Intraspecific Variation and Phylogenetic Relationships of Plenodomus tracheiphilus Causing Mal Secco Disease of Lemon

Abstract

Mal secco disease caused by Plenodomus tracheiphilus is one of the most destructive vascular diseases of lemon (Citrus limon), leading to severe yield decline and tree mortality in citrus-growing regions. The present study investigates the intraspecific variation and phylogenetic relationships of P. tracheiphilus isolates obtained from symptomatic lemon trees. Field samples showing characteristic symptoms such as leaf chlorosis, twig dieback, vascular discoloration, and progressive decline were collected from multiple orchards. The pathogen was isolated on selective media and characterized based on cultural morphology, colony pigmentation, growth rate, and sporulation features. Molecular characterization was performed using ITS rDNA and β-tubulin gene sequencing to assess genetic diversity among isolates. Phylogenetic analysis revealed the presence of distinct genetic clusters within P. tracheiphilus populations, indicating moderate intraspecific variation across geographical locations. Some clustering patterns were associated with specific regions, suggesting limited gene flow and potential local adaptation within citrus-growing areas. However, overall genetic divergence remained relatively low, confirming species-level conservation. Pathogenicity tests on healthy lemon seedlings reproduced typical mal secco symptoms, confirming the virulence of all tested isolates. Variations in aggressiveness were observed among isolates, indicating differential pathogenic potential despite genetic similarity. Environmental conditions such as humidity, temperature fluctuations, and orchard management practices influenced disease severity and spread. The study highlights the importance of integrating molecular and pathogenic variability data to understand the epidemiology of mal secco disease. Overall, the findings provide valuable insights into the genetic structure and evolutionary relationships of P. tracheiphilus populations, supporting improved disease monitoring and the development of effective management strategies for sustainable citrus production.