First report of brown spot of tangerine hybrid cultivars Minneola, Page and Fortune caused by Alternaria alternata in Iran

A new disease of the tangerine hybrid cultivars Minneola (Citrus tangerina cv. Dancy × C. paradisi cv. Duncan), Page (Minneola × C. clementina) and Fortune (C. tangerina x C. clementina) was observed in the Mazandaran province of Iran in the summer of 2002. Brown necrotic lesions surrounded by yellow haloes, characteristic of alternaria brown spot of citrus, were found on young leaves and fruit (Fig. 1).

Infected, young fruit and leaves abscised prematurely and mature fruit was unmarketable due to light brown, circular, depressed lesions on its surface (Fig. 2). A fungus was isolated from the infected leaves, fruit and twigs and was identified as Alternaria alternata based on the morphological characteristics of the conidia (Fig. 3). Pathogenicity tests were conducted by inoculating 10 detached, immature leaves of the varieties Minneola and Page with droplets of conidial suspensions (10⁵ conidia per ml) and incubating them in a moist chamber at 25°C. Necrotic spots with characteristic yellow haloes developed on the leaves after 3 days. Re-isolation of the same fungus from diseased tissue of both inoculated cultivars fulfilled Koch's postulates.

Figure 2: Lesions on fruit of tangerine hybrid cultivar Minneola Figure 3: Conidia of fungus isolated from leaf and fruit lesions. Bar = 50µm

DNA was extracted from five isolates and a partial endopolygalacturonase gene was PCR-amplified and sequenced for each (Peever et al., 2002, 2004). Sequences of all five isolates were identical and in BLAST searches of the NCBI database, the closest matches were A. alternata accessions AY295023.1 (isolate EGS 44-160), AY295021.1 (isolate BC2-RLR-1s) and AY295022.1 (isolate EGS44-159) with 99.8, 99.8 and 99.6% sequence similarity respectively. Phylogenetic analyses revealed that isolates from all three hosts clustered in Clade 3, together with brown spot isolates from Israel, Turkey, South Africa and Australia (Peever et al., 2002). These results, in conjunction with the morphological characterisation and pathogenicity tests, confirm the identity of the fungus as Alternaria alternata. To our knowledge, this is the first report of alternaria brown spot of citrus in Iran.