Most of the rhododendron diseases such as botrytis, galls, rust and powdery mildew, are quite easily controlled with good ventilation, plant hygiene and common fungicides.
Root rot, especially the fungus Phytophthora cinnamomi, can be a problem, particularly in poorly drained soil. Prevention -making sure the plants are not too crowded and that the drainage is satisfactory - is the best method of control. Soil fungicides can be used to prevent or control root rot but in most cases only one or two plants are affected and that can be tolerated.
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Phytophthora syringae is a closely related fungus that causes a die back effect on the branches. Phomopsis is another die back disease, one that mainly affects azaleas. It slowly destroys twigs and branches and can prove fatal, although this usually takes several years. If odd branches show signs of wilting and yellow foliage yet there is no sign of frost damage, suspect phomopsis dieback. The disease can be controlled by removing any affected branches and spraying with an appropriate fungicide.
Pestalotia fungi cause sunburn-like markings of the foliage, particularly around the margins and tips of the leaves. Although clearly not sunburn, which usually scorches the center of the leaf, this disease can be difficult to differentiate from fertilizer burn. If no fertilizer has been used, you should suspect Pestalotia. Many fungicides will control this problem.
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Azalea leaf gall is a fungal disease that attacks the new growth and the flowers. It causes an unsightly thickening and distorting of foliage and petals and can spread quickly in cool, moist weather. Picking off affected leaves and petals and spraying with a suitable fungicide will control the disease. However, this is mainly a cosmetic problem that generally causes little long-term damage.
Bud blast or bud blight is a fungal disease that attacks unopened buds, causing them to turn brown and fail to open. It would be difficult to tell apart from frost damage were it not for the fine black filaments (coremia) that develop on the bud. Infected buds should be removed and destroyed and the plant treated with a fungicide. Copper-based sprays are effective.
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Ovulinia petal blight is a fungus that destroys the flowers as they open. It starts as pinhead-sized, watery, brown spots and gradually the whole flower becomes a slimy mess. This disease can spread quickly in cool, moist, spring weather. Good ventilation helps to control it but spraying with a fungicide is usually also necessary. The disease spores winter over in the soil, so the soil should be drenched with a fungicide during winter in areas where outbreaks are known to occur.
Many rhododendron leaves roll along the midrib, curl down and may develop purple spots in winter. These are not signs of a disease but appear to be related to cold weather stress.
Ring-like purple spotting on the foliage is a disease: a virus common with R. griffithianum hybrids such as those of the 'Loderi' grex. Although unsightly and sometimes the cause of premature leaf drop, the disease is virtually impossible to eradicate and is something you have to live with if you wish to grow these otherwise superb hybrids.
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Few of the pests that attack rhododendrons are life threatening and provided you are prepared to put up with a few chewed leaves the damage is seldom serious.
Caterpillars, weevils, sawfly larvae and cutworms all cause damage by chewing the foliage and occasionally the buds and flowers. Most can be removed by hand or controlled with safe insecticides.
Azalea leaf miners can cause significant damage by destroying the foliage and retarding growth. These small caterpillars, which are common on evergreen azaleas, start life within the leaf and feed by burrowing (mining) between the upper and lower surfaces of the leaf. Later, when they become too large to remain within the leaf, they emerge and make a shelter by rolling over the tip of the leaf. Because the pest is inside the leaf, systemic insecticides are the only really effective control.
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Thrips, lace bugs and spider mites may all be found on the underside of rhododendron leaves. They can cause severe damage by rasping the foliage and sucking the sap. Their presence is usually indicated by leaves that develop silver-gray upper surfaces. An examination of the undersurface will reveal a sticky brown honeydew deposit, which can lead to the development of sooty mold diseases. Oil sprays and other safe insecticides can control these pests but it is very important to get complete coverage.
Damage from root-feeding weevil and beetle larvae can be a serious problem, especially with container plants. In the garden these pests are seldom present in sufficient numbers to cause great damage but in a pot they can quickly kill a plant by eating away most of the roots. An occasional sprinkle of soil insecticide will prevent such potential disasters. Rhododendron weevils will also chew the foliage. However, if the larvae are controlled, the adults are rarely a serious problem. Weevils feed at night and may be removed by hand.