Temperature and moisture content stimulate the growth of fungi on healthy stored grain over time

When grain is harvested and correctly stored it should remain free of infestation from most pests and diseases. Issues only arise when the storage conditions are breached, allowing the entry of pests, water or other contaminants. The hypothesis of this study was that a low level of fungal contamination is always present in healthy stored grain, and that these fungi may be stimulated to grow depending on the temperature at which the grain is stored, the moisture content of the grain, and the duration of storage. Wheat grain at varying moisture contents from 10.4 to 15.2%, was stored for one, two or six months at 15, 20, 25 or 32SC. Gamma-irradiated grain was included as a control. At each harvest time, for each combination of temperature/moisture/storage time treatments, 10 grains were plated on to full strength potato dextrose agar to determine changes in fungal frequency. The number of grains that exhibited fungal growth was recorded , and isolates emerging from the grain were sub-cultured for identification. After one month of storage, the highest percentage of fungal contamination occurred in grain at a moisture content of 11.4 and 13.5% that was stored at 15 and 25' C. After two months of storage, the highest percentage of fungal contamination occurred in grain at a moisture content of 13.5% stored at 25' C. The lowest contamination occurred on grain of 10.4% moisture content stored at 15, 25 and 32.5' C. The most commonly isolated genera were a putative Botryosphaeriaceae (Tiarosporella) that is a likely first report on stored wheat, and Alternaria spp. Most Alternaria spp. were found in grain of 11% moisture content stored at 20°C for one month; however the frequency of isolation of Alternaria sp declined after two months in storage. Nigrospora oryzae and Rhizopus sp. occurred at low frequency. The number of putative Botryosphaeriaceae increased after two months. It is not surprising that fungi can be stimulated to grow on stored grain , particularly when it is stored at high moisture content. What is interesting is the range of fungi that are present, as many of them are capable of producing secondary metabolites such as mycotoxins. Our work is continuing to investigate how the frequency and variation of genera change over time, with the aim of developing a rapid method for early detection of fungal contamination