Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Maple -- Bacterial Leaf Spot and Dieback
 
Cause: Pseudomonas syringae pv. syringae, the same bacterium that causes bacterial blight of lilac, fruit trees, and many woody ornamentals. It overwinters on infected plant parts or as an epiphyte on healthy tissue. It spreads with windblown rain, insects, and pruning tools. Bacteria enter through wounds or natural openings. Wound infection during budding may interfere with bud-take. Frost damage, high nitrogen fertilization (especially late summer), and heavy rains favor bacterial invasion. Most species except sugar maples are susceptible. Most Japanese maple cultivars such as 'Sango Kaku' and 'Oshi Beni' are highly susceptible.

Two common genetic traits increase the bacteria's ability to cause disease. Most produce a powerful plant toxin, syringomycin, which destroys plant tissues as bacteria multiply in a wound. Bacteria also produce a protein that acts as an ice nucleus, increasing frost wounds that bacteria easily colonize and expand.

Symptoms: Symptoms include leaf spots, vein blackening, and tip dieback of Japanese, Norway, and red maples. Leaf spots vary from pinpoint size to 0.25 inch in diameter. Most leaf spots begin as a water-soaked area and may show a chlorotic halo. Spots may coalesce, destroying leaves or young seedlings.

Year-old twigs may turn black during the dormant season. These twigs then turn ash-gray with a black band of tissue near the advancing lesion. Buds can turn black and fail to break in spring, or shoots may leaf out and then die back. Twigs may continue to die back through spring as blackened tissue continues to expand toward the shoot's base.

As shoots die back they first turn black from the bacterial infection then gray as other organisms colonize the dead tissue.

 
Cultural control:

  1. Plant resistant cultivars or avoid highly susceptible cultivars.
  2. Carefully handle young plants.
  3. If possible, do all wounding including pruning and budding in dry weather.
  4. Protect from rain and frost, if feasible, in early spring or late fall. Plastic shelters have been as good as or better than chemical methods against the same disease on other crops.
  5. Remove and destroy dead and/or blackened twigs and fallen leaves.
  6. Maintain adequate spacing for good air circulation.
  7. Avoid high-nitrogen fertilization that produces a lot of late-season growth which causes plants to be more sensitive to winter injury and thus more susceptible to bacteria.
Chemical control: Must use with cultural controls. Many bacteria resistant to copper-containing products and to antibiotics have been detected in Pacific Northwest nurseries.

  1. Fixed coppers. Apply in fall to protect wounds and leaf scars, in spring to protect new growth. Spring sprays must be applied under fast-drying conditions to avoid leaf injury.
    1. Kocide 2000 T/N/O at 0.75 to 3 lb/A (or 1 to 3 Tbsp/1,000 sq ft) dormant or at 0.75 to 2 lb/A when new growth is present. 24-hr reentry.
    2. Junction at 1.5 to 3.5 lb/A. 24-hr reentry.
  2. Phyton 27 at 1.5 to 2.5 oz/10 gal water. 24-hr reentry.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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