Effect of Water Potential on Mycelial Growth and on Production and Germination of Sclerotia ofMacrophomina phaseolina

The influence of osmotic water potential (Ψs) on growth parameters of Macrophomina phaseolina was determined at 30°C on potato dextrose agar (PDA) and potato dextrose broth (PDB) adjusted to different water potentials with KCI, NaCI, or sucrose. Radial growth of M. phaseolina on PDA was maximum at Ψs values between -1,220 and -1,880 J/Kg, but growth was reduced at lower Ψs values. Biomass of M. phaseolina grown on PDB was maximum at Ψs values between -2,030 and -3340 J/Kg. Production of sclerotia of M. phaseolina on PDB was not affected by Ψs of -670 to -3,920 J/Kg, but was completely inhibited at Ψs between -8,270 and - 12,020 J/Kg. After 2 days of incubation, the germination of sclerotia of M. phaseolina on PDA was not significantly affected at Ψs of -320 to -4,760 J/Kg, but germination was drastically reduced with further reductions in Ψs. After 10 days of incubation, the percentage of germinating sclerotia was higher than at earlier sample times, particularly at Ψs between -7,250 and -10,220 J/kg. The growth of colonies formed by germinating sclerotia at Ψs greater than -3,060 J/Kg was reduced. The influence of Ψs adjusted by KCI, NaCI, or sucrose on M. phaseolina followed a similar pattern, but growth of M. phaseolina was much greater in media adjusted with sucrose. Results of this study showed that M. phaseolina can grow and produce large quantities of sclerotia under relatively low water potentials. These findings may partially explain why diseases caused by M. phaseolina have been recognized previously as being favored by drought.