Pathogenicity and Molecular Identification of Phytophthora cactorum Causing Crown and Collar Rot of Apple Rootstocks

Abstract

Crown and collar rot caused by Phytophthora cactorum is a destructive disease affecting apple rootstocks, leading to reduced plant vigor, graft failure, and significant economic losses in nurseries and orchards. The present study aimed to investigate the pathogenicity and molecular identity of P. cactorum isolates associated with diseased apple rootstocks. Samples showing typical symptoms such as crown necrosis, bark discoloration, root decay, and wilting were collected from affected orchards and nurseries. The pathogen was isolated on selective media and identified based on cultural and morphological characteristics including colony pattern, hyphal structure, sporangial morphology, and oospore formation. Pathogenicity tests conducted on healthy apple rootstocks confirmed the ability of the isolates to reproduce characteristic crown and collar rot symptoms, thereby fulfilling Koch’s postulates. Molecular identification was performed using ITS rDNA sequencing, which confirmed the isolates as P. cactorum. Phylogenetic analysis revealed close genetic similarity among isolates, with limited but detectable intraspecific variation, indicating a relatively conserved population structure in apple production systems. Disease development was favored by high soil moisture, poor drainage, and cool environmental conditions typical of nursery and orchard sites. Infection was primarily initiated through the root and crown regions, with rapid colonization of cortical tissues leading to plant decline. The study highlights the importance of integrating morphological, pathogenic, and molecular approaches for accurate identification of P. cactorum. Overall, the findings provide valuable insights into the etiology and genetic identity of crown and collar rot pathogen in apple rootstocks and emphasize the need for improved nursery sanitation, drainage management, and use of disease-free planting material for effective disease control.