Screening of Streptomyces strains helping arbuscular mycorrhizal symbiosis against pepper (Capsicum annuum L.) Phytophthora blight

Abstract

Mycorrhiza helper bacteria (MHB) can promote the formation and functioning of arbuscular mycorrhizal (AM) symbiosis, but their role and application potential in coping with soil-borne diseases are still unclear. The aim of this 14-week greenhouse pot experiment was to obtain several actinomycetes strains which could help AM symbiosis in suppressing the Phytophthora blight of pepper (Capsicum annuum L.), using a soil inoculated with Phytophthora capsici after sterilization. Five Streptomyces strains, including S. pseudogriseolus, S. albogriseolus, S. griseoaurantiacus, S. tricolor and S. tendae, as well as the AM fungus (Funneliformis caledonium) were tested. The Phytophthora blight severity reached 66% at full productive stage in the control, and only F. caledonium, S. griseoaurantiacus and S. tricolor alone remarkably decreased (P < 0.05) it to 47%, 40%, and 35%, respectively. Compared to F. caledonium alone, only additional inoculation of S. tricolor and S. tendae which were isolated from the rhizosphere of a healthy individual in an infected field substantially elevated (P < 0.05) root mycorrhizal colonization, root biomass, fruit yield, and total K acquisitions of pepper, and further dramatically decreased (P < 0.05) the severity. According to the feature of enhancing disease-suppression by AM symbiosis, both S. tricolor and S. tendae were confirmed as MHB strains here. Specifically, S. tendae had a stronger performance in directly accelerating mycorrhization, while S. tricolor was also an antagonist to the pathogenic P. capsici. Furthermore, S. griseoaurantiacus with the independent disease-suppression function was not a MHB strain here. The redundancy analyses demonstrated that when AM fungus present, root mycorrhizal colonization replaced soil pH becoming the main factor impacting pepper Phytophthora blight. Thus, S. tricolor and S. tendae seemed to have the value of preparation and application in the future to help AM symbiosis against pepper Phytophthora blight.