Aflatoxin in maize,a multifaceted answer of Aspergillus flavus governed by weather,host-plant and competitor fungi

Aspergillus section Flavi is able to produce aflatoxins (AFs) in vitro down to 0.85 a_w with a potential maximum occurring between 0.95 and 0.99 a_w, while in the field AFs increased significantly with kernel humidity below 0.95 a_w. In order to clarify this apparent discrepancy, a 3-year field trial with artificial inoculation of maize ears with Aspergillus flavus strains was organised. The co-occurrence of Aspergillus section Flavi and Gibberella fujikuroi species complex (Gfsc) was observed. The incidence of A. flavus was significantly influenced by the year and negatively related to Gfsc incidence. In 2012, when the highest temperature and the lowest rain were registered, aflatoxin B₁ (AFB₁) content was the highest and a_w < 0.95 in kernels was measured early throughout the growing season. In 2013 and 2014, the temperature was lower and rain more abundant, and a_w decreased below 0.95 only close to harvest and AFB₁ contamination was limited. The possibility of describing/predicting reasonably well a_w dynamic based on temperature, (degree day) was confirmed. With a_w > 0.95, a positive correlation between AFB₁ production rate and a_w was found, but a negative correlation resulted with a_w < 0.95. Other factors than a_w play a role, but a_w = 0.95 should be considered as an indicator of suitable conditions for rapid aflatoxin accumulation.