MODELS TO PREDICT THE COMBINED EFFECTS OF TEMPERATURE AND RELATIVE HUMIDITY ON PECTOBACTERIUM ATROSEPTICUM AND PECTOBACTERIUM CAROTOVORUM subsp. CAROTOVORUM POPULATION DENSITY AND SOFT ROT DISEASE DEVELOPMENT AT THE SURFACE OF WOUNDED POTATO TUBERS

The main objectives of this study were to evaluate and model the influence of temperature (10, 15 and 20°C), relative humidity (86, 96 and 100%) and initial concentration of bacterial inoculum (105, 107 et 109 CFU ml-1) on the population density of Pectobacterium atrosepticum (Pba) and Pectobacterium carotovorum subsp. carotovorum (Pcc) which are important potato pathogens in temperate climates, and on the development of soft rot symptoms caused by these bacteria at the surface of wounded potatoes tubers under controlled conditions. Experiments were carried out according to a Box-Behnken experimental design, simplifying prediction of the combined effects of three controlled factors. With both bacterial species, statistical analysis showed a significant effect of temperature, relative humidity and initially applied bacterial concentration on population dynamics and soft rot development at the surface of wounded potato tubers. Multiple regression analyses and the contour plots showed that the temperature is the most important factor, followed by the initially applied bacteria concentration and relative humidity. More than 64% of the variability of the soft rot symptoms observed could be explained by the presence of Pba and Pcc at the level of wounded potato tubers under the combined effect of tested factors. The quadratic polynomial models developed in our research should integrate the heterogeneity of tested bacteria belonging to the same species (which was not evaluated in this preliminary investigation) in further research.