FIRST REPORT OF APPLE COLLAR ROT INCITED BY SCLEROTIUM ROLFSII IN TUNISIA

Abstract

Sclerotium rolfsii Sacc. [teleomorph Athelia rolfsii (Curzi) Tu et Kimbrough] is a soil- borne polyphagous phytopatho- genic fungus characterized by prolific growth and ability to produce persistent sclerotia. In Tunisia, S. rolfsii has al- ready been reported on potato (Daami-Remadi et al., 2007) and olive (Boulila, 2001). In spring 2012, a sudden death of 2-year-old apple trees (Malus domestica) cv. Royal Gala grafted on MM106 rootstock was observed in an orchard near Tunis showing leaf chlorosis, root and collar rot. Isola- tions from necrotic wood on PDA medium, yielded consis- tent white fungal colonies, producing only sclerotia and no fruiting bodies or spores. Based on morphological charac- teristics, the fungus was tentatively identified as Sclerotium sp., then confirmed by sequencing the ITS region of rDNA. The obtained sequence (accession No. KF021301) matched the sequence of Athelia rolfsii (ATCC 201126). A pathoge- nicity test was performed on 2-year-old grafted apples of cv. Royal Gala in a glasshouse at 25-28°C and 80% relative hu- midity. Mycelial plugs of S. rolfsii were inoculated into the basal stem of 10 plants, whereas an equal number of non inoculated plants served as controls. Symptoms like those observed in the field developed in inoculated plants, but not in the controls, 35 days post inoculation. S. rolfsii was consistently re-isolated from infected wood tissues fulfill- ing Koch’s postulates. Apple collar tot (ACR) was originally described as apple southern blight in the USA and has also been reported in India, China, Israel and Italy (Corazza et al., 1999). To our knowledge this is the first report of ARC in Tunisia. The Tunisian Apple industry is mostly grown on MM106 rootstocks, and therefore, ACR represents a serious threat.