ARS | Publication request: Culture Media and Sources of Nitrogen Promoting the Formation of Stromata and Ascocarps in Petromyces Alliaceus (Aspergillus Section Flavi)

Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 24, 2005
Publication Date: October 12, 2005
Citation: Mcalpin, C.E., Wicklow, D.T. 2005. Culture media and sources of nitrogen promoting the formation of stromata and ascocarps in Petromyces alliaceus (Aspergillus section Flavi). Canadian Journal of Microbiology. 51(9):765-771.

Interpretive Summary: The fungus Petromyces alliaceus is considered responsible for an important nephrotoxic mycotoxin that is occasionally observed in California fig orchards. The fungus reproduces sexually yielding fertile survival structures called stromata and this represents the principal source of genetic variation impacting mycotoxin production. There is a need to identify conditions promoting sexual reproduction in P. alliaceus stromata and the research has identified culture media and sources of nitrogen supporting a five fold increase in fertility. The work is important to genetic investigations of mycotoxin production in P. alliaceus and in understanding the significance of fertile versus non-fertile stromata to the fungal disease cycle in orchards, vineyards or crop fields.

Technical Abstract: Petromyces alliaceus is the only known sexually reproducing fungus classified in Aspergillus section Flavi. The fungus was recently associated with ochratoxin A contamination that is occasionally observed in California fig orchards. When grown on solid culture media P. alliaceus produces numerous grey-black, sclerenchymatous stromata, some of which may slowly ripen to form ascocarps. Ascospores (meiospores) represent the principal source of genetic variation among naturally occurring clonal populations of P. alliaceus. The goal of this research was to identify culture media and sources of nitrogen that best support the formation of stromata with ascocarps. Three cultures of Petromyces alliaceus (NRRL 31813, 31814, 31816) isolated from crop field soils were grown on selected agar media in Petri dishes (7 mos. dark incubation, 30C). The largest numbers of stromata were recorded for cultures grown on Czapek's agar (CZA) and a mixed cereal agar (MCA) while the percentage of stromata containing ascocarps was greatest (p = <0.05) for cultures grown on MCA (25-28%). When P. alliaceus was grown on standard CZA containing 0.3% NaNO3, only 5% of the stromata contained ascocarps. A greater percentage of the stromata (15-22%) formed ascocarps when the NaNO3 in CZA was replaced with an equivalent amount of available nitrogen supplied by ammonium tartrate, glutamic acid, or serine.