Cardinal temperature differences, determined in vitro, between closely related species and subspecies of pectinolytic bacteria responsible for blackleg and soft rot on potatoes.
Potato blackleg and soft rot cause major losses and are caused by two bacterial genera, Pectobacterium and Dickeya. Species affecting potatoes are Pectobacterium atrosepticum (Pba), Pectobacterium carotovorum subsp. carotovorum (Pcc), Pectobacterium carotovorum subsp. brasiliense (Pcb), Pectobacterium wasabiae (Pwa), Dickeya dadantii (Dda) and Dickeya solani (Dso). Pathogenicity of these species is dependent on temperature, with each species having its own optimal temperature and temperature range for growth, leading to varying degrees of losses. Pectobacterium atrosepticum, a temperature sensitive species, mainly occurs in temperate climates, Pcc in temperate to tropical, and Dickeya spp. in subtropical environments. The aim of this study was to determine the cardinal growth temperatures for the species responsible for blackleg and soft rot in vitro. Bacterial isolates were incubated in a temperature gradient shaking incubator at 30 different temperatures ranging from ±5°C to ±56°C, and growth measured at two set time intervals. Results were statistically analysed using the Gaussian function. The optimal temperature of 31°C and temperature range of 20°C to 38°C for Pectobacterium carotovorum subsp. brasiliense, was similar to those recorded for Pcc. Pectobacterium wasabiae grew at an optimal temperature of 29°C and range of 20°C to 34°C. Higher optimal temperatures of 32°C and 34°C, with ranges of 21°C to 38°C and 23°C to 41°C were recorded for Dda and Dso, respectively. The minimal variation in optimal temperatures between different species might be an indication that temperature ranges, rather than optimal temperature, play an important role in disease development. Results for Dso, which has not yet been reported in South Africa, are especially important in light of prevailing temperatures in South African potato production regions.