Antifungal activity of avocado rhizobacteria against Fusarium euwallaceae and Graphium spp., associated with Euwallacea spp. nr. fornicatus, and Phytophthora cinnamomi

Abstract

Plant rhizobacteria have been successfully used as biocontrol agents against fungal phytopathogens. However, their potential to control two important avocado diseases, namely Fusarium dieback (FD) and Phytophthora root rot (PRR), has been poorly studied. FD is an emerging disease triggered by fungi associated with two ambrosia beetle species (Euwallacea fornicatus species complex), while PRR is caused by Phytophthora cinnamomi, a soil-borne oomycete. In the present work, the antifungal activity of bacteria isolated from avocado rhizosphere was tested in dual culture assays against Fusarium euwallaceae, Graphium euwallaceae and Graphium sp., causal agents of FD, and against P. cinnamomi. In 2015, rhizosphere soil samples of FD infested and non-infested avocado trees were collected from a commercial avocado orchard in Escondido, California. In an initial screening, 72 of the 168 assessed bacterial isolates reduced mycelial growth of F. euwallaceae by up to 46%. Eight bacterial isolates showing inhibition percentages larger than 40% were then selected for further antagonism assays against the other fungal pathogens. Five bacterial isolates, determined by 16S rDNA sequencing to belong to the Bacillus subtilis/Bacillus amyloliquefaciens species complex, successfully inhibited the mycelial growth of both Graphium species by up to 30%. The same isolates and an additional isolate identified as Bacillus mycoides, inhibited the growth of P. cinnamomi by up to 25%. This is the first report of avocado rhizobacteria with antifungal activity against pathogens responsible for FD and PRR in avocado.

Plant growth-promoting rhizobacteria associated with avocado display antagonistic activity against Phytophthora cinnamomi through volatile emissions

“…Rhizobacteria have proved to be a source of biological control agents of fungal phytopathogens in numerous studies [ 16 , 31 , 43 , 45 ]. In the present work, we found that isolate A8a, a phylogenetically close relative to Bacillus acidiceler , presented strong antagonism against P .…”

“…cinnamomi , through the emission of both diffusible and volatile compounds ( Table 2 , Fig 5 and Fig 6 ). Species of the bacterial genus Bacillus have shown to inhibit the mycelial growth of several fungal pathogens, such as Fusarium euwallaceae [ 16 , 46 ], F . oxysporum and Rosellinia necatrix [ 15 ] and Phytophthora spp.…”

“…cinnamomi mycelium and placed on the center of a PDA plate. Bacterial isolates were taken from a single colony with a sterile toothpick and inoculated at a 2 cm-distance from the mycelial plug, following the method recently reported [ 16 ]. Three different bacterial isolates were tested per plate and a sterilized toothpick mark was used as a control.…”

“…cinnamomi or that are associated with its suppressiveness have shown promising results and have identified Bacillus spp., Pseudomonas spp., Streptomyces spp. and some species of Actinobacteria as potential candidates for biological control of Phytophthora root rot [ 13 , 14 , 15 , 16 ]. The antagonistic activity presented by rhizobacteria against fungal pathogens may occur through different mechanisms, such as the production of diffusible antifungal compounds or the emission of antifungal volatile compounds [ 17 , 18 ].…”

Characterization of plant growth-promoting bacteria associated with avocado trees (Persea americana Miller) and their potential use in the biocontrol of Scirtothrips perseae (avocado thrips)

“…Despite the importance of avocado around the world, there are just a few studies regarding its rhizosphere biology. Carzola et al [13] reported that four strains of Bacillus subtilis were associated with avocado antifungal activity and showed apparent enzymatic and antibiotic action against phytopathogenic fungi, whereas Guevara-Avendaño et al [14] identified Bacillus strains with the ability to control the phytopathogenic fungi Fusarium euwallaceae and Graphium sp. Méndez-Bravo et al [15] described the effects of Bacillus, Pseudomonas and Arthrobacter isolated from the avocado rhizosphere against Phytophthora cinnamomi through the production of volatile organic compounds.…”