Influence of water activity and temperature on growth and mycotoxin production by isolates of Pyrenophora tritici-repentis from wheat

Pyrenophora tritici-repentis is a phytopathogenic fungus that can infect wheat kernels and leaves, causing red smudge and tan spot, respectively. A number of P. tritici-repentis isolates have been shown to be mycotoxigenic, producing the anthraquinone mycotoxins emodin, catenarin and islandicin. The influence of water activity (aw; 0.75–0.99 aw) and temperature (5–45 °C) on growth and mycotoxin production by five isolates of P. tritici-repentis was studied. All isolates grew at 0.95–0.99 aw and 15–25 °C on a wheat-based medium, with three isolates also producing small colonies at 5 °C. The optimal growth conditions for all isolates consisted of 0.99 aw and 25 °C, and growth was significantly reduced at 0.95 aw and/or 15 °C. Emodin and catenarin were detected in cultures of all isolates, at concentrations ranging from 0.06 ± 0.04 to 11.31 ± 2.96 µg emodin/g medium, and from 0.09 ± 0.06 to 53.42 ± 4.36 µg catenarin/g medium. In most isolates, the concentrations of emodin and catenarin declined under suboptimal growth conditions. However, in some isolates, significant increases in the concentrations of both compounds were observed under suboptimal conditions. Islandicin was detected in cultures of only three isolates, at concentrations ranging from 0.07 ± 0.05 to 5.69 ± 0.76 µg/g medium. The results suggest that growth and mycotoxin formation by P. tritici-repentis are markedly influenced by aw and temperature, and that this fungus is hygrophilic. Therefore, infection and contamination of kernels by P. tritici-repentis are likely to occur in the field rather than in storage. To our knowledge, this is the first study on the effect of environmental factors on mycelial growth and mycotoxin production by P. tritici-repentis.