Modelling the effects of pH and temperature on the mycotoxin production of Alternaria alternata in a tomato-based medium.

Alternaria species are ubiquitous and includes both pathogens and saprophytes that may damage crops and cause postharvest decay. Alternaria spp. are able to grow at low temperature and may responsible for spoilage during refrigerated transport and storage. Certain species are also capable of producing mycotoxins which can contaminate plant products. Due to their thin skin, tomatoes are particularly at risk of being infected by Alternaria, and rapid growth may occur in soft tissues, causing the well-known black mold. In that context, predictive models of mycotoxin synthesis may be helpful in determining the levels of these mycotoxins for conditions supporting growth of Alternaria spp. The objective of this study was to evaluate the effects of temperature and pH on the toxin production of an A. alternata strain in a tomato-based medium A full factorial design of six pH levels (ranging from 2 to 7) and 11 incubation temperatures (ranging from 3.2 to 37°C) was undertaken. Tomato-based agar plates at aw 0.99 were centrally inoculated with a standardized inoculum of 40 spores/5µL. Each experimental condition was tested in triplicate. For each of the 35 combinations supporting fungal development, mycotoxin accumulation was measured. Measurements of TeA, AOH and AME concentrations were performed on plates fully covered by the fungus, using HPLC. Quantification was performed by measuring peak areas at toxins retention time and comparing them with the relevant calibration curves. The experimental observations suggest that increasing pH induces a linear increase in the square root of mycotoxin synthesis, followed by a logistic decrease. The equation used to describe this relationship is based on the model of Bréand et al. (1997) and is defined by the following parameters: pHminTOX (minimum pH for toxin production), pHoptTOX (optimum pH for toxin production), TQoptTOX (toxin quantity at pHoptTOX) and TQminTOX (the toxin quantity at high pH values). Secondary models were developed to describe the relationship between temperature and the parameters (pHminTOX, pHoptTOX and TQoptTOX) while TQminTOX was assumed to be independent of temperature TeA has been described as the major mycotoxin produced by Alternaria spp. on tomatoes. Perhaps not surprisingly, only TeA could be measured in this work while AOH and AME have not been detected. TeA measurements show an important contribution of acidic pH to mycotoxin synthesis, in particular near pH 4. At similar pH levels, incubation at low temperatures (from 6.5°C to 12°C) resulted in higher TeA production than at temperatures above 20°C. The developed model for TeA synthesis describes accurately the experimental measurements (R2=0.98). The predicted maximum toxin quantity TQoptTOX increases from of 180 µg/g at 6.5°C to 290 µg/g at 12°C. It then decreases until reaching 70 µg/g at 30°C. The parameters pHminTOX and pHoptTOX were also found to be temperature dependent The model for toxin production can be used together with a model of fungal growth to highlight at-risk storage conditions for which mycotoxin production is at its highest although fungal growth is rather small