Temperatures just before and during bloom will determine if fire blight becomes serious in early spring. Daily temperatures must average 65°F or higher during pink through petal fall for bacterial populations to grow enough to cause severe disease. The disease also occurs later in the season when bacteria enter late-opening blossoms or growing tips of new shoots.
Where this disease was present the previous year, we suggest the following management program:
- Prune out all cankers from limbs 1 inch or more in diameter. Cut apple limbs at least 8 inches below external evidence of the canker and cut pear limbs at least 12 inches below. Pruning tools do not need to be disinfected when temperatures are below 45°F.
- Where the disease was severe the previous year, apply a dilute Bordeaux spray plus miscible superior oil at silver tip. This spray is not warranted if only occasional infections occurred.
- When daily temperatures average 65°F or higher during pink through petal fall, make at least two applications of a streptomycin formulation. Apply the first streptomycin spray anytime after first blossoms open, when daily temperatures are above 65°F or are expected to be so within 24 hours. Repeat sprays at 5- to 7-day intervals through late bloom. A minimum of two applications is necessary to provide control. (Streptomycin formulations are much more effective when applied during slow drying conditions, such as at night.)
- When average daily temperatures fail to reach 65°F during pink through petal fall, delay the streptomycin application until the disease first appears. To detect the first appearance of fire blight, inspect trees at 5- to 7-day intervals beginning at petal fall. When the disease appears, prune out all new infections. Again, remove shoots 8 to 12 inches below the last signs of browning. Remember to disinfect pruning tools between cuts with a bleach solution or alcohol since contaminated tools can spread the disease.
We do not recommend cutting out blighted shoots after terminal growth has stopped. When growth stops, the spread of fire blight should also stop. The most important thing to do to control fire blight during the summer is to control sucking insects like aphids and leafhoppers. Applying streptomycin sprays within 24 hours after hail to prevent new infections is also a good practice.
Proper fertilization practices can help reduce the potential for fire blight. Trees that are excessively vigorous due to high nitrogen applications can be more prone to fire blight. Variety selection also can help reduce the incidence of this disease. Many of the scab-resistant varieties are resistant to fire blight (see Table 4.1). Varieties such as Jonathan and Rome Beauty are more susceptible to fire blight. Resistant pear varieties include Magness and Moonglow.