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Insects & Disease of Rhododendrons and Azaleas
Common insects & diseases, their symptoms and remedies: When choosing chemicals or controls for insects & diseases, always determine the problem first, then choose a product for use. *Always follow application instructions carefully. And remember - More is not better! There may also be biological controls available to correct many of these problems as well. Insect: Weevils Weevils are insects that are really beetles with a long beak or snout protruding from their heads which contain their mouthparts. There are many kinds of weevils that can attack azaleas and rhododendrons, among them the obscure root weevil, black vine weevil, strawberry root weevil, woods weevil, Japanese weevil, and the clay-colored weevil. Many of these weevils can be found on other plants in your landscape as well, including arborvitae, hemlock and yews, to name a few. Symptoms: Regardless of what species of weevil is attacking your plants, damage by them can be quite injurious, with the larvae of most species being the most destructive because they generally feed on the small new roots and the bark around the crown of the plant. This type of feeding can mean a slow death for the plant and the damage is usually done before the problem insect is diagnosed. The adults of most species generally attack the foliage feeding on the perimeter of the leaves, leaving a ragged edge appearance. Adults of many species feed during the day, but the adult of the black vine weevil only feeds at night and hides in the soil below the plant during the day. If you suspect damage by a this weevil, simply go out at night with a flashlight to see if they are feeding. Other weevils can be observed feeding during the day. Often you can reach your arm into the plant and shake the foliage a bit, if a little beetle appears on your arm or leg - it is probably a weevil - look for the ‘snout’. Remedy: Spray the foliage completely to kill the adults and drench the soil around the plant to kill the larvae late may through June. Choose a product marked specifically for treating borers.*
Insect: Lacebug (Do not confuse this bug with the beneficial insect, the lacewings.) Lacebugs are small flat bugs with lacy, transparent wings that feed on the underside of the foliage sucking out moisture from the leaves. Infestations are usually more severe on plants growing in full sun. Plants are rarely killed by lacebugs, but infestations left unchecked cause the plants to become sickly and eventually they decline in vigor. Symptoms: Leaves will show a yellowish speckling on the top surface and brown or black tar spots will be evident on the underside of the foliage. Remedy: Spray the foliage completely in early May - June, when active nymphs are noticed. For adults spray foliage completely, especially the under sides of the leaves starting in late June to early July. *
Insect: Borers Borers, bore into the stems and branches of plants to feed and pupate. As they feed, they create tunnels throughout the stems and make holes periodically along the stems to the outer bark and push out large amounts of frass (fine sawdust.) Symptoms: Early symptoms are similar to drought stress, leaves lose their color and become chlorotic (yellowish). Holes will be noticed in the bark, along the stems, trunk and especially in the stem crotches. Sawdust will be noticed on the ground as well as on the rough bark. Entire branches on an otherwise healthy looking plant may be dead. If caught early enough, damage will be limited closer to the top of the plant. In the later stages borer damage will be noticed near the base of the plant, 6" or so from ground level. Remedy: Cut and burn (or throw into trash bags) the infected portions of the plant. Cut the infected stems below the damaged area, well into the uninfected portion of the stem. Make clean cuts and paint or seal the wound. Spray adults in early June, make sure to spray the trunks, stems and branches thoroughly. *
Disease: Fungus (Leaf Spots) Fungus leaf spots are caused by the fungi Phyllosticta, Septoria and Pestalotia and are commonly the agents of this problem which enter through wounds on the stem or from poor pruning practices. This disease is more common during damp, wet weather. Leaf spots can also be caused by other conditions such as improper soils, poor drainage, drought, improper fertilizing, winter sun scorching, summer sun scorching or mechanical injury of some kind. If you are not sure, seek out a professional opinion, spraying needlessly may do more harm than good. Symptoms: Spots can be irregular in size and color, some with red-brown borders and silver-gray centers. Very small black dots which are actually the fruiting structures of the fungi are found in the center of these spots. Remedy: Plants should be sprayed with a fungicide.* Begin spraying after flowering and repeat every 10-14 days until dry weather begins. Use a spreader sticker to hold the fungicide on the foliage. Avoid overhead irrigation or watering from above with a hose, this could spread the fungus disease. Rake up and dispose of any fallen leaves around the plant.
Disease: Bud Blight Bud blight is caused by the fungus Briosia azaleae. Symptoms: Flower buds turn brown and fail to open, eventually tiny black spores cover the surface. Remedy: Remove and destroy the affected buds. Fungicide applications are usually not necessary.
Disease: Leaf Gall Leaf gall is much more common on azaleas, than on rhododendrons. This disease is caused by the fungus Exobasidium vaccinii and generally affects the new leaves in late spring and early summer, but also can affect the blossoms and seed pods. Symptoms: On azaleas, bladder shaped galls appear on the new foliage that change from red to brown and are covered with a powdery white film. On rhododendrons, the leaves become chlorotic (bleached to a creamy-off white or pinkish color) and become thickened. A white fungal growth can be found on the underside of the foliage. A rosette may also be noticed at the ends of the branches. Remedy: Remove and destroy affected parts of the plant. Spray with a fungicide prior to bud break and repeat in about 2-3 weeks.*
Disease: Root Rot Root rot is caused by the fungus Phytophthora which is often found in poorly drained soils. Symptoms: Leaves turn yellow, wilt, roll inward and hand down, but remain attached to the plant. The root system rots and the plant eventually dies. Remedy: Remove and destroy the infected plant, do not replant in the same hole, especially with the same kind of plant material. To help avoid root rots, always provide good drainage, maintain proper nutrition and soil pH. Continued Deficiency symptoms and their signs:
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