Garden Column for the Salisbury Post
Darrell Blackwelder
May 9, 2003

    Many have called distressed about pear and apple trees. Cool damp weather experienced a month ago has caused both ornamental and fruit trees to look as though their limbs have been singed with a torch. The problem is fire blight.
    Blossoms, fruits, fruit spurs, twigs, and branches are affected and sometimes the entire tree may be killed. They are concerned that the tree may die.
    Fire blight is a disease is a bacterial disease that can severely damage apples and pears. Bacteria over winters in diseased and dead tissue or cankers Remember, fire blight develops more rapidly when temperatures are 65-90 F combined with humid or rainy weather, and that once blight is established it is very difficult to control on the tree trunk supplying inoculum for rapid infection under ideal conditions. In the spring, bacteria are carried by wind, rain and insects to blossoms or growing, succulent shoots. The bacteria were easily spread over the past few months with above average rainfall. Bacteria enter through natural openings in the flower or through stomata in the leaves; however, wounds and injuries made by insects, hail, and wind or by pruning are important means of entrance. Under ideal conditions, the bacteria may be carried to other blossoms, twigs or fruits resulting in secondary infections.
    Control of this disease is not easy. Planting resistant varieties is the first step. Although varieties vary in resistance to fire blight, none are immune. The more susceptible apple varieties are Jonathan, Golden Delicious, Lodi, Yellow Transparent, Stayman and Gala, with Red Delicious generally less affected. The pear varieties Bartlett, Bosc, and Clapp's Favorite are highly susceptible. Kieffer, Orient, Starks Delicious, and Dawn are less so and Magness and Moonglow are relatively resistant.
    This next step is to remove of all cankers and blighted twigs before growth starts in the spring is very important. Cuts should be made 4-6 inches beyond any evidence of dead tissue. Cuts should be made 10-12 inches beyond the last evidence of disease. Care must be taken to prevent spread of the bacteria by hands or cutting tools. If infections are cut out, use bleach (Clorox) dilute 1:10 with water to sterilize pruning shears or saw blades between each cut.
    Streptomycin sprays will help control the disease; however this must be done while the tree is blooming in March. Sprays before, during and after bloom are very important for control. Streptomycin will not harm the bees.
    Take time to remove succulent, rapidly growing twigs and shoots during the summer. These are most susceptible to infection. Over fertilizing using excessive amounts of nitrogen fertilizer produces excessive growth and watersprouts which develop near the base of the scaffold limbs, crotch, or trunk.