OCCURRENCE OF ALTERNARIA SPECIES INVOLVED IN FRUIT BLIGHT OF PISTACHIO IN TUNISIA

Abstract

During summer 2007-2010, a new disease was observed on pistachio (Pistacia vera L.) cv. Mateur in several orchards of Tunisia. Symptoms on the nuts consisted of black lesions with red halos, resulting in yield losses often exceeding 20%. A fungus recovered on potato dextrose agar (PDA) from surface-disinfected symptomatic nuts, produced cottony colonies characterized by an abundant branched, brownish, septate mycelium which, with time, turned olive-green to dark olive-green due to abundant sporulation. Conidiophores were simple or branched, septate, straight or flexuous, pale olivaceous-brown, and bore several conidial scars. Conidia were 100-200 mm in size, obclavate to ellipsoid, multiseptate and formed in single or, more often, branched chains. Based on these morphological characters, the fungus was identified as Alternaria sp. (Fr.) Keissler (Ellis, 1971). Pathogenicity tests were carried out on detached fresh pistachio nuts inoculating the hull with 5 μl conidial suspensions prepared from actively growing colonies (105 conidia/ml sterile distilled water). All isolates induced symptoms similar to those observed in the field after 15 days. An Alternaria sp. was reisolated from symptomatic fruits. To identify the fungus at the species level, the ITS1-5.8S-ITS2 region was amplified with primers ITS1 and ITS4 (White et al., 1990) and the amplicons were directly sequenced. A BLAST search of GenBank database revealed 99% homology with both A. alternata (accession No. HQ873733.1) and A. tenuissima (HQ916330.1). To our knowledge this is the first report of an Alternaria species of the alternata species-group, as the causal agent of pistachio fruit blight in Tunisia.