Rhody Blahs

eose(7)July 12, 2010

This seems to be a stressful time of year for rhodys. I planted 4 rhodys 22 months ago, 2 Cunningham, 2 Boursalt. The 2 Cunninghams are doing great, one of the Boursalt is doing OK. The other...not so much. Last summer the center stem started slowly dying. Several of the new branches failed to mature. After the cooler weather arrived the die-back stopped and the plant did well all winter. It bloomed nicely this spring and looked healthy so I thought the problem had been solved. Now, for the last month the center of the plant has started wilting and dying, just like last summer. All 4 have the same culture, same light, same water. Four azaleas just next to them, which actually get more sun, are immaculate. Any suggestions on what the cause and treatment might be?

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Embothrium(USDA 8 Sunset 5 WA)

Sounds like Phytophthora.

    Bookmark   July 13, 2010 at 10:37PM
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rhodyman(SE PA, USDA Z6)

Actually Phytophthora is usually fatal. It is a root infection.

Your description is confusing. How can the part that died back last year die back again this year. Maybe it is progressing further. One possibility is borers. They will tunnel in a branch, killing everything beyond where they are tunneling. They can come back year after year.

A more likely possibility is Botryosphaeria dothidea or Botryosphaeria dieback. It causes leaves to turn dull green and then brown and roll and droop. Cankers form on branches which may girdle the branch. This is the most common disease of rhododendron in the landscape. A typical symptom of this fungal disease is scattered dying branches on an otherwise healthy plant. Leaves on infected stems turn brown, then droop and roll inward. These leaves often lay flat against the stem and will remain attached. The pathogen can infect all ages of stem tissue through wounds, pruning cuts, and leaf scars. Heat, drought stress, and winter injury can increase disease incidence. Cankers on branches can gradually grow through the wood until the stem becomes girdled. Diseased wood is reddish brown in appearance. Discolored wood viewed in longitudinal cross section often forms a wedge that points toward the center of the stem, and the pith may be darker brown than the surrounding wood. Sanitation and applying a fungicide such as metalaxyl (Subdue) after pruning my provide some control. Plants should be grown in partial shade, with mulch and kept well watered during dry periods. All dying branches should be promptly pruned out in dry weather and all discolored wood should be removed. Plants should also be protected from rough treatment during maintenance activities to prevent unnecessary wounds.

During periods of drought, the most common damage is Botryosphaeria dieback. It attacks specific branches rather than the whole plant.

Here is a link that might be useful: Rhododendron and Azalea Problems

    Bookmark   July 14, 2010 at 10:22AM
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eose(7)

Thank you for your informative reply. I guess I will treat for b. dieback. I no nothing of Subdue, perhaps it is available locally. Thank you again.

    Bookmark   July 14, 2010 at 2:35PM
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eose(7)

Thank you for your informative reply. I guess I will treat for b. dieback. I no nothing of Subdue, perhaps it is available locally. Thank you again.

    Bookmark   July 14, 2010 at 3:07PM
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eose(7)

Well, sorry, a quart of subdue for $200.00 is probably a little overkill for 1 rhody. Any other suggestions for products or treatment? Thanks again.

    Bookmark   July 14, 2010 at 3:13PM
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rhodyman(SE PA, USDA Z6)

Make sure you have eliminated borers as a possibility. They fit your description.

Botryosphaeria dieback is opportunistic and attacks plants that are stressed, usually due to drought. It is better to cut off diseased wood and water the plant enough to prevent stress. Not too much so as to create stress.

    Bookmark   July 17, 2010 at 8:36AM
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