An indication of their resistance is provided by using the excised shoot method, where pieces of shoot are tested in vitro by placing shoot cuttings in a culture of the fungus. Although resistance to fire blight is more important in scion cultivars, it is a desirable character particularly in pear rootstocks. Since the commonly used quince rootstocks are very susceptible, there is a need for resistant selections to prevent the disease spreading in the nurseries. Peter Spring A susceptible pear seedling showing typical symptoms of fire blight. Often the blossoms are the first point of entrance for the disease, from where it progresses into the spurs, branches and the stem of the tree, forming cankers and finally leading to tree death.
Infected areas become water-soaked and in warm, humid weather often show droplets of bacterial ooze.
From here the bacteria are dispersed by rain, wind, birds and most importantly by insects to other trees. Aphids, flies and leafhoppers have been associated with the disease transmission, but especially bees are very efficient disseminating agents for fire blight. The same applies for the popular cultivars of Asian pear, with Hosui and Nijisseiki being susceptible, and Kosui showing intermediate resistance. Table 1 . Pest and disease resistant apple rootstocks.