Studies on fire blight

Part 1Effects of water potential and temperature on multiplication of and pressure by Erwinia amylovora in host plantsAnalysis of field data from Eve Billing, England, on the duration of the incubation period of fire blight revealed that temperature and rainfall were positively and interactively correlated with the development rate of fire blight. Values of standard regression coefficients suggest that temperature had more impact on the variation in the development rate than rainfall.Billing studied the effect of temperature on the multiplication rate r of E. amylovora by means of laboratory experiments. Instead of r , she used the variable 'potential doublings per day', represented by PD ( PD = r / 1n(2) ). Reconsideration of her calculations of PD revealed, however, that the PD-values in Billing's table were underestimated. The relationship temperature - r was recalculated, and corrected PD-table presented.Shaw studied the effect of water potential on r of E. amylovora . This relationship appears to be applicable to E. amylovora in plant tissues and in nectar of flowers.Growth of E. amylovora in a restricted space, e.g. an intercellular hole, may create a pressure on the surrounding host tissue. Theoretically, this bacterial pressure equals the actual water potential of the host tissue minus the water potential at which the bacterial biomass would completely fill the intercellular space, but without exerting pressure. Growth of E. amylovora can be caused by multiplication and by swelling of the bacterial biomass, due to absorption of water without increase of dry weight. The maximum 'multiplication pressure' equals the actual water potential minus the lowest water potential at which E. amylovora is able to multiply at absence of bacterial pressure. The maximum 'swelling pressure' equals the change in the water potential.The volume of the cells of E. amylovora hardly changes with changing water potential, but the extracellular slime of E. amylovora , consisting mainly of extracellular polysaccharide, swells strongly with increasing water potential. The hypothesis of the swelling pressure would explain why the extracellular slime is a virulence factor.The pressure, caused by multiplication and swelling of the bacterial biomass, may lead to compression of soft host cells, to tearing of host tissues, and to formation of large slime-filled holes in the plant tissue. Moreover, the expanding biomass may force its way to the outside of the plant or to healthy parts. Cork barriers, being formed by the plant after infection, may be broken through if the mechanical pressure is high and if the cork barrier is incomplete or not yet fully developed. Sealing off is then prevented. Strong tissue may be able to resist the pressure, so that symptom progression can be prevented.Simulation runs indicate that, when the pressure increases, the extracellular slime of E. amylovora shrinks by releasing water, thus allowing further production of bacterial dry matter. The slime remains around the bacterial cells as a dense substance, low in water content, having a strong capacity to swell when the pressure induces tearing apart of the host tissue. Simulation runs show that the bacterial pressure attains its highest values at evening and night.To gain insight into the limitations imposed by temperature and water on multiplication of E. amylovora in shoots of fruit-trees under field conditions, two simulation models were designed: a short-term model for immediate effects of weather and soil water potential, and a long-term model for effects of rain and soil profiles. The relationships temperature - r derived from Billing and water potential - r derived from Shaw were incorporated into the models.In the Netherlands, in the month of June, when the soil is moist and the weather 'average', water hardly limits E. amylovora multiplication in shoots but, according to the short-term model, temperature reduces then the multiplication by about 60 %. When the soil is dry and the potential transpiration rate of the trees high, water may limit E. amylovora multiplication in shoots considerably. According to the long-term model, rain has a delayed effect on multiplication.The effect of a rain shower increases gradually in the course of time and reaches its maximum 2 to 30 days after the rain, in dependence of soil moisture content before the rain, amount of rain, and soil profile. Calculations were made for three soil profiles representing three typical fruit growing areas in the Netherlands. The results suggest different effects of rain on the behaviour of fire blight according to soil profile.According to the short-term model, r of E.amylovora in shoots of fruit trees was twice as sensitive to the daily maximum temperature Tmax , as to the daily minimum temperature Tmin , during the second half of June under dutch conditions. Because of this difference in sensitivity, and because the standard deviation of Tmax was larger than that of Tmin , the variation of r due to Tmax was three times larger than that due to Tmin . The sensitivity to daily global radiation was negligible when the soil was moist.Part 2The effectiveness of flowering prevention of hawthorns to control fire blight in pear orchardsSince 1984, when a new Ministerial Regulation on fire blight came into force, there are in the Netherlands 20 'protected regions', where nurseries and pear and apple orchards are extra protected against fire blight. This policy is also necessary to meet the EC-requirements on fire blight. One of the measures in the protected regions is the prohibition of flowering of the native hawthorns ( Crateagus monogyna and C.laevigata ).Five research areas of about 3 x 3 km 2were chosen with hawthorns and pear orchards. Two of these areas were located in protected regions and three in nonprotected regions. The more than 50,000 hawthorns in the areas were grouped into 1125 hawthorn objects.2.3 % of the non-flowering and 16.4 % of the flowering (or berry carrying) hawthorn objects had fire blight at least once in 1987, 1988 and/or 1989. The flowering prohibition for hawthorn in protected areas was rather well observed, so that in protected areas a smaller proportion of hawthorn objects had fire blight (4.1 %) than in non-protected areas (14 %). Moreover, there were less hawthorn objects per km2 in the protected areas (13) than in the non-protected areas (26).In protected areas 53 % and in non-protected areas 59 % of the pear orchards had fire blight during 1987, 1988 and/or 1989. The difference was not significant. The first reason for the ineffectiveness of the flowering prevention in hawthorn to control fire blight in pear orchards is the inadequate sanitation of the pear orchards. The second reason is that in this study fire blight hardly spread from hawthorn to pear, assuming that a new focus is most probably initiated by the nearest existing focus. Spread of fire blight within pear orchards and between pear orchards occurred frequently.