FIRST REPORT OF DIEBACK OF OLIVE TREES CAUSED BY PHOMA FUNGICOLA IN TUNISIA
S. Krid Hadj Taieb, M.A. Triki, I. Hammami, A. Rhouma
During spring 2013, a new disease was observed on olive (Olea europea) cv. Chemlali in orchard in Sahlin (Sahel of Tunisia). Symptoms included abundant dead branches and wilted leaves and shoot necrosis. Shoots showing dieback symptoms were disinfected with 2% sodium hypochlorite, rinsed in sterile distilled water and air dried. Several fragments (3 x 3 mm) of infected shoots were cut and placed on potato dextrose agar medium (PDA). All plates were incubated at 25°C for 4 days under continuous fluorescent light. A pycnidial fungus was consistently isolated from branch cankers and identified as Phoma fungicola Aveskamp, Gruyter et Verkley, based on morphological characteristics and analysis of the ITS gene region (White et al., 1990). The sequence showed high identity (99%) with a reference sequence (strain H11 H10; accession No. KF29376 3.1). Pathogenicity tests were conducted on 2-year old olive plants (cv. Chemlali). A mycelial plug cut from the margin of an actively growing colony of the fungus was placed into a shallow wound (0.4 cm2) inferted with a sterilized scalpel on the stem base. Inoculated wounds were wrapped with Parafilm. In control plants, sterile PDA plugs were placed into artificial wounds. Ten replicate inoculated plants were used and maintained in a greenhouse at 25°C. Two months after the inoculation, the inoculated trees reproduced stem browning symptoms observed in the field, while control plants remained healthy. Koch’s postulates were then verified and P. fungicola was isolated from inoculated stems, whereas the controls were free of the fungus.
Phoma sp. and P. incompta have been reported as responsible for branch dieback of olive tree in Tunisia and Italy, respectively (Rhouma et al., 2010; Ivic et al., 2010). To the best of our knowledge, this is the first report of P. fungicola as a causal agent of dieback of olive trees in Tunisia.