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Rhododendron
Problems

Rhododendron fungus

By: Sandy Feather 2009
Penn State Extension


Q. My rhododendron has leaves that are turning yellow and some have spots that resemble "black spot" on roses. I have recently used Miracle Grow fertilizer and extra watering. Up until this year, I never had any problem with my rhododendrons. Am I doing something I should not, or not doing something I should?

 

A. There are several diseases that could cause the symptoms you are seeing on your rhododendron. The first one that comes to mind is Cercospora leaf spot, a fungal disease caused by Cercospora handelii. It causes irregular brown spots on the leaves, usually attacking lower leaves first. The spots turn lighter tan in the center, and you may see small dark pimples about the size of a pinprick inside the larger spots on close inspection. These are the fruiting bodies of the fungus that will produce spores that spread the infection to healthy leaves.


Steps to Take

Rake and destroy fallen leaves to remove as much of the source of infection as possible. Rhododendrons are shallow rooted plants and certainly appreciate extra water when we are not receiving sufficient rainfall. However, drip irrigation and soaker hoses that keep water on the ground and off the plant are best ways to water. If that is not likely, at least use a watering wand and direct the water to the soil as much as possible.

Irrigation Practices

Although plants get wet when it rains or when there is heavy dew, there is no need to make the problem worse by overhead watering. It is best to water first thing in the morning so that plants dry as quickly as possible when the sun comes up; overhead watering in the evening guarantees that leaves will stay wet all night, increasing the chance of disease development. You may want to apply a fungicide next spring to protect the new growth from infection, beginning as new growth starts and making repeat applications as directed by the label of the product you are using. Ortho Max Garden Disease Control (chlorothalonil), Spectracide Immunox (myclobutanil), Bayer Advanced Disease Control for Roses, Flowers and Shrubs (tebuconazole), and Green Light Fung-Away Systemic Fungicide (triadimefon) are labeled to control leaf spot on rhododendrons in Pennsylvania.
  

white flowering rhododendrons

  
You did not mention if any of the stems are wilting in connection to the leaf spotting and yellowing. If they appear to be wilting, there are two other diseases that may be responsible: Botryosphaeria canker or phytophthora root rot. Botryosphaeria canker is a fungal disease characterized by sunken, dark brown cankers on the stems. The leaves on affected stems wilt as this causal fungus destroys the vascular tissue in the stem. You can see tiny black spots, which are the fruiting bodies of the fungus, in the dead bark over the cankers. Drought stress predisposes susceptible rhododendrons to this destructive disease. Although you have been watering this year, dry weather in previous years may cause an accumulated stress that made them more susceptible.

Continue to water as long as the weather is dry, but stop when we are getting sufficient rain. Prune out and destroy dead stems, and make sure you cut back to whirls of leaves where there are buds that will begin to grow. Do not leave large dead stubs that invite infection by this disease. There is no chemical control for botryosphaeria canker.

Root Rot

Phytophthora root rot causes rhododendrons to wilt, and affected plants may be stunted. The wilted leaves may be marked with dark brown spots, and usually turn a dull yellow-green color before becoming entirely brown. The shoots appear to die from the tip back as the fine feeder roots are killed and can no longer take up water. Phytophthora is more likely to occur in rhododendrons planted in heavy soil that drains poorly. You can check to see if phytophthora is to blame by examining an affected stem at ground level. Peel some of the bark back and look at the underlying tissue. You will see a distinct boundary between healthy white tissue and diseased brown tissue. While fungicide applications can protect uninfected plants, plants that are already infected inevitably die. If phytophthora is confirmed, infected rhododendrons should be removed.
  


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