Erwinia amylovora, the causal agent of fire blight, carries the common plasmid pEA29 of 29 kb. To screen for occurrence of natural strains without plasmid pEA29, we applied PCR analysis with primers from the plasmid and the chromosomal ams region. In addition, a described TaqMan probe from pEA29 and newly designed primers from the amsregion were used for identification by real-time PCR. One strain isolated in Iran, one strain from Spain and two strains from Egypt lacked plasmid pEA29. From a recent screening series in southern Germany, in 123 E. amylovora strains from necrotic fire blight host plants, one strain was found without the common plasmid. The strains without pEA29 were virulent in assays with immature pears and on apple seedlings, but showed a reduced growth level in minimal medium without amino acids and thiamine. Transposon-labelled pEA29 was transformed into the plasmid-free strains resulting in restoration of this growth deficiency. The plasmid was stably maintained in these E. amylovora cells. The newly designed chromosomal primers for conventional and for realtime PCR identified E. amylovora strains in field samples lacking pEA29. These variants are apparently rare, but were detected in isolates from different regions in the world with fire blight.

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Yeasts are potential antagonists of microorganisms in the phyllosphere. Due to their osmotolerance, they should also be able to colonize apple flowers. In field experiments, we applied yeast agents against fireblight at two different sites in the southern part of Germany. They showed efficiencies 0–20% below Plantomycin (streptomycin). Co‐culture experiments in liquid basal media with synthetic nectar resulted in suppression of Erwinia amylovora by yeast. This effect could not be confirmed with population studies of yeasts and E. amylovora in flower clusters. Field and laboratory experiments indicated that yeasts have antagonistic properties against fireblight but further research is needed to investigate this potential.

Xanthomonas fragariae spreads in symptomlessly infected strawberry plantlets and a method for detection of latent infections is necessary. It is a very slow-growing bacterium in culture and is easily overgrown by saprophytic bacteria. Therefore, plating is not a suitable method for detecting low numbers of bacteria in symptomless plants. In addition, selective media are not available. Serological assays like immunofluorescence are useful for testing in-vitro plants, but they are not suitable for field-grown plants, as cross reactions are common with the available antisera. For these plants, nested PCR with primers from Pooler (Pooler et al ., 1996) andZimmermann (Zimmermann et al ., 2004) has proved to be a valuable method. The method was successfully applied for a survey of strawberry plants from fields in Germany and for testing imported plants (frigo and green plants).

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