FIRST REPORT OF DIPLODIA SERIATA AS CAUSAL AGENT OF ALMOND TREE BRANCH DIEBACK IN TUNISIA

Abstract

A branch dieback of almond trees (cv. Achaak) was noticed in 2016 in the region of Sfax. 10% of trees showed dead branches with sunken areas. Infected tissues were abnormally dark and cross sections revealed a light-brown and irregularly shaped canker developing from the graft-union. Numerous pycnidia were observed on the surface of infected branches with severe gummosis on the buds. To identify the pathogen, 35 small pieces of infected tissues were plated on potato dextrose agar. After incubation at 25°C for 5 days, 15 isolates were consistently isolated. Colonies were grayish white, producing dark melanised hyphae and pycnidia after 10 days of incubation. Conidia were first hyaline, and then turned brown as they matured. They were aseptate, rounded at both ends with thick melanised cell walls, 19 to 25.7 × 9.5 to 11.03 µm. Based on colony characteristics, all isolates were identified as Diplodia seriata (Phillips et al., 2007). Molecular identification was performed by sequencing the ITS, the EF-1-α and the β-tubulin genes. BLAST searches of ITS (KY275259), EF-1-α (KY284865) and β-tubulin (KY275260) sequences, respectively revealed 98%, 99% and 100% identity to Diplodia seriata. Pathogenicity tests were performed on detached stems, wounded with a scalpel and inoculated with mycelial plugs (Yan et al., 2011). Controls were inoculated with sterile agar plugs. Inoculated stems were placed in polyethylene boxes and incubated at 25°C for 30 days. While mock-infected stems remained healthy, inoculated stems developed brown discoloration, with small dark pycnidia growing on the surface. The pathogen was isolated from inoculated stems, thus fulfilling Koch's postulates. To our knowledge, this is the first report of canker disease caused by Diplodia seriata in Tunisia