Sorghum grain mold: identifying fungal colonisers, quantifying concomitant mycotoxins and exploring conducive weather periods.

Sorghum grain mold (SGM), a pre-harvest biotic constraint, negatively affects the crop yield and quality across the globe. Our study had two aims, to: i) identify mycotoxigenic fungi in sorghum grain and to quantify concomitant mycotoxins and ii) explore conducive periods during anthesis required to initiate infection and grain colonisation. Harvested grain, from previous field trials (2017-2019), as well as historic field trial data (2009-2013), from KwaZulu Natal and Limpopo (South Africa) were used. Fungi were identified through morphological characteristics and SEQ-PCR. Twenty-four representative samples were sent to the Southern African Grain Laboratories for accredited mycotoxin analysis. Non-linear regression analyses of meteorological parameters, Fusarium graminearum species complex (FgSC DNA) biomass and mycotoxin accumulation were conducted. Mycotoxigenic fungal genera, i.e., Alternaria, Curvularia, Fusarium and Phoma were identified in sorghum grain, additionally, Fusarium graminearum and F. culmorum were confirmed to species level. Mycotoxins were only detected in nine of the 24 samples. One of four samples that contained zearalenone had a concentration of 1250 μg/kg. Five samples contained deoxynivalenol (DON) although they were all lower than 1000 μg/kg. Aflatoxins and fumonisins were not detected. Weather parameters required to initiate infection and increase the mean FgSC DNA biomass in sorghum grain were identified as, i) five conducive days of maximum temperature between 20 and 28 °C (R2 = 0.81), ii) three to eight days of rainfall greater than 1 mm (R2 = 62) and iii) six conducive days of 70% relative humidity (R2 = 0.98). There was significant positive correlation between FgSC DNA concentration and DON and nivalenol. Fungi associated with aflatoxins and fumonisins were not identified, hence these toxins were not detected. However, caution should be taken since aflatoxins producing fungi could contaminate the stored sorghum. All mycotoxins detected are Fusarium spp. toxins where one sample exceeded the maximum legislative limit. Our study indicated a risk of increased deoxynivalenol and nivalenol when greater concentrations of FgSC were detected. Defining conducive periods of temperature, rainfall and relative humidity associated with FgSC infection of grain provides potential future risk indicators for producer intervention.