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THIS DISEASE HAS BECOME A SIGNIFICANT PROBLEM ON IMPORTANT CROPS SUCH AS LILAC. |
Bacterial blight is a serious disease affecting a large number of deciduous woody ornamentals in nurseries. Common disease symptoms include
Economic losses vary between years. Because the pathogen can survive on healthy woody plants and weeds, inoculum is always present in the nursery. In the spring, when weather conditions are favorable, bacterial populations can increase very rapidly and disease losses can be severe. Factors that favor disease include frost during early shoot development; cool, wet weather in spring or fall; pruning wounds; and heavy vegetative growth early or late in the season brought on by high nitrogen.
Bacterial blight is caused by Pseudomonas syringae pv. syringae. It is a relatively weak pathogen that requires a large population size and plants with wounds or in poor health. First identified on lilac, the host range of these bacteria is very broad, including most of the deciduous woody plants and ornamental fruit trees grown in nurseries. Frequent and severe losses occur on apricot, blueberry, flowering cherry, flowering dogwood, cherry laurel, forsythia, lilac, maple, flowering pear, magnolia, mountain ash and viburnum.
The bacteria overwinter in cankers or dormant buds of infected plants and in low numbers on healthy plants and weeds. Infection requires a wound, such as a leaf scar or a pruning cut, or a natural opening like a stomate or a nectary. Wind-driven rain, insects and contaminated pruning tools all spread the bacteria from plant-to-plant.
The bacterial populations within a nursery vary in their host range and ability to cause disease. Local groups of the bacteria are usually more damaging on a particular type of plant, but are capable of infecting many other plants. Plant parts differ in their susceptibility with newly expanded leaves and flowers most severely damaged.
The ability of the bacteria to cause disease is increased by two common genetic traits. Most produce a powerful plant toxin, Syringomycin, which destroys the plant tissues as the bacteria multiply inside a wound. There is also a protein produced by the bacteria that acts as an ice nucleus, increasing frost wounds that are easily colonized and explanded by the bacteria.
Bactericides containing metallic copper help reduce disease, but control can be inconsistent between types of plants and over time. A survey has shown that resistance to copper and the antibiotic streptomycin is widespread in commercial ornamental nurseries in the Pacific Northwest. Of bacteria collected in 1982/83, 25% were copper-resistant, 7% were streptomycin-resistant and 68% were sensitive to both copper and streptomycin. Of bacteria colled in 1992/93, 24% were copper-resistant, 6% were streptomycin-resistant, 24% were resistant to both copper and streptomycin and only 46% were sensitive to both chemicals. Also, the amount of copper that bacteria can resist more than doubled between 1983 and 1993.
Current chemical recommendaions for control of bacterial blight include a fall application of copper sulfate or copper hydroxide. This type of a program is designed to kill the overwintering bacteria and protect wounds and leaf scars from infection. Most of the copper applied in the fall will be washed away by the time the bacteria start to multiply and spread in the spring. An additional application of copper, provided just before the buds break in late winter/early spring, provides the best level of disease control. Copper must be applied to young tissues with care after bud break because of the risk of burning.
Copper bactericides come in many different formulations. The labels give the active ingredient as "metallic copper"; however, the metallic copper must be dissolved in water to be toxic. Most of the copper sprayed onto the plants will not be able to kill bacteria because it does not dissolve easily. Using a higher than recommended rate of copper, or spraying more often, will not necessarily fix this problem or improve control of copper resistant bacteria.
In laboratory and field experiments with lilacs, all copper formulations tested gave better disease control of copper-sistive bacteria than did streptomycin sulfate. Some copper formulations were better than others against copper-resistant bacteria. Basic copper sulfate + hydrated lime (Bordeau 4-4-100), tribasic copper sulfate, and copper hydroxide tank mixed with mancozeb, or copper hydroxide plus ferric chloride gave the best blight control. For copper hydroxide, wettable powder formulations were better than the liquid and dry flowable formulations.
Keeping plants under plastic shelters when young tissues are most susceptible (February-May) provides protection from frost and rain, thereby reducing disease severity. High nitrogen fertility that produces a lot of early and/or late season growth should be avoided.
Genetic resistance to bacterial blight has not been a part of woody ornamental breeding programs. Most cultivars of lilac (Syringa vulgaris) are very susceptible. However, these cultivars have been observed with less disease when planted in a garden: 'Edith Cavell,' 'Glory,' and 'Pink Elizabeth.' Other Syringa species may have some resistance to bacterial blight. In trials in western Washington, Syringa josikaea, Syringa komarowii, Syringa microphylla, Syringa pekinensis and Syringa reflexa had less disease than Syringa vulgaris. Most maples (Acer sp.) are susceptible to bacterial blight except for sugar maple (Acer saccarum). Japanese maple (Acer palmatum) cultivars 'Sango Kaku' and 'Oshi Beni' are hightly susceptible.
Written by Heather J. Scheck and Jay W. Scheidt, Department of Botany and Plant Pathology, Oregon State Unversity , Corvallis, OR 97331.
originally published in Yankee Nursery Quarterly. Winter 1998. Volume 7, No. 4, pp.6-8.