Antibiosis, Endophytic Colonization, and Efficacy of Trichoderma Against the Laurel Wilt Pathogen in Avocado

Laurel wilt, caused by Harringtonia lauricola (HL), is a lethal vascular disease threatening the global avocado industry. HL, a nutritional symbiont of several ambrosia beetles, is present in 12 states and continues to spread through beetle vectoring, root grafting, and human-mediated transport of infested wood. Disease management relies on removing symptomatic trees, reducing insect vector populations, and using fungicides administered as injections. The ineffectiveness of the only labeled fungicide has prompted a search for alternative products. A total of 17 Trichoderma isolates and six commercially available strains were grown in potato dextrose broth for 7 days, and 10% of the filtered-sterilized broth was used to amend potato dextrose agar plates where HL was grown. Seven isolates showed significant (>70%) pathogen growth inhibition; however, none demonstrated fungicidal capabilities, even at a 50% v/v filtrate concentration. These isolates were evaluated for endophytic capability through a seed-inoculation assay. Three months later, all seedlings presented Trichoderma in at least one tissue type, but the extent of endophytic colonization varied among isolates and was overall low across treatments and tissues. Seedlings pre-colonized with Trichoderma were inoculated with HL, but no significant disease suppression was observed. The pathogen was recovered from all inoculated plants, while Trichoderma was recovered from only one treated plant. Although results indicated that a single seed inoculation with antagonistic Trichoderma is insufficient for extensive colonization of internal tissues, their ability to become endophytic, coupled with the high pathogen growth inhibition observed in vitro, warrants further investigation.