First Report of Root Rot of Cocoyam Caused by Pythium myriotylum in Sri Lanka

Abstract

Root rot of cocoyam (Xanthosoma sagittifolium L. Schott) caused by Pythium myriotylum Drechsler is a major disease of this crop in Africa (1,2) but is unreported from other regions of the world. During September 1999, commercially grown cocoyam (cv. Ratu-kiri-ala) in Gampaha (7°05'N, 80°00'E), Sri Lanka suffered from severe root rot. Initial symptoms were water-soaked lesions at the root tips that gradually enlarged to rot the entire root system and tuber. Wilting and yellowing of leaves were observed in advanced stages of disease. A Pythium sp. was regularly isolated from the affected roots and an isolate, SC5, was identified as P. myriotylum on the basis of morphology and the internal transcribed spacer (ITS) rDNA sequence. Characteristics of isolate SC5, grown on a grass-leaf water culture (3) were main hyphae up to 8.5 μm wide, oogonia terminal or intercalary (22.5 to 33.8 μm in diameter), antheridia diclinous occasionally monoclinous, one to eight per oogonium, stalks branched, often more or less loosely enveloping the oogonium, antheridium clavate or crook-necked, making apical contact with the oogonium, breadth of antheridium 2.5 to 7.0 μm, oospores aplerotic (17.0 to 22.5 μm in diameter), oospore wall 0.8 to 2.0 μm in thickness, sporangia terminal or intercalary, filamentous, inflated lobulate, and digitate, of variable length, breadth of sporangia 7.0 to 17.5 μm, formed in water; zoospores formed at 25°C, and diameter of encysted zoospores 10.0 to 12.5 μm. Cardinal temperatures on potato carrot agar 8°C minimum, 34°C optimum, and 37°C maximum with daily radial growth rate for 34°C at 32.8 mm. The ITS rDNA sequence of the isolate matched the sequences of P. myriotylum in GenBank (Accession Nos. AB095051 and AF452156) and isolate CBS254.70 used for the species description by van der Plaats-Niterink (3). The sequence of SC5 has been deposited in GenBank, Accession No. DQ102701. Pathogenicity tests used potted cocoyam plants (20 cm high), planted in an autoclaved potting mix. Four agar disks (8 mm in diameter) of isolate SC5 grown at 25°C for 48 h on potato dextrose agar was mashed and injected at a depth of 2 to 3 cm in the soil around the roots. Inoculated plants were placed in transparent plastic bags and kept for 7 days in a growth chamber maintained at 24 to 26°C with continuous light (52 to 98 μmol m-2·s-1). The experiment was carried out twice with three replications for each test. Dark brown rotting on roots and wilting of leaves were observed in 7 days after the inoculation. P. myriotylum was reisolated from diseased tissues and found to be morphologically identical to the original isolate SC5. Noninoculated control plants remained healthy. On the basis of the symptoms, morphological and molecular characteristics and confirmation of pathogenicity, P. myriotylum is the causal agent of root rot of cocoyam. To our knowledge, this is the first report of P. myriotylum causing root rot of cocoyam in Sri Lanka.