Effect of interacting conditions of water activity, temperature and incubation time on Fusarium thapsinum and Fusarium andiyazi growth and toxin production on sorghum grains.
This study examined the effect of interacting conditions of water activity (aW, 0.995, 0.98 and 0.95) and temperature (15, 25 and 30 °C) on growth rate of two Fusarium thapsinum and one F. andiyazi strains isolated from sorghum in Argentina. In addition, the effect of interacting conditions (aW × temperature × incubation time (7, 14, 21 and 28 days)) on mycotoxin production (moniliformin (MON), fusaric acid (FA) and fusarin C (FUS C)) on a sorghum grain substrate was evaluated. Statistical analysis showed that aW and temperature significantly affected growth of both species, mainly the aW. Incubation time significantly influenced mycotoxin production by both species as well, mostly for FA. Maximum growth rates of the F. thapsinum strains were obtained at the highest aW (0.995) and 25 °C and growth rate decreased as aW and temperature were reduced. The same growth profile was observed for F. andiyazi RCFA09 (maximum growth rates at 0.995-25 °C). Mycotoxin production by both species was detected at the highest aW levels whereas at 0.95 aW only low amounts of MON were produced by F. thapsinum. Maximum MON and FUS C production by both F. thapsinum strains was observed at 0.995 aW and 25-30 °C after 28 days of incubation. Also, F. thapsinum strains showed maximum FA production at the highest aW and temperature but after 14 days; after this incubation time toxin levels significantly decreased. The responses to aW and temperature of F. andiyazi were similar to that of F. thapsinum strains in relation to FA and FUS C production. Maximum levels of FA were detected at the highest aW after 14 days of incubation at 25-30 °C. Fusarin C was produced at all assayed temperatures but maximum levels were detected at 30 °C and 0.995 aW after 28 days of incubation. Two-dimensional profiles on the interactions of aW by temperature were developed from these data to identify conditions that indicate a significant risk from MON, FA and FUS C accumulation on sorghum grains. The results of this study suggest that sorghum grains could be colonized by these species and toxin production can occur, especially during development stages under field conditions at high water activity of grains or during grain storage if the drying process is slow or deficient. To our knowledge, this study described for the first time FUS C production by F. thapsinum and F. andiyazi under interacting conditions of aW, temperature and incubation time on sorghum grains.