2.5.8
DEVELOPMENT OF POPULATIONS OF SOFT ROT BACTERIA ON WITLOOF CHICORY LEAVES DURING THE ROOT PRODUCTION IN THE FIELD

BM SCHOBER and JC ZADOKS

Department of Phytopathology, Wageningen Agricultural University, PO Box 8025, 6700 EE Wageningen, The Netherlands

Background and objectives
Bacterial soft rot of witloof chicory causes major damage during forcing of the chicory heads, although colonization and infection of the crop occurs in the root production prior to forcing. During a 4-year experiment, witloof chicory plants grown for root production, were surveyed for the presence of the softrot bacteria Erwinia carotovora and Pseudomonas marginalis on the leaves. Populations were quantified throughout the growing season to establish correlations with environmental variables and to test the hypothesis of a population build-up in the field.

Materials and methods
Chicory plants were sampled from a root production field on the Experimental Station for Arable Farming in Lelystad, The Netherlands. Pectolytic bacteria collected from the leaves were identified to genus level and the influence of environmental variables on their occurrence was calculated. Linear regression was used to establish relationships between temperature, humidity, radiation, rainfall and the average number of pectolytic isolates.

Results and conclusions
A significant correlation (R2=0.47) was found for the regression model relating log-transformed monthly counts of bacterial isolates to the air temperature and the month of the growing season. The number of isolates found per month was positively correlated with the monthly average temperature (Pearson's r=0.41; P=0.072) and time expressed as months (Pearson's r=0.70; P=0.001). The average monthly growth rate (r=0.022) of the bacteria differed significantly from zero during the field season, indicating a continuous increase of bacteria owing to growth and migration. The ratio between E. ; carotovora and P. ;marginalis isolates changed during the growing season from a predominance of P. ;marginalis from May until July owing to their strong saprophytic capabilities [1] to an equal ratio of both pathogens from August until harvest in late September. Only populations of E. ;carotovora were effected by the global radiation level (Pearson's r=-0.58 ). The number of E. ;carotovora isolates increased later in the growing season when the bacteria were protected from high radiation levels and adverse temperatures in the microclimate of the dense canopy. Future analysis of the incidence of softrot during forcing of the chicory heads will show whether high numbers of bacterial isolates on the roots at harvest can influence the occurrence of bacterial softrot during forcing.

References
1. Jaques, MA, Kinkel, LL, Morris, CE, 1995. Applied and Environmental Microbiology 61, 899-906.