Effects of culture media and environmental factors on mycelial growth and pycnidial production of Potebniamyces pyri.

Potebniamyces pyri (anamorph Phacidiopycnis piri) is the causal agent of Phacidiopycnis rot of apples and pears. The disease has recently been recognized in pears in the USA. Little information on the basic biology of the fungus is available. In this paper, we report the effects of culture media, temperature, water potential and pH on mycelial growth, and the effects of media and light on pycnidia production. Prune juice agar was the best for rapid mycelial growth. Pear juice agar, apple juice agar, potato dextrose agar, and oatmeal agar (OMA) also favoured mycelial growth. Czapek-Dox agar was not suitable for mycelial growth. The fungus was able to grow at temperatures from -3 to 25 degrees C. Optimal mycelial growth occurred between 15 and 20 degrees. The average radial growth rate on OMA was 3.9 mm d(-1) at 15 degrees and 4.4 mm d(-1) at 20 degrees. Mycelial growth was not observed after 10 d at 30 degrees, but growth resumed at 20 degrees. The fungus failed to resume growth at 20 degrees after being incubated for 10 d at 35 degrees. The fungus was able to grow at water potentials as low as -4 MPa but no growth took place at -7.3 MPa. Active mycelial growth was observed on OMA at pH between 3.2 and 6.1. Optimal growth was observed at pH around 4 and no growth was observed at pH 7.1. No pycnidia or very few formed on the nine media at 20 degrees in the dark. Fluorescent light significantly stimulated formation of pycnidia. OMA was the best medium for production of pycnidia and macroconidia. Pycnidia that formed on 8-wk-old OMA cultures incubated at 20 degrees under 12 h dark/12 h light produced abundant macroconidia and the technique is recommended for inoculum production of conidia for research.