Synergistic effect of organic and inorganic fertilization on the soil inoculum density of the soil borne pathogens Verticillium dahliae and Phytophthora spp. under open field conditions

Background: The increasing demand of food causes an excessive exploitation of agricultural lands, often inducing phenomena of soil sickness accompanied by the development of soilborne diseases. The use of residual biomasses together with inorganic fertilizers can be considered a good agricultural practice for controlling the inoculum density of soilborne phytopathogens since soil conditioners can release inorganic nitrogen, polyphenols and fatty acids that, especially in vitro, have demonstrated various degree of suppressiveness against such pathogens. Further, soil organic amendments can also modify the population of soil culturable bacteria and fungi that, in turn, can affect the soilborne diseases in several ways. With this study, Authors aim to evaluate the impact of the synergistic application of different biomasses and inorganic fertilizers on the soil inoculum density of Verticillium dahliae and Phytophthora spp. during two potato cycles under open field conditions. The biomasses used for the fertilization of the potato crop were olive pomace residues (OPR), composts from municipal solid wastes (CMW), spent mushroom compost (SMC), and livestock manures based compost (BRX). Results: The inoculum density of Verticillium dahliae appeared inhibited by BRX due to its low C/N ratio that caused a quicker release of inorganic nitrogen with respect to the others soil conditioners. In contrast, OPR was conducive to the aforementioned soilborne pathogen since that biomass was characterized by a very high percentage of unsaturated fatty acids that, rather, stimulate the inoculum density of V. dahliae. Finally, polyphenols did not influence the same pathogen because they apparently turned into no toxic compounds very quickly. The inoculum density of Phytophthora spp. was reduced equally by all the biomasses used in combination with the inorganic fertilizers, regardless of their composition and quantity, mainly because of the development of general microbial suppression. Therefore, the chemical characteristics of the soil conditioners apparently did not affect the inoculum density of Phytophthora spp. Conclusions: The results of this work underline the behavioral diversity of the different pathogens towards the different means adopted. Phytophthora spp. are sensitive to any kind of biomasses combined with inorganic fertilizers while the inoculum density of Verticillium dahliae should be reduced using soil conditioners characterized by low C/N ratio and low quantity of unsaturated fatty acids.