Rapamycin Plays a Pivotal Role in the Potent Antifungal Activity Exhibited Against <em>Verticillium dahliae</em> by the Endophytic Strains <em>Streptomyces iranensis</em> OE54 and the Novel Species <em>Streptomyces lacaronens

Verticillium wilt, caused by Verticillium dahliae, represents a significant threat to olive trees (Olea europaea L.). The isolation of endophytic Streptomyces strains from olive roots has led to the identification of several strains exhibiting strong antifungal activity against V. dahl-iae, as demonstrated through in vitro and small-scale soil trials. Molecular analyses con-firmed that strain OE54 belongs to Streptomyces iranensis. The primary antifungal com-pound identified in this strain was rapamycin. Rapamycin exhibited a potent antifungal activity, significantly inhibiting conidiospore germination (IC₅₀ = 87.36 μg/mL) and hyphal growth of V.dahliae, with a minimum inhibitory concentration MIC₅₀ of 3.91 ng/mL. Moreover, a second rapamycin-producing strain, OE57T, was isolated. Phenotypic and genotypic analyses indicated that OE57T represents a novel species, for which the name Streptomyces lacaronensis sp. nov. is proposed, with OE57T designated as the type strain (=DSM 118741T; CECT 31164T). The discovery of two endophytic rapamy-cin-producing Streptomyces strains residing within olive roots is particularly notheworthy, given the rarity of rapamycin production among microorganisms. These results highlight the potential of rapamycin-producing Streptomyces strains in the development of bioferti-lizers aimed at managing V. dahliae and mitigating the effects of Verticillium wilt on olive trees and other crops.