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Azalea (Rhododendron spp.) - Leaf Burn
Cause Leaf scorch on rhododendrons is a response to stress. Water stress can occur under both extremes of flooded, overly-saturated soils or under drought conditions when too little water is present. Root or stem damage due to disease or transplant shock can also cause scorch symptoms. High soil pH and exposure to drying winds are other possible causes of leaf scorch. Too much salt in soil or irrigation water, or using large amounts of inorganic fertilizers, or a combination of all three can cause symptoms as well. Plants in containers or not receiving enough water are often salt damaged, which also predisposes them to other problems such as Phytophthora root rot.
Symptoms Symptoms include browning of tips or margins of leaves, with the damage sometimes spreading to the center of the leaf. The south/southwest side of the plant is the most likely to be damaged by direct sunlight, but plantings also might be damaged by reflected sunlight.
Cultural control Prevention is the key as there is no remedial action that can be taken once leaves are scorched by the sun, but effects are not fatal and new leaves will eventually cover damaged ones.
- Adjust soil pH to 4.5 to 6 by adding sulfur or organic matter, especially conifer needle mulch, which is highly acidic.
- Transplant bushes or new plants in the fall or spring when a water is not limiting.
- Provide adequate soil moisture by irrigation where dry soil is a problem.
- Use raised beds or improve soil drainage with tiles if saturated soil is the problem.
- Shelter susceptible plants from drying winds, either with other plantings or by planting near structures.
- Use an appropriate amount of inorganic fertilizer. Home gardeners should fertilize with products marketed as azalea, camellia and/or rhododendron food.
- Irrigate heavily on occasion to leach out excessive salts.
- Check the salt content of your water source.
Chemical control Use before hot summer weather. Always follow pesticide label directions. Anti-transpirants or anti-desiccants are not recommended since they have not been shown to be effective at preventing heat or cold-induced desiccation of plants beyond their normal adaptations.
- Surround CF at 6.25 to 37.5 lb/A plus a surfactant. A dry white film will result on all treated plant parts. 4-hr reentry. O
Reference Linderman, R.G., and Benson, D.M. 2014. Compendium of Rhododendron and Azalea Diseases and Pests, 2nd ed. St. Paul, MN:APS Press.
Chalker-Scott, L. 2010. The myth of antitranspirants, pp. 183-189. In The Informed Gardener Blooms Again, University of Washington Press, Seattle, WA.