Erwinia amylovora is one of the most frequently occurred plant pathogenic bacterium. It causes necrosis and blight symptoms on host plants and it lead to considerable yield losses throughout the world on apple trees. There is no effective chemical treatment is currently available against fire blight. The purpose of the present study was to search a new, alternative control method. The evaluation of the plant protection potential of an enriched fraction of Xenorhabdus budapestensis cell-free conditioned media was investigated. Purified samples were tested in vitro and in planta against the phytopathogenic bacterium. A reproducible method for isolation of a peptide-rich fraction from Xenorhabdus cell-free conditioned media was established. The process resulted in 400 mg of dry sample prepared from three litres of Xenorhabdus cell culture. Significant correlation was found between the concentration of the purified preparation and the induced inactivation zones against Erwinia amylovora Ea1 in agar diffusion test method. The minimum inhibitory concentration and minimum bactericidal concentration of the purified fraction against Ea1 strain were 8 μg/mL and 16 μg/mL, respectively. In planta experiments were tested on an apple cultivar ('Watson Jonathan') susceptible to fire blight. The effective range of concentration was 62-200 µg/mL, while treatment with 300 µg/mL and larger amounts caused necrotic symptoms on the petals of flowers. Current study pointed to the effectiveness of the compounds produced by X. budapestensis against fire blight. The development of a commercially applicable formulation of these compounds would allow growers to effectively control fire blight in apple and pear orchards.
Bacterial blight is one of the most serious diseases affecting Persian walnut (Juglans regia L.). Susceptibility to this disease was evaluated by artificial inoculation in an in vitro experiment for cultivars developed in Hungary and for selections from Transylvania. Thirty pieces of immature fruit of 11 walnut cultivars and 10 selections were inoculated by punction using a suspension containing a mixture of Xaj-isolates of controlled virulence. As control, a moderate resistant (mR) 'Pedro' and a highly susceptible (hS) 'Milotai intenzív' cultivars were used. After ten days the diameter of the necrotic area around the inoculation points was measured and the disease rate (DR) was noted. For the calculation of indexes it was taken the structure of the lesions (diffuse or defined margin) into consideration. None of the 21 cultivars/selections analysed were found to be hardly resistant, although all of them showed a higher degree of resistance than the susceptible control ('Milotai intenzív'). The majority of the analysed cultivars fit into the moderate susceptible (mS) and moderate resistant (mR) group. 'Milotai kései' and SZEN-10, had a notable reaction as considered to be moderate resistant (mR), showed a similar degree of resistance compared to the control cultivar 'Pedro'. It was concluded that these cultivars, based on their resistance to Xanthomonas arboricola pv. juglandis, could be proposed as resistance gene sources, as well as for production purposes in the environmental conditions of the Carpathian Basin.
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