Soil and Plant Diagnostic Services

Azalea and rhododendron

Iron chlorosis (nonparasitic)

Interveinal chlorosis of youngest leaves. Leaves eventually may become yellow, cream colored or white. Usually caused by reduced availability of iron in high pH soil. Correct site-related conditions, such as high soil pH, water logging and poor aeration. Iron chelate applied to foliage may provide temporary greening, but for long-term control, lower the soil pH.

Leaf and flower gall (fungus)

Leaves become thickened or fleshy galls that turn pale green or white. The entire blossom may become a fleshy gall covered with a whitish bloom. Usually, chemical control is not warranted. Pick off and destroy galled parts before they become white, if feasible. Reduce humidity around plants by increasing airflow and avoiding prolonged moisture on leaves. Some cultivars are more susceptible than others. If infection is severe, an application of triadimefon the following year may help, although fungicidal controls have not always been found to be successful. Begin at bud break and reapply at 10-day inter vals through the infection period.

Phytophthora dieback and root rot.

Phytophthora dieback (fungus)

Terminal buds and leaves turn brown. Evergreen leaves roll up and droop as though in a winter condition. Cankers are formed on the stems. If stems are girdled, all parts above will wilt and die. Prune and remove affected branches. Improve site drainage. Avoid overhead watering, watering late in the day or other practices that prolong periods of leaf wetting. To prevent re-infection and protect the remaining plant, drench or apply a foliar spray of Fosetyl-Al or mefenoxam.

Note
Use lower labeled rate of mefenoxam for cv. Coral Bell.

Repeat in two to three months, if necessary. Chlorothalonil or mancozeb may be applied on a protectant basis for shoot infections. Apply at new leaf emergence and at 7 to 14 day intervals.

Note
Chlorothalonil may discolor blossoms.

Phytophthora root rot (fungus)

Discoloration associated with the base of an azalea affected by Phytophthora See photo above

Discoloration associated with the base of an azalea affected by Phytophthora.

Leaves wilt. On evergreen species, the leaves become chlorotic and roll downward parallel to midrib and eventually wilt. Growth is stunted. Fine roots decay, and cankers may develop at the base of the stem. Phytophthora-specific fungicides are available. Generally, their use is not recommended for established plantings. Remove and destroy affected plants. If practical, improve soil drainage and replant with a more tolerant variety.

Resistant or tolerant azalea varieties
The most resistant cultivars include Formosa, Fakir and Corrine Murrah. As a group, Indica hybrids are most resistant. Resistant cultivars of rhododendron include Caroline, Professor Hugo deVries and Red Head. Note: extremes in soil moisture often affect susceptibility.
Resistant rhododendron cultivars
Caroline
Professor Hugo deVries
Red Head.

Note
Extremes in soil moisture often affect susceptibility

Winter leaf scorch (nonparasitic)

Evergreen leaves turn brown at edges and near tips. There may be large, brown blotches as well. More prevalent on older leaves. Be sure soil is moist in the fall. Damaged plants should be fertilized. Provide wind breaks around plants. Antidesiccants applied every 6 weeks in winter may provide some help.

Update6/10/09