JARS v38n3 - Rhododendron Diseases

Rhododendron Diseases
R. K. Jones, Extension Plant Pathologist
D. M. Benson, Plant Pathologist

Phytophthora root rot ( Phytophthora cinnamomi is a serious, widespread and difficult-to-control fungus disease affecting a wide range of plants in North Carolina. Plants susceptible to root rot include azalea, rhododendron, dogwood, Camellia japonica , Pieris , Taxus (yew), deodar cedar, mountain laurel, heather, juniper, high-bush blueberries, Fraser fir, white pine, shortleaf pine, leucothoe, and others.
The symptoms of Phytophthora root rot vary with the cultivar. Some cultivars fail to grow or grow very slowly with pale green foliage and may die after several years. Others suddenly wilt and die within a few weeks. Roots are reddish-brown, brittle and often limited to the upper portion of the media in a container or very close to the soil surface (upper 2 inches). The reddish-brown discoloration advances to the larger roots and eventually to the lower part of the main stem.
Phytophthora root rot is favored by high soil moisture and warm soil temperatures. The disease does not occur as frequently and may not be as severe on well-drained sandy soils as in heavy clays or poorly drained soils, etc. The disease is common and severe in areas where run-off water, rain water from roofs, etc. collects around plant roots. Setting woody plants deeper than the soil level in the nursery or container, over-watering plants, or long periods of heavy rain also favor disease development especially in shallow soils with underlying rock or compacted hard pans.
Phytophthora root rot must be prevented as chemicals are often ineffective in controlling this disease after above-ground symptoms appear. The following suggestions may aid in the prevention of root rot:
1.  Purchase disease-free plants from a reputable nursery. Avoid plants that lack normal green color, appear wilted in the morning, or have dark, discolored roots.
2.  Plant in well-drained areas. If excess water from any source collects in the planting site, avoid planting root-rot susceptible plants. If soil is clay, set plants in raised beds (Fig. 1) and thoroughly mix a porous material such as bark (not sawdust or peat) into the bed. The material should be incorporated to a depth of 8-12 inches below the surface to help reduce excess soil moisture.
3.  Do not set the new plant any deeper than the soil level in the container or the soil line in the nursery. Firm the soil beneath the soil ball so that the plant will not settle into the bed.
4. In areas where plants have died from root-rot, replant with plants that are not susceptible to root rot.
5.  For trained applicators, methyl bromide (sold under various trade names) used at the rate of two pounds per 100 square feet and released under an air-tight cover such as plastic, will reduce the fungus population. The soil should be well prepared for planting and the soil temperature should be 55°F or above. Leave the plastic cover in place for 48 hours. Plants can be set one week after removing plastic.
6.  Within cultivars of various types of plants, some cultivars are highly susceptible (very likely to be killed by the fungus) and others are resistant. Resistance of the many cultivars of rhododendron to the Phytophthora root rot fungus are given in Table 1.

Table 1. Reaction of Rhododendron Hybrids to Phytophthora Root Rot
Caroline Professor Hugo de Vries
Martha Isaacson Red Head
Pink Trumpet
Moderately Resistant
Brickdust Madame Carvalho
Broughtonii Aureum Mrs. AT. de la Mare
Dr. A. Blok Mrs. C.B. Van Nes
Dr. Arnold W. Endtz Prize
English Roseum Bosley Dexter 1020
Lucky Strike Rocket (Shammarello)
Van Veen

Fungicide Drenches
The spread of Phytophthora into or among plants also can be reduced through the use of metalaxyl (Subdue), but this chemical may not kill the fungus in infected plants. For individual small rhododendron plants in the landscape, approximately 10 square feet of soil around the plant should be drenched. For large rhododendron and other large shrubs, 20-30 square feet of soil should be drenched. Metalaxyl can be applied with a hozon applicator at the rate of 11 cc per 1 gallon of water stock solution for 90 sq ft. The rate for rhododendron and other woody ornamentals is higher than for azaleas. Using this rate on azaleas may cause some injury. In the nursery metalaxyl can be applied through the irrigation system.
In summary, for long-term control of root rot the following control measures should be utilized in this order:
1.  avoid poorly drained areas,
2.  plant in raised beds except in deep sandy soils,
3.  use highly resistant or resistant cultivars,
4. chemical control as a last resort in the landscape.

Leaf gall ( Exobasidium vaccinii ) is very common and widespread in the early spring on azaleas and occasionally occurs on rhododendrons. Some native Rhododendron sp. (azaleas) are more susceptible than hybrid rhododendrons. The leaves and entire shoots become thickened, curled, fleshy and pale green to white. In the latter stages of the disease the galls are covered with a white powdery substance. The disease is more alarming than damaging. If you only have a few plants, control the disease by hand picking and destroying diseased leaves. Leaf gall seldom causes enough damage to justify spraying. It also can be reduced by spraying with ferbam or zineb at the rate of 2 tsp. per gallon. Make first application just before and again near the end of the flowering period and repeat 6 weeks later. Leaf gall also occurs on camellia but is caused by a different fungus.

Rhododendrons are susceptible to stunt nematode ( Tylenchorhynchus claytoni ) damage. Leaves turn yellow and plants are stunted and gradually die. They fail to respond to fertilizer and water. There are no chemicals (nematicides) available to control nematodes on rhododendrons established in the landscape. Provide good growing conditions, i.e. mulch, partial shade, fertilize plants, according to a soil test report, and water during dry periods. (If plants die, purchase nematode-free plants and set in another area free of stunt nematode).

Petal blight ( Ovulinia azaleae ) can cause considerable damage to the flowers. The occurrence and severity of the disease is highly dependent on wet weather conditions during the flowering period. The symptoms are tiny, pale or whitish to rust-colored spots on petals. The spots enlarge rapidly and the infected tissue becomes soft and watery. Entire flower heads may be completely rotted within 2-3 days. The disease also occurs on azaleas. The fungus produces hard, black resting structures (sclerotia) in the blighted petal tissue that survives on the soil surface until the next spring.
Control petal blight by applying 20 percent Terraclor (2 lb per 100 sq. ft.) on the soil beneath the plants. Apply 1 to 2 weeks in advance of flowering of early cultivars. Plants can also be sprayed with triadimefon (Bayleton 25% WP) at 8-16 oz per 100 gals of water (1-2 tsp per 1 gal). Spray plants thoroughly just as flower buds begin to show color. A second application may be made if disease has been severe in past years. Later blooming varieties should be sprayed as they begin to show flower color. Other fungicides including benomyl (Benlate 50WP) at lb per 100 gal of water (1 tsp per 1 gal) or zineb (Dithane Z-78 75WP) at 1 lb per 100gal (1 TBS per 1 gal) can be sprayed on the open flowers 2-3 times each week as a protectant. Blighted flower heads should be picked and removed from the garden before the petals fall to the ground.

Twig blight ( Botryosphaeria dothidiae ) usually appears on larger branches of established plants or newly set plants without a well established root system. Occasionally Phomopsis sp. will cause twig die-back very similar to Botryosphaeria . Infected twigs first show wilting and death of leaves on one or more branches. A reddish-brown discoloration can be found under the bark on dying branches, often on one side of the stem (Fig. 2). This discoloration may extend from several inches to several feet along the stem. The most common dieback in the landscape is caused by Botryosphaeria (see Table 2).
Twig blight can be controlled by promptly pruning out and disposing of the diseased branches. Carefully check to be sure all the discolored stem tissue has been removed. Fungicides are of little value.

Dieback of hybrid rhododendron is a foliar disease caused by several species of the fungus Phytophthora . In North Carolina, P. nicotiana var. parasitica , P. heveae , P. cactorum , and P. citricola have been isolated from diseased plants. Rhododendron dieback is primarily a problem on container-grown hybrid rhododendrons in nurseries. This disease is very rarely observed in the landscape in North Carolina (see Table 2).

Table 2. Diagnosing rhododendron dieback caused by:

1.  mostly in landscape
2.  entire branch or section of plant wilts quickly (1 -2 days) and dies
3.  attacks older wood
4.  entire leaf rolls downward parallel to mid rib
5.  entire leaf turns gray-green then brown
6.  leaves remain attached to dead stem
7.  brown discoloration in wood on one side of stem
8.  moves fast in old stems
9.  branches die at any time

1.  mostly in container nurseries
2.  attacks individual shoots
3.  attacks succulent leaves and stems
4.  leaf does not roll
5.  dark brown to black discoloration progressing in "V" shaped pattern from mid-rib to leaf margins
6.  mature leaves drop from stem quickly
7.  brown discoloration in wood
8.  moves fast in succulent stems
9.  shoots usually die during growth flush

Lesions first appear as chocolate-brown spots on young, expanding foliage and stem. Infected areas may appear water soaked at first; later leaves dry out and may drop off. The fungus grows from initial lesions down the stem into older leaves. Lesions on mature leaves are characteristically wedge shaped, extending from the petiole toward the leaf margin. Infection spreads rapidly in warm weather and one-year-old plants can be killed within a few days. The fungus progresses slowly in older woody stems, but can kill any size plant.
Factors Favoring Infection and Spread of Dieback
Phytophthora dieback is favored by hot, wet weather during the summer months. The fungus is spread from infected plant debris on the container base and between plants by spores in splashing water. Free water on the leaves is needed for infection by the spores. Infection occurs on the lower surface of expanding leaves. Nurseries where rhododendron are grown under shade and irrigated by overhead sprinklers provide ideal conditions for Phytophthora infection. Puddling of water around plants often occurs in nurseries using pine bark or plastic as a container base and increases the spread of the fungus by splashing. Excessive use of nitrogen fertilizer, which increases plant succulence, increases plant susceptibility to Phytophthora .
Control of Phytophthora Dieback
Several practices can be employed in the nursery to prevent Phytophthora dieback (see Plant Pathology Information Note 227).
Control of Phytophthora dieback in the landscape by the following practices:
1.  Purchase only plants that do not show Phytophthora dieback or root rot symptoms.
2.  Remove and destroy all severely infected plants and plant debris promptly.
3.  If overhead sprinklers are used, avoid late afternoon irrigations, to allow the foliage to dry before night. If plants are watered by hand, do not wet the foliage.
4.  Avoid over-fertilizing plants. Dieback develops and spreads more rapidly in very succulent tissue.
5.  The spread of Phytophthora die-back can be reduced by spraying plants with protective fungicides. The spray must completely cover the leaves to be effective, especially the lower leaf surfaces and younger foliage. A fungicide spray program is not usually recommended unless Phytophthora dieback is known to be a problem in the nursery.
6.  Provide adequate spacing between plants to promote better air circulation around plants to dry foliage. Prune off lower branches to get better air circulation under the plant to help the soil surface dry under the plant. Proper spacing will also allow better fungicide spray penetration and coverage.
7.  On larger rhododendron plants, dieback can be selectively pruned out if it is detected in the early stages. All brown discolored wood in diseased stems must be removed. Prune back to a healthy branch. Pick up and destroy all diseased leaves and stems.

Leafspot may develop on older rhododendron leaves during fall and winter months causing premature leaf drop but usually causes very little or no damage to the plants. Control is usually not necessary.