First report of Cladosporium colocasiae as a pathogen causing leaf spot disease on taro (Colocasia esculenta) in Iran.

Abstract

During the Summer 2013, a new foliar disease was observed on taro (Colocasia esculenta) in tropical areas of Iran, including Hormozgan, where many farmers were alarmed by complete destruction of their taro cultivation. Leaf spots, yellow to dark brown, were subglobose and oval. A fungus was consistently isolated from diseased tissues on Potato Dextrose Agar (PDA) at 23± 2°C. Morphological observations were carried out on Synthetic Nutrient Agar (SNA) medium as described by Bensch et al. (2012). Conidiophores were macronematous, cylindrical-oblong with several nodes being quite apart from each other, very long, up to 350 μm or even longer. Conidia ellipsoid-subcylindrical to cylindrical, forming short, unbranched or branched chains. Terminal conidia were 9–16 × 5–8 μm and aseptate or 1-septate. A portion of the translation elongation factor 1-α gene was amplified (Bensch et al., 2012) and sequenced (GenBank Accession No. KP893386). A BLAST search showed 99% similarity with Cladosporium colocasiae sequences deposited in GenBank. To verify Koch’s postulates, pathogenicity was tested by spraying a conidial suspension prepared from cultures on PDA (1 x106 spores ml ) in sterile water on healthy leaves of four test plants (5- 7 leaf stage, cv. Niue). Control plants were sprayed with sterile water. Plants were kept in a greenhouse (average minimum temp. 24°C, max. 37°C and relative humidity 64%). Symptoms appeared 2 weeks post-inoculation on inoculated plants, whereas control plants remained asymptomatic. Reisolation of the pathogen with the same morphological characters was achieved from symptomatic plants only, hence confirming the causal agent as C. colocasiae. To our knowledge, this is the first report of C. colocasiae on C. esculenta in Iran.