The effect of temperature and relative humidity on the development of Diaporthe phaseolorum var. caulivor in culture

The ascigerous state of Diaporthe phaseolorum var. caulivora (DPC), the causal organism of soybean stem canker, was studied under laboratory conditions to determine the effects of temperature and relative humidity on the development of perithecia and subsequent ascospore discharge. Discharge of the ascospores under controlled conditions of temperature and humidity was monitored in order to clarify aspects of the epidemiology and distribution of the soybean stem canker pathogen. Development of the perithecia, on sterile soybean stems, embedded in water agar culture, occurred between 20-26 C with an optimum at 22 C. Growth of DPC in culture increased with increasing temperatures from 20 to 26 C. Discharge of ascospores from three-week old perithecial cultures was stimulated by relative humidities above 55% at 22-23 C. Humidities between 70-100% favored a greater number of ascospores to be released. Histological studies were made in an attempt to learn more about the growth of the pathogen within the host. Ascospores were germinated on excised soybean stems in moist chambers and samples were taken at two day intervals to observe the stages of perithecial development. Light microscopy and histological studies revealed hyphal tips penetrating the epidermis and ramifying throughout the parenchyma cells of the stem tissue. The first recognizable structures to be formed were stromatal initials.