Pathogenicity of Fungi Associated With the Cocoyam Root Rot Blight Complex in South-Eastern Nigeria

Abstract

Disease surveys were carried out in major cocoyam (Xanthosoma sagittifolium (L) Schott) growing communities of Igbariam, Lowa and Umudike in the humid tropics of South-eastern Nigeria in 2009 and 2010 cropping seasons. The highest mean disease incidence of 46.7% and severity of 2.85 occurred in Lowa while the lowest figures of 33.3% and 2.33, respectively were recorded in Umudike. Isolations of associated `fungi made from infected cocoyam root tissues and rhizosphere soils yielded eight different fungal isolates. These were Aspergillus niger Thom and Raper, Botryodiplodia theobromae Pat., Fusarium solani (Mart.) Sacc., Pythium myriotylum Dresch., Rhizoctonia solani Kuhn, Rhizopus nigricans Ehhrenb., Trichoderma harzianum Rifai and Sclerotium rolfsii Sacc. The results showed that P. myriotylum was the most frequently isolated organism in infected cocoyam roots in the three locations with percentage frequency of occurrence of 82–90% and 60–72% abundance in infected soils. Pathogencity tests on susceptible cocoyam variety – Xanthosoma sagittifolium cv ede ocha (NXS001), using all fungal isolates, showed that only P. myriotylum was pathogenic on the test crop. The results of this investigation showed that the cocoyam root rot disease was incited by a single organism viz: Pythium myriotylum and not by a complex of organisms as suggested by earlier workers.

Keywords: Cocoyam root rot blight complex; cocoyam root rot disease; Pythium myriotylum, Cocoyam, Xanthosoma sagittifolium