Is this a new plant? How old and tall is it? Do you know the name of this variety?

Is it potted or planted on the ground?

If you insert a finger into the soil, does the soil feel moist? Or does it feel dry or wet?

A newly planted shrub may not need any fertilizers if the potting soil that it came in has fertilizer pellets. Rhododendrons and azaleas are not heavy feeders and will feed off the decomposing mulch. Unless your soil has some type of mineral defficiencies, the shrubs will be fine with just mulch. If you need soil amendments, try a slow release fertilizer with a NPK Ratio close to 4-3-4 like Holly-tone or use an organic fertilizer like cottonseed meal.

Here is a link that might be useful: Rhodo Info from the ARS Toronto Chapter

luis pr has given good advice to which I would add the following: yellowing of the oldest leaves before they fall off is perfectly natural and nothing to worry about. Rhododendrons really detest heavy clay soil. If that's what you have, it's much better to plant on top of it than in it. The dry dusty peat available at garden centers in bales is really pretty useless and may do actual harm in clay soils. It's far better to use aged soft wood bark both as an amendment and a planting medium. Advice about growing rhododendrons always emphasizes good drainage. This is important, but lots of a air space in the planting medium is at least equally important. Clay plus peat produces a mix without sufficient air space.

You have received some good advice.

The plant will tell you what is wrong to some degree. Here of some of the possibilities:

1) Powdery Mildew: Light green or yellowish patches on leaves sometimes accompanies by brown spots on the back side of leaves is a sign of Powdery Mildew (Microsphaera azaleae). One of the puzzling aspects of this fungal problem is the fact that two different affected rhododendrons vary in appearance. Rhododendron cultivar 'Unique,' for instance, shows almost no upper leaf changes, other than occasional very faint lighter yellowish areas, while the underside of the leaves will be completely covered in brown spots. A deep green leaf may begin to show lighter green patches, and these areas will gradually become more yellow. Another cultivar, 'Virginia Richards,' gets brownish purple spots on both tops and bottoms of leaves. This common disease is named Powdery Mildew despite how little the symptoms resemble the familiar fungal disease often seen on roses and azaleas. Usually the disease doesn't produce the familiar white powder-like spores, although late in the summer some may become visible. The disease manifests instead as color changes in the leaves, followed by defoliation toward the end of the growing season. Many rhododendrons, if basically healthy, will coexist with the disease and seem to outgrow or at least survive the symptoms. Last year's leaves, once they have been hit by the disease, will always have it, with symptoms persisting from year to year until the leaves drop off. High relative humidity at night and low relative humidity during day with 70-80 F (22-27 C) temperatures is ideal for the disease to flourish.

2) Chlorosis: 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, magnesium deficiency or too much phosphorus in the soil. 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. For a quick but only temporary improvement in the appearance of the foliage, ferrous sulfate can be dissolved in water (1 ounce in 2 gallons of water) and sprinkled on the foliage. Some garden centers sell chelated iron, which provides the same results. Follow the label recommendations for mixing and applying chelated iron. 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.

3) Nitrogen Deficiency: 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.

4) Normal yellowing: 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.

5) Sunburn: 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. After pruning, rhododendron leaves that were shaded burn very easily.

6) Low soil moisture: 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.

7) High soil moisture: Small yellow leaves and stunted growth are signs of water stress brought on by water-logged soil or wet/dry fluctuations in soil moisture.

8) Insect problems: 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. Dick Murcott had a simple remedy to control the numbers of white flies. He would hang pieces of stiff plastic or 12" square metal sheets painted with a bright yellow/orange paint and then covered with petroleum jelly or any clear, sticky material. The white flies will fly to the colored material and get stuck in the sticky stuff!

Here is a link that might be useful: Rhodododendron problems and solutions

Goodness. I posted a response yesterday but that seemed to have disappeared.

Thank you all for the very informative response. Let me first say that I'm a seasoned gardener - I just don't have that much experience with Rhodos. rhodyman's detailed list of possible problems do give me some clues on what may be going on.

This is a small leafed rhodo now in it's third year in my garden. Except for the yellowing leaves, including the emergent leaves, it's looking fine. It's got a lot of buds about to bloom. Not looking stunted. It's planted in clay but which has been amended with compost and composted manure over the years. And this is near a westwall and so it's in mostly light shade and part sun condition. The addition of peat was something I did just recently to try to acidify the soil just in case soil ph is the source of the problem. The clay mound which it is in is moist but not overly moist and this is is because water does pool nearby when it rains but this drains out almost as quickly since I have installed a hidden drain. It's got no disease of any sort nor any pests. The possibility that this is chlorosis had crossed my mind but have ruled out. There's no veining.

I'm inclined now to think that it's either 2 issues that's going on. The plant is about to shed it's leaves enmass (that this is normal yellowing) although all I've seen were few leaves being shed. Perhaps a greater number of leaves will fall later in the season. or that the soil's nitrogen level needs to be raised. Now I do have to ask because I'm getting conflicting information about this.. my understanding is that adding wood to soil only deplete's the soil of nitrogen - so maingrower's suggestion to use softwood chips as an alternative to peatmoss somewhat puzzles me. Please correct me if I am wrong about this. I don't usually mulch my beds and so I'm not too clear as to what wood chips will provide as nutrients. I know they are good for either water retention, winter protection, etc... but not much knowledge on nutrients.

You are 100% correct about wood chips. The Rhododendorn Species Foundation in Federal Way, Washington, used saw dust to amend the soil when they first planted. It was a disaster after a few years. The wood rotted which used nitrogen and also created a slimy mess which had poor drainage. They had to replant everything. Oddly enough they went to bark dust as well as peat moss. Bark dust is much more stable.

I personally like composted peat. It is hard to find now, but used to be available everywhere.

Thanks. I think I understand now. There's a difference between interior wood and bark. Interior wood not being recommended as a soil amendment, often because it contains resinous material. Bark being a safe additive to the soil. Thanks for helping me clarify this.

We have peat moss here but I'll see if I can located composted peat.

Ace hardware sells compost peat humus.

Catalog and Web only: ACE# 72436: MFR# AOP40
Price: $2.79 for 40# bag.

Thanks. I'll look for that.

Oh - I think I can give a diagnosis of this problem. It's almost certainly lack of nitrogen. Can you recommend how much nitrogen to offer?

ianna

I would get Hollytone and apply it once at the recommended rate this year and in any future year where the leaves still have this problem. Then after that at half the recommended rate if at all.

They are not heavy feeders. A littler fertilizer goes a long way.