SummaryThe 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 (ψosm) 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 ψ= ‐0.12 MPa to ψ= ‐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 ψ= ‐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°C, of Erwinia carotovora subsp. atroseptica (Eca) at 15°C and of Pseudomonas marginalis (Pm) at 5°C. Chicory heads of two cultivars, Rumba and Salsa, inoculated with Ecc, had a significantly higher disease severity at 30°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.
During a 4.-year field experiment at ihe Experimental Slatton for Arable Farming in Lelystad, the Netherlands, witloof chicory plants, grown for root production, were surveyed for Ihe presence of epiphylic or saprophytic populations of the softrot bacteria (Enriniu carotovora and Pseudomonus marginalia) on their leaves. Isolates of pectolytic bacteria collected frotn the leaves were identified and the influence of environmental variables was assessed testing the hypothesis of an increase of the number of isolates on plants in the field. A significant linear correlation (R-= 0,47, n = 20, P = 0.002)" was found for the regression of the number of pectolytic isolates on titne (tnonth) and temperature (monthly average). The average monthly growth rate of the softrot bacteria was significantly greater than zero during the field seasoti, indicating an increase of the populations of softrot bacteria. The ratio between the number of Envinia and Pseudomonus isolates changed during the growing season from a predominance of pseudomonads frorn May until July to ati equal ratio from Atigust until harvest. The number of isolates of Erwinia carotovora was negatively correlated with total global radiation. The increase in the number of Ecc later in the growing season was attributed to protection from high radiation levels and adverse temperatures in the tmicroclimate under the canopy,
During the cold storage of witloof chicory roots, changes in the populations of the softrot bacteria Erwinia carotovora ssp. carotovora (Ecc), Erwinia carotovora ssp. atroseptica (Eca) and Pseudomonas marginalis (Pm) were investigated from October (directly after harvest) until March of the next year. Colonization incidence of Ecc and Pm changed during the early weeks of storage in each of the 3 years of the experiment and increased significantly after December, approximately 6 to 8 weeks after the roots had been placed in storage. After inoculation, during storage in 1996, populations of Eca decreased continuously from 5.8 × 105 CFU/ml to 103 CFU/ml 4 weeks later in mid‐November without any further statistically significant increase during the rest of the storage period. Timecourse analysis of the monthly colonization incidence of Ecc and Pm over the 3 years revealed that the incidence was significantly dependent on the incidence 2 weeks earlier. Also, high colonization incidences during storage coincided with high disease incidence during the following forcing period of the roots. Therefore, a forecast of disease incidence during forcing of the witloof chicory heads may be envisaged.
No abstract
Immunofluorescence colony staining (IFC) and a new technique using spiral plating combined with IFC were evaluated for the soft-rot pathogen Erwinia carotovora subsp. atroseptica in witloof chicory. Target bacteria could be detected in platings at various dilutions of plant washings. Brilliance of the stained colonies of E. carotovora subsp. atroseptica was high. Spiral plating, used for both the plating of the bacteria and for the delivery of the conjugated antiserum, had a positive effect on the reduction of the background compared with the staining of the bacteria. The combination of spiral plating and IFC proved to be a functional tool for the quantification of target and nontarget bacteria and the isolation of target bacteria as pure culture from IFC-positive colonies. The method uses less conjugated antiserum than traditional IFC and produces results with very small variation within replications. The recovery of the bacteria in both pure culture and plant washing is significantly higher than the recovery using crystal violet pectate medium. RCsumC : Pour fin de diagnostic, la coloration de colonies par immunofluorescence (IFC), ainsi qu'une nouvelle technique utilisant 1'Ctalement en spirale combink i I'IFC, ont Ct C CvaluCes sur Erwinia carotovora subsp. atroseptica, l'agent pathoghe de la pourriture molle chez l'endive. Les batteries cibles ont pu Ctre dCtectCes sur plaques B diverses dilutions des lavages des plantes. La luminescence des colonies colortes d'E. carotovora subsp. atroseptica Ctait forte. L'Ctalement en spirale utilisC tant pour les bactCries que pour l'addition de I'antisCrum conjuguC a eu pour effet de reduire le rayonnement de fond comparativement B la coloration des bactkries. La combinaison de la culture en spirale et de I'IFC s'est rCvC1Ce un outil fonctionnel pour la numCration des bactCries cibles et non cibles et pour l'isolement des bactkries cibles en culture pure B partir des colonies IFC positives. La mCthode requiert moins d'antiskrum conjuguC que I'IFC traditionnelle et produit des rCsultats avec des variations minimes entre les repiquages. Le recouvrement des bactCries dans les cultures pures et dans les lavages de plantes est significativement plus tlevt que le recouvrement sur milieu de pectate avec cristal violet.
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