Influence of soil moisture on formation of perithecia and pycnidia and spore release in Diaporthe phaseolorum var. caulivora

The formation of perithecia and pycnidia and the release of ascospores and conidia, respectively, by Diaporthe phaseolorum var. caulivora was determined on soil at matric potentials (psi m) between 0 and -0.32 MPa. D. p. caulivora infected internodes of susceptible soybean cv. Bedford were incubated at 24 +/- 1 C on the surface of soil samples adjusted to psi m = 0.0, -0.01, -0.02, -0.04, -0.08, -0.16, or -0.32 MPa. The number of perithecia and pycnidia formed and the number of sporulated perithecia and pycnidia that released ascospores and conidia, respectively, per square centimeter of soybean internode were counted at weekly intervals. Large numbers of perithecia (24-93/cm2) formed at psi m between 0 and -0.08 MPa after incubation for 7 days, whereas fewer perithecia (3-37/cm2) formed at -0.16 and -0.32 MPa after incubation for 21 days. Maximum production of perithecia occurred at 14 days of incubation at 0 to -0.02 MPa, at 28 days at -0.04 MPa, and at 35 days at -0.08 MPa. The highest number of sporulated perithecia was produced at -0.04 MPa after incubation for 28 days. Ascospores and conidia were released from the perithecia and pycnidia, respectively, in a gelatinous matrix. The gelatinous matrix dried on the surface of perithecia after 7 days. Ascospores in the dried gelatinous matrix remained viable for 35 days. The basal internode produced the highest number of perithecia and sporulated perithecia. The number of pycnidia formed and the release of conidia were erratic at different matric potentials. These results suggest that factors other than soil moisture govern the production of pycnidia