Is the recently described Macrophomina pseudophaseolina pathogenically different from Macrophomina phaseolina?

Charcoal rot disease causes heavy yield losses to many hosts including cowpea (Vigna unguiculata) and groundnut (Arachis hypogea) in Senegal. The causal agent of the disease was a longtime considered to be Macrophomina phaseolina. However, a new Macrophomina species, Macrophomina pseudophaseolina was reported recently to also cause charcoal rot disease alone or in association with M. phaseolina on several hosts in Senegal. Since 1969, charcoal rot has become increasingly important in cowpea and other crops in the Sahel. It was not known if this status is correlated with the occurrence of the new Macrophomina species. This study therefore aimed to investigate the pathogenicity of M. phaseolina and M. pseudophaseolina on three varieties of cowpea under two temperature regimes. Ten M. pseudophaseolina and nine M. phaseolina isolates were tested on three cowpea varieties in a complete randomized design. Plants were grown in infected soil at two growth temperatures (24/34 and 26/36°C) in a climatic chamber. Disease incidence, level of tissue infection and potential primary inoculum produced in cowpea plants were determined 45 days after planting. By and large, the two Macrophomina species showed the same results, except that at 36°C, M. pseudophaseolina induced more disease development than M. phaseolina in the susceptible cv. Mouride. In conclusion, M. pseudophaseolina induces less disease incidence on cowpea at 34°C than at 36°C. At this temperature, it becomes as damageable as M. phaseolina on cowpea. Key words: Macrophomina phaseolina, Macrophomina pseudophaseolina, growth temperature, disease incidence, potential primary inoculum.