Azaleas are generally healthy plants when their basic cultural requirements are met. However, they are subject to a number of problems caused by infectious agents, insect pests, weather and nutrition deficiencies. Several of the more common problems are described here in detail.
An excellent reference to azalea diseases is provided by Penn State University at: http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/azalea-rhododendron-diseases
Plants are stunted and wilted. Leaves yellow, and the entire plant eventually dies. Roots with few feeder roots die. Stem wood at the soil level has red-brown discoloration. In the top dieback phase, leaves have dark-brown spots. Shoots die from the tips back as dark-brown cankers form. No obvious fungal fruiting structures are formed.
Purchase disease-free plants. Especially inspect southern-grown, containerized material before planting. Use clean, disinfected tools. Avoid overhead watering. Following a diagnosis confirming that the plant is infected, remove infected plants. Apply a fungicide to protect plants. Fungicide applications to infected plants merely delay eventual death.
Leaf and flower gall is caused by Exobasidium spp., typically Exobasidium vaccinii on ericaceous plants in the United States. It is not a serious disease, but can appear unsightly and is more prevalent under moist conditions. Overhead irrigation provides an excellent environment for infection.
The disease first appears as a thickening and fleshy appearance of a plant structure, particularly expanding leaf buds, flower buds and new shoots. As shown by some pictures here, the galls range from small thickenings on leaves to large irregular to round galls up to 2″ in diameter, extending beyond the leaves and flower buds. The galls enlarge rapidly, and are at first soft and succulent, typically light green in color. Later a whitish fungus covers the gall, and the gall then slowly turns brown, shrinks and becomes quite hard.
The fungus apparently overwinters as spores in bud scales of host plants. Spores germinate and infect young bud tissue as the buds begin to expand in the spring. Infection increases with moisture in the form of humidity and rain at budbreak, with poor soil aeration, and is more prevalent on lower leaves. Spores from the galls can be carried by wind and rain to expanding buds on nearby stems and plants to form secondary infections, and can also adhere to new buds and overwinter.
Prevention measures include reducing humidity and moisture at budbreak, particularly moisture on leaves, and increasing soil aeration. The primary control measure is to remove and destroy the galls before they become white, to reduce the potential for spread of the infection and its overwintering. Use of fungicides has not generally been successful.
First reported in the 1930s on the Belgian-Indica hybrids, petal blight can attack most azaleas under favorable weather conditions. Infected flowers first exhibit small spots, about 1 mm in diameter, which appear water-soaked. The spots enlarge rapidly and become very slimy, causing entire petals to become slimy and limp, usually within 2-3 days after initial infection. Infected areas of flowers soon become tan or light brown, and eventually entire flowers turn prematurely brown. Infected flowers dry and generally cling to the plant longer than uninfected flowers do. One to 20 (commonly two to five) small, black sclerotia (a fungal resting stage) form on an infected flower 6-8 weeks later. They appear first as small white areas on the dried petals and slowly become dark as they mature.
Caused by the Ovulinia azaleae fungus, the disease develops during periods of moist weather at flowering time. Early- and late-flowering cultivars or species tend to escape the disease. The fungus overwinters as sclerotia on diseased petals adhering to plants, on the soil surface, or in leaf mulch under the bushes. At the beginning of the host’s blooming period, the sclerotia germinate and in 3-5 days give rise to small stalked, cup-shaped fruiting bodies filled with spore sacs, which are discharged with sufficient force that they strike the lower blooms above the sclerotia or are carried in air currents to nearby blooms on adjacent plants.
The rate of infection is higher when periods of frequent precipitation and warm weather coincide with flowering. Heavy dew or extended periods of misty weather are particularly favorable. Low temperatures at the beginning of the flowering period and dry conditions toward the end of it most likely reduce the infection potential.
Spraying the infested soil with fungicides and removal of the leaf litter beneath infected plants have not been reliable methods of disease control. Sprays applied at or just before bud opening have offered the best means of control. In general, sprays of triadimefon or benomyl applied at weekly intervals throughout the flowering period have controlled the blight and inhibited sclerotium formation. Making sure diseased or disease-carrying plants are not introduced into the growing area reduces the disease potential.
Diagnosing plant nutrient deficiencies and excesses is difficult, and is best done by knowing the history of fertilization, soil, location and analysis of the leaves. This discussion focuses on visual examination of the leaves.
Nitrogen Nitrogen deficiency is seen by stunted plant growth and small, pale green or greenish yellow leaves. Eventually, older leaves develop red blotches or red edges. Additional nitrogen should be in the form of ammonium sulphate. Bacteria convert nitrogen to ammonium, and other bacteria convert ammonium to nitrate. These latter bacteria are less active at higher pH. High levels of organic matter and low pH can reduce the availability of nitrogen to plants by fixing the nitrogen in the microorganisms.
Toxicity due to an excess of nitrogen shows as chlorosis and leaf drop of young leaves. This is due to nitrate forms of nitrogen. Nitrogen in the form of ammonium produces better color and more growth and is not as toxic. The chlorosis may be induced by an alkaline environment at the roots, and the leaf drop may be caused by an unavailability of trace elements at the high pH.
Phosphorus The leaves of plants with phosphorus deficiency become dark green, sometimes almost black, and reddening later develops on the lower surfaces, especially along the midribs. The reddening may occur in splotches, which become reddish purple on both surfaces of the leaves. Leaves may turn dark brown or black and drop off. Lower leaves on vigorous shoots may die, with reddish bronze leaves remaining at the ends of the shoots.
Phosphorus may be tied up by the microorganisms that decay the organic matter in which the azaleas are grown, and under these acid conditions can be leached out, so a deficiency results. However, azaleas do not need as much phosphorus as other plants. Mycorrhizal fungi absorb phosporus and transport it to the azalea roots.
Potassium The initial symptom of potassium deficiency is interveinal chlorosis of young leaves, like that of iron deficiency. If it continues, marginal scorch and dead areas develop on recently matured leaves. Later, leaves become bronzed and chlorotic and new shoots die back. Leaves may roll inward, others drop off, and terminal growth stops. This is rare, as azaleas need very little potassium.
Calcium Early symptoms of calcium deficiency resemble those of iron deficiency. Later, stunting and chlorosis of new leaves occurs, followed by tip burn of young expanding leaves. Tips of other leaves become twisted. Later, the terminal and lateral buds die. This is rare, as the azaleas require very little calcium.
High levels of calcium inhibit the distribution of iron, resulting in iron chlorosis and the accumulation of iron along veins. Mature leaves seldom recover from the chlorosis.
Magnesium Symptoms are chlorosis of older leaves, beginning at the leaf tips. Veins remain green at first but later lose color. Brown, dead areas appear on the tips and margins of lower leaves which soon drop off. They may develop reddish purple blotches on their upper surfaces. While azaleas apparently require more magnesium than calcium, that amount is small compared to other plants.
Iron Iron deficiency causes interveinal chlorosis of young leaves, with young leaves becoming yellow, cream colored or white while the veins remain green. The lower leaves are the last to be affected. The symptoms resemble those of lack of calcium, potassium or magnesium.
Iron deficiency is one of the most common causes of chlorosis and is produced by various conditions, including high soil pH, soils high in calcium, waterlogged soil with poor aeration, and reduced availability of iron in the soil. It can be diagnosed by spraying the leaves with iron chelate. If the chlorosis is caused by iron deficiency, the foliage will become green within a week. This is not a permanent solution, however, which can only be done by lowering the soil pH.
Manganese Symptoms of manganese deficiency are similar to those of iron deficiency, but are less severe. Spraying the foliage with magnesium sulphate can help to diagnose the deficiency. This should be done before treating the plant for the deficiency, although azaleas have a high tolerance for manganese.
Boron Boron deficiency is not a frequent problem but may occur in alkaline soils. The symptoms are a sprinkling of brown dots on new leaves, later becoming translucent flecks, and finally becoming brown dead areas which grow and distort the leaves. Eventually the growth buds die. Browning and dead areas develop on the flowers at or near the base of the petals.
Boron can build up in potted plants from frequent fertilization. Symptoms of boron toxicity are yellowing of leaf tips, followed by marginal chlorosis and premature leaf drop. Prevention consists of using fertilizer low in boron. Treatment includes pruning off the affected parts, and irrigating with a solution of copper sulfate and limewater.
Copper This may occur in plants potted in a medium low in copper, such as a mixture of sphagnum moss and peat. The first symptoms are marginal chlorosis of a few leaves at the tips of the main shoots, followed by brown dead spots. If the deficiency is not corrected, the leaves are severely restricted in size, and leaf drop and dieback of shoots occur.
* Excerpted and summarized by permission from Compendium of Rhododendron
and Azalea Diseases, second edition, American
Phytopathological Society, ISBN 0-89054-436-0.
The table of contents of the Compendium is at azalea problems reference.
azalea problems (parent page)