Soft rootrot of cassava (Manihot esculenta Cranz)

Abstract

A Phytophthora species was isolated from the transition layer tissues of rotten cassava tuber roots. Its sporangia were irregular in size and shape. They were able to germinate or liberate zoospores at certain temperature and moisture conditions. Its oogonium was globose; the amphigynous antheridium adhered to its base. The fungus formed also some oospores in tap water. The characteristics of the sporangia and oospores being considered, the species can be identified as Phytophthora drechsleri. Inoculation tests into small vertical incisions made on the stem proved the pathogenicity of the fungus. The plants which were inoculated died due to the putidness around the inoculated wounds. Plants which were wounded but not inoculated remained healthy. When the fungus was introduced into the root incision it caused tissue deterioration and sometimes death of the plants. Drawings of the main structures of the Phycomycete are presented. Some control measures are suggested considering primarily physical conditions of the soil. Resistant varieties would certainly be the most appropriate control process in areas where soils are highly infested with spores of the pathogen.