Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001693
11/12/99
Phytophthora Root Rot of Rhododendron
The roots of rhododendrons growing in wet soil are
attacked and rotted by water molds.
Symptoms: Symptoms are wilting and death of the top of the
plant. To diagnose the cause as root rot, examine the
finer roots of plants just showing wilting. If the roots
are black and soft and the outer skin easily pulls away
from the inner wood, the likely cause of disease is a
root rot fungus. Plants affected by root rot show an
inward curling of the leaves. Drought alone, however,
causes the same top symptoms. With root rot, a single
branch near the soil surface may develop curled leaves,
then die and drop its leaves. This branch dieback occurs
when the root rot fungus is splashed up onto a leaf,
infecting the leaf and the branch.
Cause: Root rot of rhododendron is often symptomless in
the landscape until stress from drought or flooding brings
on disease. Fertilizers are also important in
predisposing the plants to disease. High amounts of
nitrogen fertilizer are the most important factor in
bringing about the root rot disease in the urban
landscape.
Control: Plant in a well-drained site and take care to
water during summer droughts. Avoid use of fertilizers;
if needed, apply a low rate of slow release fertilizer.
It is particularly important to not add high nitrogen
rates at transplanting. Fungicides for controlling root
rots are available for use as soil drenches.
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This file was generated from data base ZZ on 01/19/00.
Data base ZZ was last revised on 11/12/99.
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