Development of populations of softrot bacteria on witloof chicory leaves during root production in the field

During a 4-year held experiment at the Experimental Station for Arable Farm ing in Lelystad, the Netherlands, witloof chicory plants, grown for root pr oduction, were surveyed for the presence of epiphytic or saprophytic popula tions of the softrot bacteria (Erwinia carotovora and Pseudomonas marginali s) on their leaves. Isolates of pectolytic bacteria collected from the leav es were identified and the influence of environmental variables was assesse d testing the hypothesis of an increase of the number of isolates on plants in the field. A significant linear correlation (R-2 = 0.47, n = 20, P = 0. 002) was found for the regression of the number of pectolytic isolates on t ime (month) and temperature (monthly average). The average monthly growth r ate of the softrot bacteria was significantly greater than zero during the field season, indicating an increase of the populations of softrot bacteria . The ratio between the number of Erwinia and Pseudomonas isolates changed during the growing season from a predominance of pseudomonads from May unti l July to an equal ratio from August until harvest. The number of isolates of Erwinia carotovora was negatively correlated with total global radiation . The increase in the number of Ece later in the growing season was attribu ted to protection from high radiation levels and adverse temperatures in th e microclimate under the canopy.