Alternaria alternata fruit rot of pomegranate (Punica granatum) in Greece

Abstract

During September and October 2005 and 2006 a fruit disease was observed in pomegranate orchards, in the region of Larissa, Central Greece, causing significant yield losses of approximately 40–50% especially on cv. Kapmaditika. Internal symptoms consisted of a black rot of the fruit core beginning from the calyx area, while the hard, leathery rind appeared healthy and fruits remained firm. Alternaria alternata was consistently isolated from infected tissues, and the identification of the fungus was based on morphological characteristics of mode of sporulation and conidia (Simmons, 1967). Conidiophores were short, septate, branched or unbranched, and green to brown. The conidia were obpyriform with conical or cylindrical beak, ovoid or ellipsoidal. They were produced in long, single but most often branched, chains. Conidia ranged from 10 to 21 µm in length and from 4 to 10 µm in width at the broadest point (average 17 × 6 µm). Cultures grew rapidly on potato dextrose agar (PDA), and they were initially white turning to grey later. The isolate was deposited in CBS International Culture (CBS 120829). Pathogenicity tests were conducted on fruits and were repeated three times. PDA-plugs, 5 mm in diameter, with actively grown mycelium were transferred into the fruit calyx as well as on wounds made by a scalpel on previously sterilized fruit surfaces. Following inoculation, the fruits were placed in plastic bags and kept at 23oC for 10 days. Fruits inoculated in the same way using PDA disks were kept as control. Although there were no external symptoms on any of the fruits, decay of the core was observed when inoculated fruits were cut vertically. Control fruits showed no decay. Alternaria alternata was reisolated from inoculated fruits, confirming Koch's postulates. Alternaria alternata has been reported to cause a leaf spot disease on pomegranate in India (Madhukar & Reddy, 1976) however, fruit rot was not reported. Fruit rot caused by Alternaria sp. has previously been reported in USA, Mexico (Farr et al., 2007), and as a postharvest disease in Greece (Pantidou, 1973). Although this disease has been detected before in Greece, it was noticed again only recently probably due to the recent expansion of pomegranate cultivation and changes in weather conditions. This is the first report of A. alternata causing fruit decay in pomegranate orchards in Greece.