sitka mountain ash

Discussion in 'Woody Plants' started by pchew, May 16, 2003.

  1. pchew

    pchew Member

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    Whistler, B.C.
    I have several Sitka Mountain Ash tree in my garden. For the past several years, I have noticed that the new growth is affected with many green pimples on the leaves. I have pinched off all of the affected leaves and ensured that the area is cleaned up around the soil, particularly in the fall when leaves fall. I think it may be some kind of rust, but I have not let these pimples mature to see what they mature into. Not all of these trees are affected, in fact I have two side by side, one affected, and the other one not. I have a quince bush that I planted several years ago that looks healthy, and I am wondering, if in fact it is rust, could that be the host.

    I was hoping that by eliminating all of the affected leaves, this spring would bring me clean trees, but I guess I'll be denuding these poor things again. Please let me know what I should be doing.


    Thanks
     
  2. Douglas Justice

    Douglas Justice Well-Known Member UBC Botanical Garden Forums Administrator Forums Moderator VCBF Cherry Scout Maple Society 10 Years

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    Sorbus species (mountain ash) are subject to a variety of pests and diseases. Blister or gall-forming (eriophyid) mites are common among these. They are microscopic mites that feed on developing leaf tissues. Their presence causes abnormal growth of the leaf tissues.

    According to Johnson and Lyon (Insects that Feed on Trees and Shrubs, revised ed. Comstock Publishing/Cornell University Press, 1981), the "biologies and habits of these pests are complex and all to often unknown." The following description is paraphrased from that text:

    These mites probably overwinter on twigs and under bud scales and migrate to leaves in spring. Typically, eriophyid mites begin feeding on the undersides of leaves. The epidermis ruptures where feeding occurs and mites enter the opening to deposit eggs. As eggs hatch, more and more mites destroy increasing numbers of cells. A pimple-shaped swelling appears on the upper surface and develops into a blister, which enlarges and may coalesce with adjacent blisters.

    Application of dormant oil in winter may suffocate overwintering mites, and leaf removal may help to reduce reproducing populations. However, these mite infestations are probably cyclical (high some years and low in other years) and may be related to other factors, such as drought, winter temperature, soil fertility. Infestations are seldom detrimental to the tree's overall health.
     

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