Declining Rhododendron

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Posted by Horticulture Guy - Peter Punzi | Posted in Southwest U.S. Gardeners | Posted on 04-03-2006

Q. I have a rhododendron. Dark green upon planting. Western exposure. Leaves and new growth turn pale green and droop. Tries to set buds and flower but buds do not mature and flowers are extremely small and die back quickly upon opening. Paul Faro – San Leandro CA

A.  Sounds to me like you have a fungal root rot disease (Phytophthora spp.).  Although pale leaves could be caused by improper pH/nutrient problems, the drooping and die back sound more like this disease, which is encouraged by poorly drained soils and planting deeper than it was in the container.  Root rot disease is tricky to diagnose since the symptoms can be confused with cultural problems like low pH/nutrient problems (pH should be between 5.5 and 5.6 to allow minerals like iron and magnesium to be absorbed) which would result in pale green leaves or drought which would cause drooping and difficulty in absorbing nutrients.  The way you presented the symptoms makes me thing it is more likely root rot disease.  This is not easy to treat with fungicides and you may be faced with replacing the shrub.  Rhododendrons are not the only species vulnerable to this disease so you will need to ask for a resistant variety of Rhododendron or a species of shrub that is not susceptible. You may be able to bring a sample to your local County Extension for accurate diagnosis and also to find out what a suitable replacement shrub may be for your area.