Water and temperature relations of softrot bacteria: growth and disease development

The effect of water availability and the temperature of the growth substrate on growth and disease development of softrot bacteria were studied using artificial media and plant material. Water availability was measured as the osmotic potential of a solution (psi(osm)) and was assessed for solutions of PEG4000 and KNO3 as artificial osmotica and for plant tissue of chicory heads. Growth of softrot bacteria was found at water potentials from psi = -0.12 MPa to psi = -8.0 MPa but the lag phase of the growth curve increased with decreasing water potential. The relative growth rates of the three softrot pathogens showed a sigmoidal relationship with water potential, the relative growth rates decreasing rapidly at water potentials lower than psi = -1.5 MPa. The water potential of harvested chicory heads decreased with storage time of the harvested crop but was still within the growth limits for softrot bacteria. In relation to temperature, the relative growth rate of Erwinia carotovora subsp. carotovora (Ecc) was highest at 10 degrees C, of Erwinia carotovora subsp, atroseptica (Eca) at 15 degrees C and of Pseudomonas marginalis (Pm) at 5 degrees C. Chicory heads of two cultivars, Rumba and Salsa, inoculated with Ecc, had a significantly higher disease severity at 30 degrees C (0.72 for Rumba and 0.47 for Salsa) than at lower or higher temperatures. In conclusion, temperature and water availability during forcing of chicory were not factors limiting populations of softrot bacteria. Possibilities for crop protection thus only avail during chicory root storage. During storage a high death rate combined with a low growth rate of the softrot bacteria may result in a decrease of bacterial populations below the minimum densities needed for infection during the forcing of chicory heads.