Morphological and Molecular Identification of Armillaria spp. Causing Root Rot of Fruit Trees in Temperate Orchards

Abstract

Root rot caused by Armillaria species is a destructive disease of fruit trees in temperate orchards, leading to gradual decline, reduced vigor, and plant mortality. The present study aimed to characterize Armillaria spp. associated with symptomatic fruit trees using integrated morphological and molecular approaches. Diseased root and basal stem samples exhibiting white mycelial fans, rhizomorph formation, and bark decay were collected from affected orchards. Fungal isolation was performed on selective media, and cultures were examined for colony morphology, rhizomorph structure, and basidiocarp characteristics. Microscopic observations revealed clamp connections and septate hyphae typical of Armillaria species complexes. Molecular identification was conducted using ITS rDNA sequencing, which confirmed the presence of multiple Armillaria species associated with root rot symptoms. Phylogenetic analysis revealed distinct clustering of isolates into separate lineages, indicating genetic diversity within orchard populations. Sequence comparisons showed close relationships with known pathogenic Armillaria taxa reported from temperate fruit-growing regions. Variability in growth rate and pathogenicity was observed among isolates during artificial inoculation assays on fruit tree seedlings, confirming differential virulence. Disease development was favored by cool, moist soil conditions that enhance rhizomorph spread and host root colonization. The study highlights the complex etiology of root rot disease involving multiple Armillaria species and underscores the difficulty of managing this soilborne pathogen due to its persistent survival structures. Integrated management strategies, including removal of infected stumps, use of resistant rootstocks, and improved soil drainage, are recommended. Overall, the findings provide important insights into the morphological diversity and molecular identity of Armillaria spp. and contribute to improved understanding of root rot epidemiology in temperate orchard ecosystems.