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Disease Management

Effective management of fire blight is multi-faceted and largely preventative. The grower must utilize a combination of sanitation, cultural practices, and sprays of chemical or biological agents to keep the disease in check.

Cultivars

Selection of a resistant cultivar is the most effective method of controlling fire blight. In apple, for example, some cultivars exist that are moderately resistant to the disease (e.g., Red and Golden Delicious). For pears, cultivar choices are more limited because superior horticultural traits (e.g., taste, storage, and marketing qualities) have been difficult to combine with higher levels of disease resistance. In recent years, fire blight has become more common in apples because the spectrum of cultivars grown commercially has expanded and shifted toward those with greater susceptibility to the disease (e.g., Braeburn, Fuji, Gala, Pink Lady). With this shift has come the recognition that popular dwarfing rootstocks for apple, Malling 9 and 26, are highly susceptible to fire blight. Dwarfing rootstocks with resistance to fire blight are being developed and evaluated (e.g., the Geneva rootstock series from Cornell University). Many ornamental cultivars also show high levels of fire blight resistance.

Elimination of overwintering inoculum

Vigilant sanitation through the removal of expanding and overwintering cankers is essential for control of fire blight in susceptible cultivars. Removal of overwintering ("holdover") cankers is accomplished by inspecting and pruning trees during the winter.

Prevention of blossom blight

Prevention of blossom infection is important in fire blight management because infections initiated in flowers are destructive and because the pathogen cells originating from blossom infections provide much of the inoculum for secondary phases of the disease, including the infection of shoots, fruits, and rootstocks. Management actions to suppress blossom blight target the floral epiphytic phase. Sprays of antibiotics, streptomycin or oxytetracycline, have effectively suppressed blossom infection in commercial orchards. (Figure 12)

Figure 12

Copper compounds also are effective but not used widely because copper can be phytotoxic to the skin of young fruits. E. amylovora has become resistant to streptomycin in some production areas, limiting the effectiveness of this chemical. Non-pathogenic, bacterial epiphytes sprayed onto blossoms can preemptively suppress fire blight by colonizing the niche (stigmatic surface) used by E. amylovora to increase its epiphytic population size. The bacterium Pseudomonas fluorescens strain A506, is registered and sold commercially for this purpose (BlightBan A506).

Mid-season suppression of established infections

In summer, established infections are controlled principally by pruning. Effective control through pruning requires that cuts are made 20-25 cm (8 to 10 inches) below the visible end of the expanding canker (Figure 13) and that between cuts the pruning tools are disinfested with a bleach or alcohol solution to prevent cut-to-cut transmission. Repeated trips through an orchard are necessary, as some as infections are invariably missed and others become visible at later times (Figure 14). Prunings harboring the pathogen are usually destroyed by burning (Figure 15).

Figure 13

Figure 14

Figure 15

In severely affected orchards, cultural practices that slow the growth rate of the tree will also slow the rate of canker development. This includes withholding irrigation water, nitrogen fertilizer, and cultivation. Similarly, practices that reduce tree wounding and bacterial movement can reduce secondary infection. This includes controlling insects such as plant bugs and psylla, limiting use of limb spreaders in young orchards, and avoiding the use overhead sprinklers. Chemicals such as streptomycin or copper can suppress trauma blight if applied immediately after a hailstorm.

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