Rates of epiphytic growth of Erwinia amylovora on flowers common in the landscape

We evaluated epiphytic growth of the fire blight bacterium, Erwinia amylovora, on flowers of plant species common to landscapes where pears and apples are grown. The plants were from genera regarded as important nectar and pollen sources for pollinating insects: Acer, Amelanchier, Brassica, Cytisus, Populus, Prunus, Rubus, Salix, Taraxacum, Trifolium, and Symphoricarpos. Floral bouquets were inoculated with E. amylovora and incubated in growth chambers at 15 degrees C for 96 h. Regardless of their susceptibility to fire blight, all species from the rose family except Prunus domestica (European plum) supported epiphytic populations of E. amylovora that exceeded 1 X 10(6) CFU/flower with relative growth rates for the populations that averaged 7% per hour. Nonrosaceous plants were generally poor supporters of epiphytic growth of the fire blight pathogen with relative growth rates averaging < 4% per hour. In two seasons of field inoculations, the rosaceous non-disease-host plants, Prunus avium (sweet cherry) and Rubus armeniacus (Himalayan blackberry), yielded mean population sizes of E. amylovora that exceeded 1 x 10(6) CFU/flower; in contrast, at 8 days after inoculation, mean population sizes of the pathogen were in the range of 5 x 10(3) to 5 x 10(4) CFU/flower on Cytisus scoparius (Scotch broom) and < 1 X 10(2) CFU on Acer macrophylum (big leaf maple). Because vectors of E. amylovora, principally bees, visit many kinds of flowers in landscape areas between pear and apple orchards, flowers of rosaceous, non-disease-host species could serve as potential sites of inoculum increase during their periods of bloom.