Rhododendron Diseases
Ornamental Disease Note No. 12
R.K. Jones, Plant Pathologist (retired)
D.M. Benson, Plant Pathologist


[Phytophthora Root Rot] [Leaf Gall] [Nematodes] [Petal Blight] [Twig Blight]
[Phytophthora Dieback] [Leafspot] [Fertilizer Injury]


Rhododendrons are a popular spring flowering shrub grown in North Carolina nurseries and landscapes. Generally these plants grow and bloom quite well across the state, particularly in the piedmont and mountain sections. Occasionally diseases can be a problem on rhododendrons in North Carolina.

Phytophthora Root Rot

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, pieris, taxus (yew), Deodar cedar, mountain laurel, heather, juniper, high-bush blueberries, Fraser fir, white pine, shortleaf pine, leucothoe, and others.

Symptoms

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 develop especially in shallow soils with underlying rock or compacted hard pans.

Prevention

Phytophthora root rot must be prevented as chemicals are often ineffective in controlling this disease after aboveground 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 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. 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.

Resistant
Caroline Professor Hugo de Vries
Martha Isaacson Red Head
Moderately Resistant
Brick Mrs. A. T. de la Mave
Broughtonii Aureum Mrs. C. B. Van Nes
Disca Prize
Dr. A. Blok Bosley Dexter 1020
Dr. Arnold W. Endtz Rocket (Shammarello)
English Roseum Wilbrit
Lucky Strike Van Veen
Madame Carvalho


Fungicide Drenches

The spread of Phytophthora into or among plants also can be reduce through the use of metalaxyl (Subdue) or fosteyl-A1 (Aliette), but these chemicals may not kill the fungus in infected plants.

Fungicide Rate Frequency of Application
Metalaxyl (Subdue 2E) 1-4 oz/100 gal water to 400 sq ft or 1.5-2.5 ml (cc) per 2 gal to 80 sq ft Drench at 2-3 month intervals April through November outside, or when media temperature is about 50 F.
 
Rate of Subdue 2E
(fl oz/100 gal)

1 gallon

5 gallons
10 gallons
25 gallons
0.5 oz
3 drops
15 drops
30 drops (1.5 ml)
3.75 ml (3/4 tsp)
1.0 oz
6 drops
1.5 ml (1/4 tsp)
3.0 ml (1/2 tsp)
7.5 ml (1.5 tsp)
2.0 oz
12 drops
3.0 ml (1/2 tsp)
6.0 ml (1.0 tsp)
15.0 ml (1 tbsp)
3.0 oz
18 drops
4.5 ml
(1 tsp)
9.0 ml (2 tsp)
22.5 ml (1.5 tbsp)
4.0 oz
24 drops (1.2 ml)
6.0 ml (1.0 tsp)
12.0 ml (2.5 tsp)
30.0 ml (2 tbsp; 1 oz)
Fosetyl-Al (Aliette 80WP) 1-2 lb/100 sq ft use 0.5-1.5 pts per sq ft Drench or spray monthly April through November or when media temperature is above 50 F.

For individual, small rhododendron plants in the landscape, approximately 10 square feet of soil around the plant should bedrenched. For large rhododendron and other large shrubs, 20-30 square feet of soil should be drench. 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 higher rate on azaleas may cause some injury. In the nursery metalaxyl can be applied through the irrigation system. Fosteyl-A1 can be used with a hozon applicator, but the stock must be constantly stirred to prevent the wettable powder fungicide from settling out. 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. Use chemical control as a last resort in the landscape.

5. Avoid rhododendrons and other root rot susceptible plants in areas where the disease has been a problem.

Leaf Gall

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 ferban 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.

Nematodes

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 from nematode damage, purchase nematode-free plants and set in another area free of stunt nematode. Nematode infested sites can be fumigated before planting with SMDC (Vapam) at the rate of 22-29 oz./100 sq. ft.

Petal Blight

Petal blight (Ovulinia azaleae) can cause and 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 head may be completely rotted within 2-3 days. The disease also occurs on azaleas. The fungus produces hard, black resting structure (sclerotia) in the blighted petal tissue that survives on the soil surface until the next spring.

Control petal blight by applying 20 percent Terraclor (21/2 Ib 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 4-8 oz per 100 gals of water (1/2 1 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. Blighted flower heads should be picked and removed from the garden before the petals fall to the ground.

Twig Blight

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 dieback 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 disease in the landscape is dieback caused by Botryosphaeria (see Table 2).

Twig blight can be controlled on cultivars that are not severely stressed nor highly susceptible by promptly pruning out and disposing of the diseased branches. Carefully, check to be sure all the discolored stem tissue has been correct pruning practices are not followed. Fungicides are of little value.

Phytophthora Dieback

Dieback of hybrid rhododendron is a foliar disease caused by several species of the fungus Phytophthora. In North Carolina, 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).

Symptoms

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 days. The fungus progresses slowly in older woody stems, but can kill any size plant.

Table 2. Diagnosing Rhododendron dieback caused by Botryosphaeria and Phytophthora.

Botryosphaeria
Phytophthora
1. Mostly in landscapes. 1. Mostly in container nurseries.
2. Entire branch or section of plant wilts quickly (1-2 days) and dies. 2. Attacks individual shoots.
3. Attacks older wood. 3. Attacks succulent leaves and stems.
4. Entire leaf rolls downward. 4. Leaf does not roll.
5. Entire leaf turns gray-green then brown. 5. Dark brown to black discoloration progressing into "V" shaped pattern from mid-rib to leaf magins.
6. Leaves remain attached to dead stem. 6. Mature leaves drop from stem quickly.
7. Brown discoloration in wood on one side of stem. 7. Brown discoloration in wood.
8. Moves fast in old stems. 8. Moves fast in succulent stems.
9. Branches die at any time. 9. Shoots usually die during growth flush.


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 new leaves. Nurseries where rhododendron are grown under shade and irrigated by overhead sprinklers provide ideal 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, particularly during August, 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 277). Phytophthora dieback is not a common disease problem in the landscape.

Control 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 dieback 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

Leafspots 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.

Fertilizer Injury

Rhododendrons have fine roots that grow very close to the soil surface. It is easy to bum the roots with fertilizer. Several light applications during spring and summer are better than one heavy application. Spread the fertilizer around the drip-line of the foliage.

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For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service

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Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical. For assistance, contact your county North Carolina Cooperative Extension Service agent. 10/90/1000


Published by North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Reformatted Dec. 2000 by A.V. Lemay