Relationships between populations of Pseudomonas syringae pv. persicae determined by restriction fragment analysis

Pseudomonas syringae pv. persicae , the pathogen causing bacterial decline of stone-fruit, was first noted almost simultaneously in France and in New Zealand, together with a closely related pathogen from myrobalan plum in England. The relative similarity of 31 strains from these three countries was examined by comparing DNA restriction endonuclease fragment patterns. Fragment patterns produced from digested DNA samples which were electrophoresed on polyacrylamide gels and visualized by silver-staining, were analysed using the software package Gelcompar. The fragment patterns produced by strains from France and England formed homogeneous but separate groups, while those from New Zealand were relatively heterogeneous. This finding suggests that the New Zealand population of P.s. persicae is older than those found in Europe. Problems in explaining the distribution of the pathogen are discussed.     

Temporary partial breakdown of Mlo-resistance in spring barley by the sudden relief of soil water stress

Seedlings of spring barley cultivars expressing mlo genes for resistance to mildew (Erysiphe graminis f.sp. hordei) in different genetic backgrounds were subjected to degrees of water stress under different nutrient and soil conditions. The temporary breakdown in the host of the expression of Mlo-resistance, reported occasionally under field conditions, was successfully simulated when stress treatments were relieved. Differences in the degree of temporary breakdown were attributable to nutrient status, soil compaction and genetic background of the mlo gene.