Underground stem canker of cocoa caused by Pythium vexans

Species of Pythium occur world-wide as plant pathogens and saprophytes in soils. They seldom attack mature tissues of woody perennials except under conditions conducive to disease development. Pythium vexans de Bary was recenty isolated from the underground stem canker of cocoa (Theobroma cacao L.) in Peninsular Malaysia. The fungus was also isolated from necrotic roots and soils around infected trees. A disease survey of 46 hectares of 4-year old clonal cocoa trees in April 1987 showed that 14percent of the trees were affected with canker which ranged in size from 14 to 215 cm2. The canker never extends above the soil line, but in advanced cases, affected trees wilt and die. The disease is closely associated with heavy rainfall. Sporangia were profusely formed when 24-hr old mycelial mats were placed in 10percent soil extract suspension for 48 hrs. Pathogenicity studies showed that the fungus (P 161) was pathogenic to cocoa seedlings, but less virulent than Phytophthora palmivora (P 159) isolated from the above-ground stem canker. Current practice to control the disease involves regular inspection to identify infection loci, particularly following period of wet weather. Trees showing foliar symptoms are often beyond saving and are excavated and removed. Neighbouring trees are inspected by removing the soil around the collar and drenching the scraped lesions with a suspension of 0.25percent metalaxyl. Studies also showed that the disease could be controlled by scraping the affected bark off and allowing the tissues to dry