Physiological and vegetative growth variations of cacao thread blight disease causing morphotypes in Ghana

Thread blight disease (TBD) is caused by several morphologically distinct pathogens classified into morphotypes. The morphotypes were identified as Marasmius crinis-equi, (Morphotype A); Marasmius tenuissimus, (Morphotypes B and C); Marasmiellus scandens (Morphotype D) and Marasmiellus palmivorus (Morphotype E). Evaluation of 52 isolates revealed consistent distinguishing growth characteristics in culture. The vegetative growth of 10 representative isolates from distinctive morphologies was assessed on commercial and cacao-based media under different temperatures, pH, and light. Mycelial growth in cacao-based media was similar to commercial media. On commercial media at 25°C, it was possible to distinguish among the TBD isolates based on cultural characteristics. Marasmius species (Morphotypes A, B, C) grew best at 25°C with no growth at 35°C. Marasmiellus spp. (Morphotypes D and E) grew best at 30°C but at 35°C the rate reduced. The isolates grew over pH 4 to 12 with optimum growth at pH 8. Isolate GH 12 (M. palmivorus) grew best in both continuous darkness and light. Isolate GH 37 (M. tenuissimus) behaved like a day-neutral pathogen. GH 76 (M. crinis-equi) grew better under continuous darkness. These imply that the TBD isolates are physiologically distinct and a single-prong approach to their control will be difficult.   Key words: Environment, fungi, Marasmiellus, Marasmius, nutrition.