alone, no consistent differences were observed. In V. dahliae-M. hapla, Ct values were higher in the stem than in the stem base, whereas in V. dahliae-P. projectus, variable responses were observed depending on the inoculation order. In conclusion, the results demonstrated the cooccurrence of V. dahliae with different plant-parasitic nematodes on peppermint and indicated the potential synergistic interactions with M. hapla and P. projectus. However, whether such synergistic effects also occur under field conditions still needs to be proven.