Influence of nutrient sources and environmental conditions on vegetative growth of fungus, Erythricium salmonicolor (Berk. & Broome) Burdsall in cocoa

Abstract

Erythricium salmonicolor is the cause of pink disease in cocoa in the humid tropics. The growth of 4 isolates (ES-WR006, ES-WR008, ES-ER009 and ESER010) of the fungus on different sources of nutrient (carbohydrate, amino acid and vitamin) and environmental conditions (temperature, pH and light) were evaluated in the laboratory. The pink disease isolates utilised the nutrients to different extent, with the best radial mycelial growth rate achieved with isolate ES-WR008 followed by ES-WR006. None of the isolates produced spores in the laboratory. The isolates utilised polysaccharides as best as disaccharides and monosaccharaides. Among 7 amino acids assayed, aspartic acid was the most promising for vegetative growth. The best radial mycelial growth on vitamins was attained on folic acid and riboflavin. The minimum, optimum and maximum temperatures for growing the fungus were 18°C, 28°C and 34°C respectively. Survival of E. salmonicolor isolates within the temperature range of 18-34°C is consistent with temperatures for growing cocoa in Ghana. The isolates also grew well within a pH range of 4-8, with the best growth at pH 6. Light quality and duration influenced the vegetative growth of the isolates. Pink disease isolates maintained in 24 hr darkness or 24 hr light performed better than in alternating 12 hr darkness and 12 hr light. There was a significant (p < 0.05) repressive effect of the alternating dark and light exposures on vegetative growth of the fungus. The implications of the results for efficient management of the cocoa disease are discussed.