Yellowing and spotted in not root rot. Yellowing of a leaf between dark green veins is called chlorosis and is usually caused by an iron deficiency. Many conditions can be responsible for an iron deficiency. Poor drainage, planting too deeply, heavy soil with poor aeration, insect or fungus damage in the root zone and lack of moisture all induce chlorosis. After these conditions are eliminated as possible causes, soil testing is in order. Chlorosis can be caused by malnutrition caused by alkalinity of the soil, potassium deficiency, calcium deficiency, iron deficiency or magnesium deficiency. Iron is most readily available in acidic soils between pH 4.5-6.0. When the soil pH is above 6.5, iron may be present in adequate amounts, but is in an unusable form, due to an excessive amount of calcium carbonate. This can occur when plants are placed too close to cement foundations or walkways. Soil amendments that acidify the soil, such as iron sulfate or sulfur, are the best long term solution. Foliar sprays of iron sulfate or chelated iron can reduce symptoms. A combination of acidification with sulfur and iron supplements such as chelated iron or iron sulfate will usually treat this problem. Chlorosis caused by magnesium deficiency is initially the same as iron, but progresses to form reddish purple blotches and marginal leaf necrosis (browning of leaf edges). Epsom salts are a good source of supplemental magnesium. Chlorosis can also be caused by nitrogen toxicity (usually caused by nitrate fertilizers) or other conditions that damage the roots such as root rot, severe cutting of the roots, root weevils or root death caused by extreme amounts of fertilizer. There is a tonic that remedies some cases of chlorosis. Yellow mottling on the upper surface of leaves and black sooty mold and transparent insects on the bottom are symptoms of Azalea Whitefly (Pealius azaleae.) and Rhododendron Whitefly (Dialeurodes chittendeni.). These may also cause the following symptoms: Small white spots on the underside of leaves and small white flies on under-surface of leaves is also an indication in infestation of Azalea Whitefly (Pealius azaleae.) and Rhododendron Whitefly (Dialeurodes chittendeni.). They are more prevalent on certain varieties and on plants grown in protected areas. These small white flying insects look like an aphid with wings and suck on the underside of foliage, leaving white spots where it has been. Heavy infestations cause the margins of terminal leaves to cup. These infested leaves will eventually turn yellowish and appear wilted. The lower leaves become covered with honeydew, followed by sooty mold (a black coating). To check for the presence of whiteflies, shake the terminals of white azaleas to flush out adult whiteflies which look like tiny white moths. Examine the lower surfaces of leaves for the presence of nymphs, which are flat, yellowish green, and resemble scale insects. All stages occur on the under sides of leaves. This whitefly is usually limited to varieties of the snow azalea, Rhododendron mucronatum. If the infestation is light, little or no plant symptoms are evident, and if beneficial insects are present, spray the undersides of leaves with insecticidal soap or a horticultural oil at the 2%summer rate. If the infestation is heavy use a registered residual insecticide such as Malathion, Diazinon or Orthene. Yellowing of leaves surfaces, often with brownish burned areas, occurring on leaves that are more exposed to sun, is caused by more sun exposure than the plant is able to tolerate. Some varieties need shade, while all plants that have been protected from direct sun will be tender until hardened off by gradual exposure to sun light. Possible solutions are to give the plant more shade or move it to a more protected site. Yellowing and dropping of leaves is normal toward the end of the second summer on the small-leaved lepidote rhododendrons. These should have dense enough habit that this doesn't matter. The larger-leaved elepidote rhododendrons keep their leaves for 3 or 4 seasons. Like all evergreen plants, rhododendrons and azaleas periodically lose some of their foliage, and the leaves may turn yellow, red, or purple before they fall. Often the only leaves that remain are those that surround the flower buds at the tips of the branches. This characteristic is linked to the genetics of the parents used to breed garden azaleas. The degree of leaf coloration or loss is a function of parentage and not the severity of the winter. In unusually cold winters, certain azaleas may lose more leaves than they would in mild winters. Uniformly yellowish-green leaves is often just the need for more nitrogen. This will be more noticeable in the full sun. Some less sun tolerant varieties will always be light green in full sun. Yellowing of leaf edges has been noted in gardens where sandy soil conditions or root competition with other plants caused insufficient soil moisture and nutrients. Usually incorporating organic material in the soil and removing the plants with the competing roots solved the problem. Care must be taken not to disturb the roots of the rhododendrons and azaleas. Hence it is best to prepare the soil adequately before planting. The tops of most competing plants can be removed leaving the offending roots in the ground and the offending roots will simply decay and pose no problem.

There are two weevils that attack rhododendrons and azaleas: Black Vine Weevil or Strawberry Root Weevil They create two entirely different situations: Adult weevils feed on the leaves at night. Leaves chewed on edges is a symptom of the Black Vine Weevil and Strawberry Root Weevil, 1/4"long black beetles. They are nocturnal feeders and can be found at night with a flashlight for identification. Foliar sprays are very effective at controlling adult weevils when leaf notching starts. Foliar sprays of Orthene should be sprayed at about three week intervals from about May to October, depending on the weather, until no adults emerge. Since weevils feed at night, you can hand pick adult weevils at night using a flashlight. Since weevils spend the daytime in the soil and come out at night to feed, you can paint the trunk with Tanglefoot to stop them, but make sure no branches are touching the ground. Foliar sprays are very effective at controlling adult weevils when leaf notching starts. Foliar sprays must be repeated until no adults emerge. The major damage is caused by weevil larvae which girdle the roots and kill the plant. Prior to killing the plant the plant shows signs of chlorosis (yellowing of a leaf between dark green veins.) Larvicidal drenches may be used to kill them but are of limited effectiveness. A more effective approach is to use nematodes. They are very effective against weevils when applied in the fall to control the larvae. While this approach is promising, it has limitations in that the beneficial nematodes are very sensitive to temperature and moisture extremes and will not live over winter. If applied to soil that is too cold, too wet, too dry or too hot, they will die and provide no control. Best control is achieved by using both chemical and nematode methods with proper timing.

Whitish specks on the upper surface of leaves and dark spots varnish-like on the bottom are symptoms of Rhododendron Lace Bugs, Stephanitis rhododendri, and Azalea Lace Bugs, Stephanitis pyrioides, small insects with transparent wings on under-surface of leaves.. This insect hatches early in spring as the new foliage begins to mature and its numbers may build to damaging levels with successive generations. Lace bugs reach their peak in late summer and do their worst in sunny, exposed sites. Spiders are important predators of lace bugs and since they shy away from sunny, hot places, plant your azaleas where there is some shade. Insecticidal soap, horticultural oil, or a systemic insecticide may spare your azaleas the damage if applied in spring when the first hatchlings are noticed. Care must be taken to spray the lower surfaces of the leaves where the lace bugs live. Moving a plant to an area with more shade may alleviate the problem. Lace bugs are more prevalent on certain varieties. The following azalea cultivars have resistance to azalea lace bug: Dawn, Pink Star, Ereka, Cavalier, Pink Fancy, Dram, Seigei, Macrantha, Salmon Pink, Elsie Lee, Red Wing, Sunglow and Marilee. Rhododendron Lace Bugs, Stephanitis rhododendri, and Azalea Lace Bugs, Stephanitis pyrioides adults are about 1/8-inch long. The body is pale yellow. The lacy wings (very distinctive) are held flat over the back and are transparent with two dark spots present. The nymphs are black, spiny and smaller than the adults. The eggs overwinter partially embedded in leaf tissue. The eggs hatch in May. The nymphs mature into adults in June and lay eggs during late June and July. The second generation of nymphs appears in August. The overwintering eggs will be laid when these nymphs become adults. Lacebug adults and nymphs feed on the undersides of leaves by piercing the leaves with their mouthparts and sucking the plant juices. This causes a mottled, silvery or white discoloration, known as stippling, on top of the leaf where the chlorophyll has been removed. The undersides of leaves are covered with dark brown to black, sticky spots of excrement. Plant rhododendrons in shade to maximize the activities of beneficial insects. Varieties that are attacked by lacebug will recover if moved to shadier areas. They only get lacebugs in full sun. The National Arboretum warns: "Excess nutrients (fertilizer) may promote larger than normal populations of azalea pests like lace bugs Thrips, Thrips imaginis: This insect is a seasonal pest, causing most damage in mid to late summer. Signs of damage include silvering of the foliage and distortion of growing points and flower buds. The underside of leaves may be covered with excreta of the insect, which are like brown tar droplets. There are no effective specific biological controls and thus control can be difficult to achieve. Thrips are preyed upon by ladybirds and lacewings but these predators cannot control thrips in plague situations.

In the proper situation, a healthy azalea or rhododendron is relatively carefree. About the only insects that attacks rhododendrons and azaleas are weevils and lace bugs. Lace bugs attacks rhododendrons that are planted in full sun and are best held at bay by moving affected plants into the shade. The weevils come up at night and eat the edges of leaves. They seldom hurt the plant while eating leaves but the larvae can girdle the roots and kill them. It is best combated with a nematode that keeps it in check. WEEVILS: Leaves chewed on edges is a symptom of chewing insects. Most are nocturnal feeders and can be found at night with a flashlight. The most prominent chewing insects on rhododendrons and azaleas are the Black Vine Weevil and Strawberry Root Weevil, 1/4"long black beetles. Feeding is done at night. Specimens may be collected at night for identification. The major damage is caused by weevil larvae which girdle the roots and kill the plant. Larvicidal drenches may be used to kill them but are of limited effectiveness. A more effective approach is to use nematodes. They are very effective against weevils when applied in the fall to control the larvae. While this approach is promising, it has limitations in that the beneficial nematodes are very sensitive to temperature and moisture extremes and will not live over winter. If applied to soil that is too cold, too wet, too dry or too hot, they will die and provide no control. Best control is achieved by using both chemical and nematode methods with proper timing. Foliar sprays are very effective at controlling adult weevils when leaf notching starts. Foliar sprays of Orthene should be sprayed at about three week intervals from about May to October, depending on the weather, until no adults emerge. Since weevils feed at night, you can hand pick adult weevils at night using a flashlight. Since weevils spend the daytime in the soil and come out at night to feed, you can paint the trunk with Tanglefoot to stop them, but make sure no branches are touching the ground. LACE BUGS: Whitish specks on the upper surface of leaves and dark spots varnish-like on the bottom are symptoms of rhododendron lace bugs, small insects with transparent wings on under-surface of leaves.. They are more prevalent on certain varieties and on plants grown in sunny areas. When damage first appears, it may be controlled by any of a number of contact insecticides such as Malathion or Orthene. Care must be taken to spray the lower surfaces of the leaves where the lace bugs live. Moving a plant to an area with more shade may alleviate the problem. Here is a link that might be useful: How to care for rhododendrons and azaleas.