Influence of Water Potential on Survival of Sclerotia in Soil and on Colonization of Bean Stem Segments by Macrophomina phaseolina . Gilberto Olaya, Former Graduate Research Assistant; Cornell University, New York State Agricultural Experiment Station, Geneva 14456-0462. George S. Abawi, Professor, Department of Plant Pathology; and John Barnard, Computer Services, Cornell University, New York State Agricultural Experiment Station, Geneva 14456-0462. Plant Dis. 80:1351-1354. Accepted for publication 16 August 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1351.

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 300C, 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 ψ m 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 (ψ m = -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 KC1 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.

Keyword(s): ashy stem blight, charcoal rot, Phaseolus vulgaris, water stress