October Color Appears at the Washington Park Arboretum

October 7th, 2013 by Pat Chinn-Sloan

Selected cuttings from the Washington Park Arboretum (September 30 - October 13, 2013)

Selected cuttings from the Washington Park Arboretum (September 30 – October 13, 2013)

The Good, The Bad and The Ugly (Minus the Good)

1)  Cherry Brown Rot

  • A fungal disease of the Prunus species caused by Monilina fructicola and Monilina laxa.
  • The first symptoms often seen are browning and collapse of the blossoms, followed closely by death of the small twigs.

2)  Dogwood Anthracnose

  • Dogwood anthracnose is a disease of flowering and pacific dogwoods (Cornus florida and C. nuttallii).
  • An anthracnose fungus, Discula sp., has been identified as the causal agent.
  • Infection of dogwoods is favored by cool, wet spring and fall weather, but can occur throughout the growing season.

3)  Elm Leaf Miner

  • Elm leaf miner, Fenusa ulmi, is a pest that feeds on the tissues in between the outer layers of elm leaves, causing browning and leaf drop. Although primarily an aesthetic pest, leaf miner damage can stunt or weaken a tree when the population in the tree is high.
  • The elm leaf miner has been in the Northwest for a few years, but recent expansion has been noticeable in Washington and Oregon recently.

4)  Powdery Mildew on Rhododendron

  • The fungus Microsphaera azalae is found throughout the Pacific Northwest on Rhododendron species and hybrids.
  • Contrary to popular opinion, powdery mildew outbreaks are not favored by rainy weather. Steady rain tends to wash mildew spores off the foliage before they have a chance to penetrate the tissue. Mildew is more commonly associated with high relative humidity and the light coating of dew that forms on leaves when cool nights follow warm days.

5)  Sorbus Sawfly

  • The Sorbus Sawfly (Pristiphora geniculate) is a new pest in western Washington. It was first noticed in the spring of 2009 in the Everett, Lynnwood and Monroe areas.
  • Sawflies that are new to an area tend to build up in large numbers and can cause significant defoliation. Sawfly larvae typically eat continuously and then drop out of sight (to pupate in the soil). Damage appears to occur overnight. Control of the first generation will reduce the number and severity of defoliation by the second and third generations.

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