Armillaria Species Causing Kiwifruit Vine Decline and Root Rot in Northeastern Türkiye: A Growing Concern for Kiwifruit Health

Kiwifruit (Actinidia deliciosa) production in northeastern T & uuml;rkiye is facing an emerging threat from Armillaria root rot. Surveys conducted in August 2021 and 2022 revealed that 10.5%-17.5% of the 200-500 vines within each of 35 kiwifruit orchards in the Per & scedil;embe, G & uuml;lyal & imath; and Alt & imath;nordu districts of Ordu province exhibited symptoms of Armillaria root rot, including complete defoliation. Diseased plants exhibited diagnostic signs on their root collars and woody roots, including white mycelial fans, extensive wood rot and rhizomorphs. A single Armillaria isolate was obtained from each of the 35 symptomatic orchards. Molecular identification, employing DNA sequencing of the large subunit (LSU) region of rDNA, the translation elongation factor subunit 1-alpha (TEF1) gene and the second largest RNA polymerase II B-subunit (RPB2) gene, identified 20 isolates as Armillaria gallica and 15 as A. mellea. Phylogenetic analyses, based on TEF1 alignments, further supported the species identification with a 100% bootstrap value. To examine genetic diversity, start codon targeted (SCoT) marker 13 was used, successfully differentiating the two Armillaria species and revealing three distinct clades. Clades I and II comprised 15 and 5 isolates of A. gallica, respectively, while Clade III contained all 15 A. mellea isolates. Pathogenicity testing on kiwifruit seedlings (cv. Hayward) revealed the ability of both A. gallica and A. mellea isolates to induce disease. Rhizomorph-producing isolates in kiwifruit seedlings displayed high virulence, inducing severe leaf necrosis and seedling mortality, while isolates lacking these structures were non-pathogenic. This study identifies the first global instance of A. gallica causing kiwifruit root rot. This finding, along with the identification of A. mellea as a causal agent in the country, highlights the emerging threat of Armillaria root rot to kiwifruit production in the region.