Influence of Water Potential on Survival of Sclerotia in Soil and on Colonization of Bean Stem Segments byMacrophomina phaseolina

Survival of sclerotia of Macrophomina phaseolina was studied in both nonpasteurized (natural) and pasteurized very fine, sandy loam soils. The soil matric water potentials (Ψm) tested were 0, -10, -30, -100, -300, -500, and -1,500 J/Kg. An air-dried soil treatment also was included in this study (about -40,000 J/Kg). At a Ψm of 0 J/Kg, viability of sclerotia was 40 and 0% after 2 and 4 weeks of incubation at 30°C, respectively. Survival of sclerotia decreased with time in the soil samples adjusted from -10 to -1,500 J/Kg and remained about 100% viable in the air-dry soil treatment. After 20 weeks of incubation, viability of sclerotia was reduced to 10% in the Ψm treatment of -10 J/kg and was more than 50% at Ψm of -500 and -1,500 J/Kg. Survival of sclerotia followed a similar trend in the pasteurized soil. However, the rate of decrease in survival of sclerotia in the pasteurized soil (Ψm = -10, -30, -100, -300, -500, and -1,500 J/Kg) was less than that in the natural soil. The influence of water potential on colonization of bean stem tissues by M. phaseolina was studied in small constant-humidity chambers controlled with KCI solutions. Segments of bean stems inoculated with sclerotia of M. phaseolina were incubated in the chambers for 10 days at 30°C. The extent of colonization was increased as osmotic water potential (Ψs) decreased and was most severe at Ψs = -3,990 J/Kg. However, further decreases in the Ψs resulted in a decrease in the extent of colonization, which was significantly diminished at Ψs lower than -7,150 J/Kg. The number of sclerotia of M. phaseolina produced per square millimeter of colonized tissues was also increased as the Ψs in the incubation chamber was decreased from 0 to -5,330 J/Kg. Maximal number of sclerotia was produced at Ψs = -3,990 J/Kg and sclerotial production was still relatively high at Ψs = -8,080 J/Kg. These results show that M. phaseolina can survive and colonize beans under relatively dry conditions.