Occurrence of mal nero disease on mandarin and orange trees in Tunisia

The Tunisian citrus industry covers 18000 ha localized mainly along the coast of the Mediterranean sea. The major phytosanitary problem is mal secco of lemon trees, a dieback caused by Phoma tracheiphila. This fungal disease has been observed in Tunisia since 1953 (Crossa-Raynaud, 1960). Different symptoms occur on the tree depending upon whether P. tracheiphila attacks via the roots or shoots. The disease is named mal nero when infection begins at the basal part of trees (trunk, roots) with a rapid progression of symptoms and browning of the hardwood may occur (Perrotta & Graniti, 1988; Solel & Salerno, 2000).

During surveys in Cap Bon region of Tunisia in 2007, decline of mandarin (cv. Cassar) and orange (cv. New Hall) grafted on sour orange rootstock was observed in some orchards characterized by a heavy soil. Symptoms included a general collapse of the tree with leaves remaining attached (Fig. 1A). A transversal section of the dead tree trunk showed necrotic coloration of the hardwood (Fig. 1B). The same symptoms have been described in Italy by Perrota and Graniti (1988). Consistent fungal colonies, isolated from necrotic woody tissue producing pycnidia and pycnidiospores, were identified as P. tracheiphila (Fig. 1C, 1D). The identification of the pathogen was confirmed by PCR technique using the primer pair Pt-FOR2 + Pt-REV2 developed by Balmas et al. (2005) for detecting P. tracheiphila on infected lemon tissues (Fig.1E).

A pathogenicity test was conducted using susceptible 1-year-old sour orange plants grown in pasteurized potting medium. Mycelial plugs of two isolates each were inoculated separately into the basal stem of 10 sour orange plants. Ten non-inoculated plants were used as controls. Plants were grown in the greenhouse at 25°C, where they all developed typical disease symptoms after 45 days. Non-inoculated controls did not develop disease symptoms. P. tracheiphila was re-isolated from all diseased plants.

To our knowledge, this is the first report of mal nero disease occurring naturally on mandarin and orange trees and the first time this disease has been reported in Tunisia. The Tunisian citrus industry is mostly grown on sour orange rootstocks, and therefore, mal nero represents a serious threat.