Occurrence of Alternaria brown spot on citrus in Tunisia

Abstract: Alternaria brown spot is a serious disease of fruit and foliage of citrus and is responsible of serious losses on susceptible cultivars in the north of Tunisia. A severe outbreak of brown spot was observed on tangerine (Citrus reticulata Blanco) and hybrids of tangerine for the first time in 2008. Symptoms at the initial stage of the infection showed brown, small, scattered spots on the leaves that gradually become round or irregular. Spots coalesce to affect large areas of the leaves and cause defoliation. The disease first appeared in April and remained till June when temperature ranged from 15 to 27 °C. Fungus isolates obtained from infected leaf tissues, grown in pure culture, were identified as Alternaria alternata (Fr.) Keissler on the base of morphological characteristics of colony and conidia and confirmed by sequencing. The fungus produced olivaceous black colonies with dark olive green margins, and abundant branched septate, golden brown mycelium. The conidiophores were branched, straight, pale brown to olive brown. The conidia of the isolates were catenated in long, sometimes branched chains of 4-8 spores. Leaves and fruits of 18 citrus cultivars were inoculated with conidia of three isolates of Alternaria alternata. Pathogenicity tests indicated that mandarins were susceptible to tested isolates. All lemon cultivars as well as the Star Ruby were not susceptible. As far as oranges are concerned, Valencia Late and Navel Late were not susceptible, while Lane Late and Maltaise were slightly susceptible.