Biological control of fire blight of hawthorn (Crataegus monogyna) with Erwinia herbicola under protected conditions

Isolates of Erwinia herbicola , obtained from flowers and leaves of hawthorn ( Crataegus monogyna) , were screened as potential control agents of fire blight disease (caused by Erwinia amylovora) using an immature pear fruit assay. Selected isolates were subsequently tested for disease control by infection of hawthorn blossom in the laboratory, and by shoot infection of hawthorn plants grown under controlled (glasshouse) and fluctuating (polythene tunnel) environmental conditions.
Although the immature pear fruit assay provided a general screen for the selection of antagonists for the control of both blossom and shoot blight, it had two major limitations when quantitatively applied. Firstly there were inconsistencies in the relative effects of different isolates on the pear-slice surface, with some isolates being more suppressive than the standard antagonist Eh252 in the first screening and less in the second. Secondly the assay was not able to predict accurately the level of control in the intact plant-as no correlation occurred between the level of control in the pear fruit assay and the percentage control of either blossom blight or shoot blight.
Two isolates of E. herbicola , WL9 and WL40, reduced both blossom- and shoot-blight. WL9 provided over 80% control of blossom blight, equivalent to that provided by chemical agents, and also gave total control of shoot blight when applied at a WL9: pathogen ratio of 10:1.