Fireblight, a bacterial disease that infects many trees and shrubs in the rose family, showed such a horrendous degree of infection this year that I wrote two columns on it last spring (May 16 and 30). Now as the growing season winds down and I see hundreds of badly affected Aristocrat pears around town still showing the symptoms — dead branches with dead leaves still attached — I want to revisit the subject.
As I said in the May 30 column, outbreaks such as this one have happened before in other areas. One reference mentions heavy damage to Aristocrat flowering pears in 1986 in the Ohio River valley on trees 6 to 12 inches in diameter that had shown few or no disease symptoms the previous 15 years. It stated the disease is favored by long frost-free periods before bloom followed by humid weather with 65- to 70-degree temperatures during and after bloom with occasional rains during that period. That sounds a lot like last winter here.
The bacteria (Erwinia amylovora) lives in lesions on the branches and trunks but also on apparently healthy branches, leaves, and buds. Because it is endemic, able to survive and multiply on many species of trees and shrubs, and readily spread by insects, the disease cannot be eradicated.
Management strategies may include pruning, chemical disease control applications, or tree removal and replacement with trees that are not susceptible to the disease. Judging by the huge number of standing, symptomatic trees, it appears that inaction is the most common response.
Some owners choose to remove infected trees and replace them with something else. Since Aristocrat pear has so many desirable qualities — adaptability to a wide range of soil conditions, manageable size, showy spring flowers and nice autumn color— selecting replacements is a challenge. One option is its close relative, the New Bradford pear, a cultivated variety of the same species but resistant to the disease. However, its tendency for weak branch attachments makes it a questionable choice.
Other replacement options lack some of Aristocrat’s good qualities, and every species has its own problems. Some options include:
Washington hawthorn (Crataegus phaenopyrum): Easy to grow here, resistant to fireblight, showy white spring flowers, colorful small red fruit, good autumn color. Drawback: wicked thorns.
Emerald Sunshine hybrid elm (Ulmus propinqua Emerald Sunshine): Immune to fireblight, fast and easy to grow, moderate size for an elm, and resistant to common pests and diseases of elms. Drawback: No showy flowers. Not yet widely tested locally.
Chinese flame tree (Koelreuteria bipinnata): Yellow flowers in late summer, showy seed capsules, yellow autumn color, widely adaptable. Drawbacks: Needs a lot of pruning to develop structure. Susceptible to Verticillium wilt.
Aside from removal and replacement, or pruning to remove dead and dying branches, treatment options may include application of chemical disease control compounds. The most proven and traditional is copper in various formulations.
As P.P. Pirone mentions in Tree Maintenance, copper sulfate spray applied to the dormant twigs and branches kills over-wintering bacterial inoculum. Additional copper sprays during bloom time help prevent infections, which commonly enter through open flowers. He cautions that one or two sprays will not prevent infection during severe outbreaks.
A few weeks after the May 30 column was published, I learned from a colleague, Mike McDonough of True North Sustainable Landscapes, of another treatment approved for use in California. On June 9, the California Department of Pesticide Regulation approved the label of Reliant® (Quest Products) for fireblight control. The label includes two options:
1. Spray at pre-bloom plus sprays at 7-day intervals through the blooming period.
2. Basal bark application at bud swell. The latter is a 50/50 mix with water and 1 percent Pentrabark.
Time will tell how effective that one will be. At least, something new is available to help fight this devastating disease.