Rhododendron Disease - Ask Extension
We bought this house a year ago. It had been in foreclosure and the landscape was in poor shape. A lot of the Rhododendrons looked bad but we pruned...
Knowledgebase
Rhododendron Disease #393593
Asked April 13, 2017, 9:02 PM EDT
We bought this house a year ago. It had been in foreclosure and the landscape was in poor shape. A lot of the Rhododendrons looked bad but we pruned and some seem to be coming back, several are not. There is still this leaf symptom. I was wondering if it was a common thing and was readily identifiable.
Marion County Oregon
Expert Response
Hi:
There are signs of both nitrogen deficiency and iron deficiency on these leaves. In general as you point out they do not look healthy. It may be that drought stress is an issue as well, and I would ensure the drainage of the plants is good. The iron deficiency is evident from the "interveinal chlorosis" on some of the leaves, where the veins are green and the tissue between is yellow. This occurs on the newest leaves at the shoot tips when iron is deficient. This occurs in Rhododendron when the soil pH is too high and is very common,. Rhododendron prefer a soil pH of between 4.5-5.5 and our typical soil pH is about 6.0. The nitrogen deficiency is evident from the green leaves at the shoot tips and yellow leaves lower down, or lack of leaves lower on the stem at all. Nitrogen deficiency is the most common nutrient deficiency in any plant, and shows up as the pattern I described. Two things you will want to do is lower soil pH in that flower bed and fertilize appropriately with nitrogen. You can learn more about acidifying soil from the OSU Extension publication EC 1560, "Acidifying Soil for Blueberries and Ornamental Plants in the Yard and Garden: West of the Cascade Mountain Range in Oregon and Washington". The link to this is below.
https://catalog.extension.oregonstate.edu/ec1560
Unless there is a root disease caused by poor drainage contributing to this problem, what I can see in the photos seems to be related to soil pH, nutrients and possibly watering practices. Whether you retain the existing plants or improve the soil and replant with new ones is probably worth considering. Feel free to write with additional questions.
There are signs of both nitrogen deficiency and iron deficiency on these leaves. In general as you point out they do not look healthy. It may be that drought stress is an issue as well, and I would ensure the drainage of the plants is good. The iron deficiency is evident from the "interveinal chlorosis" on some of the leaves, where the veins are green and the tissue between is yellow. This occurs on the newest leaves at the shoot tips when iron is deficient. This occurs in Rhododendron when the soil pH is too high and is very common,. Rhododendron prefer a soil pH of between 4.5-5.5 and our typical soil pH is about 6.0. The nitrogen deficiency is evident from the green leaves at the shoot tips and yellow leaves lower down, or lack of leaves lower on the stem at all. Nitrogen deficiency is the most common nutrient deficiency in any plant, and shows up as the pattern I described. Two things you will want to do is lower soil pH in that flower bed and fertilize appropriately with nitrogen. You can learn more about acidifying soil from the OSU Extension publication EC 1560, "Acidifying Soil for Blueberries and Ornamental Plants in the Yard and Garden: West of the Cascade Mountain Range in Oregon and Washington". The link to this is below.
https://catalog.extension.oregonstate.edu/ec1560
Unless there is a root disease caused by poor drainage contributing to this problem, what I can see in the photos seems to be related to soil pH, nutrients and possibly watering practices. Whether you retain the existing plants or improve the soil and replant with new ones is probably worth considering. Feel free to write with additional questions.
Thank you for the information. I have been increasing the Nitrogen feeding but just assumed the pH was acceptable because the problem was not common to all the Rhodys, there are about 15 all the same age. The house was built in the 80's and the landscape is probably 20 to 30 years old, well established but neglected the last 2 to 3 years.
Does the Iron deficiency or pH cause the necrotic black/gray edges? Thank you for the assistance.
Chuck Achberger
Does the Iron deficiency or pH cause the necrotic black/gray edges? Thank you for the assistance.
Chuck Achberger
Hi Chuck, I think that when the plants get so stressed as these, they may start to display all sorts of odd symptoms. Marginal necrosis like that might be consistent with excess salts in the growing environment (though that is usually from over-fertilization, which seems not to be an issue here!) or sometimes drought stress. I'm inclined to think it's just the result of prolonged stress from multiple causes. It is not disease, in any case.
Thanks Neil, I have downloaded the publication you mentioned and will see if I can make them happier.
Chuck
Chuck
