Speckled Rhodie Leaves

[Anonny Moose]

A few of my rhododendrons' leaves are showing a loss of color this winter. They look almost white with green speckling. Anyone know what this is and what I can do for it?

Thanks.

Karen portland, or

[David Hill]

Sounds as if they would benefit from a height nitrogen feed

[Anonny Moose]

Does this mean applying a nitrogen solution to the leaves?

karen

[paghat]

When SPOTS appear on leaves in winter, it can be any number of problems causing ill health to the root system -- such as borers or fungus -- & winter spottiness is just the first sign of worse to come. But if the whole leaf is just fading to yellow between the leaf-veins, the shrub is chlorotic, which is repairable. This happens to all plants that prefer acidic soil that end up in poor or alkaline soil. Some people induce the problem if they add fireplace ashes to gardens, believing this is beneficial when it can be very harmful to rhodies, roses, blueberries, apple & cherry trees, by alkalinizing soil. If you've gone the "ashes are good" route then you've injured the soil for rhodies. If that is the case, rain & waterings will eventually wash out the alkalinity, unless your local soils are naturally alkaline, then it's going to be a recurring harrassment to ammend soils to acidic conditions unnatural for your area.

Over fertilizing or the wrong fertilizer can also cause chlorotic conditions by interferring with rhodies' capacity to take in iron. Heavy feeding of phosphorus intefers with iron uptake even if the iron quantity in the soil is sufficient. Run-off from newly laid concrete or cement, OR unstable deteriorating old cement, can alkalinize the immediate area (stable well-cured cement is NOT a problem though). If you lime a nearby lawn, this can leech into areas of woody shrubs, damaging the soil for many perennials & shrubs & fruit trees -- what's good for lawns is not always good for much else, so take care not to lime lawns too near gardened areas.

The usual "fix" is sulfer or iron sulfate. This assumes the soil does have iron in it but that alkalinity or other factor hinders uptake of iron. A mere tablespoon of sulfer in the soil around the base of each shrub will help it take in the iron it needs. If there's something continuously keeping the area to the alkaline side of the pH scale, it will need a tiny bit of sulfer EVERY spring. But this is a compensating method, not a real restoration if the soil has been alkalinized. A great amount of peat worked into the soil & heavy topcoatings of leafmold & composted manure will help acidify the soil itself & increase plant health. Be VERY careful with fertilizers that shoudl be low in nitrogen & low in phosphorus; until alkalinity can be reduced, no amount of even a properly balanced rhody/azalea fertilizer will help, because the problem is that the shrub cannot take in iron when soil is limey.

Very rarely excessively acidic soil is the culprit, as even acid-loving plants such as rhodies want only moderate acidity. In such a case a little lime actually helps rather than increases the problem. So you need to know your specific pH conditions before knowing which action to take, as lime will increase the problem if the soil is alkaline, & help the problem if the soil is super-acidic.

Though naturally alkaline soil or soil damaged by excesses of phosphorus or lime or ashes are commonest culptrits, sometimes the soil is fine & some other thing causes chlorosis, but the other causes would usually mean the shrub did not do well for a few years before began to show stress, but always struggled in the given location. Compacted, poorly draining, clayey, over wet soil destroys a rhody's capacity to take in iron, as does being planted too deep (their roots must remain "in the shallow." These conditions usually cause root stress & disease or attract insect damage the first sign of which will be either winter spottiness OR chlorosis. A greater amount of peat or other organic material in the ENTIRE garden, well-loosened for better aeration.

If soil is literally iron deficient or magnesium depleted, this can be narrowly resotred. A feeding of liquid chelated iron according to product directions will be the proper fix or iron deficiency. For magnesium depletion, a high-magnesium low-nitrogen evergreen fertilizer, or just epsom salts (magnesium sulfate) will fix that. Some recommend a combination chelated iron with magnesium sulfate together as kind of broad-spectrum approach. But tinkering with soil chemistry repeatedly will in the long run do more harm than good & the original cause of the problem should not be permitted to continue, so that organically rich slightly acidic soils remain in prime condition by means of healthful annual topcoatings of leafmold or manure compost. Without knowing the soil conditions beforehand, & without knowing why the soil went wonky if it was previously just fine, any action you take could turn out to be wrong. You'll know fast if the action you take was helpful, as it takes only about two weeks to begin turning back to green.

-paghat the ratgirl

[paghat]

Nitrogen is not likely to help since rhodies are low-nitrogen users to begin with. Spottiness (in winter) is evidence of root stress or illness; paleness of leaves between veins (at any time of year) is evidence of the plant's inability to take in iron.

-paghat the ratgirl

[David Hill]

Whilst I cant fault phagat's instruction for Rhodo growing.....I find that some of mine show yellowing of the leaves every couple of years and I am on a soil with a ph of around 5. I use a handful or two (Depending on the size if the bush) of Nitro chalk...which is a very high nitrogen fertilizer......... If you do try this route then on no account let it get onto the leaves, it will burn them fast. Apply to the ground around the bush. You could water it on to the soil. but I would do nothing now till the spring. I find I have to do the same with my camellias and Azaleas....

If you can test the soil to find out the ph it will help decide on how to treat them.

[David J Bockman]

Check the undersides of the leaves for evidence of Lacebug poop... will look like black pepper.

Dave

[Pam - gardengal]

I agree with David that what you describe sounds like the results of a lacebug infestation, but winter is an unusual time of year for this to show itself. Lacebugs are typically present during the growing season, but winter stress from excessive cold (like you experienced several weeks ago) may have amplified what was overlooked earlier in the season. Eggs overwinter on the underside of the leaf close to the midribs. Hosing off or applying an insectidicidal soap is your best control method. Will need to repply when they hatch in May and again in late June or early July. Rhododendrons grown in much sun seem to be more prone to damage from lacebugs than those grown in a cooler, shadier position.

pam - gardengal

[Anonny Moose]

Kind gardeners,

I just pulled a couple of leaves for a better look and have to admit my initial description was a bit off. The topside of the leaves has yellow-white very fine speckling or mottling, and there is also dark mottling on top and bottom that looks like physiological leaf spot. There is no notching (root weevil) but close inspection reveals a couple of tiny light-colored insects on the underside (maybe one or two on each leaf), too small to make out details using a 10X loupe. The pictures of rhodie lace bug damage look quite a bit like what I'm trying to describe, along with the physiological leaf spot. It doesn't look to me like powdery mildew or gray blight, whitefly, rust or virus; definitely not chlorosis.

There has been no lime or wood ash added to the soil. I mulch with leaves and compost and added one or two applications of a rhodie food over the season. A soil test taken early last year returned a pH of 6.2. The sample was very low in sulfur, very high in iron and phosphorus, high in organic matter, potassium and zinc. The Comments mention low levels of sulfur may cause yellowing and lack of vigor but that sulfates may have leached below the sampling depth. Boron level was 0.4 and Comments suggest aiming for levels above 0.5 ppm to avoid deficiency.

What would be the best way to add sulfur and boron? One of the posts suggested treatment for lace bug, so I'll copy that and give it a try. Hope I haven't lost them yet! Any other thoughts would be appreciated.

Thanks all.

Karen Portland, OR

[Anonny Moose]

There is dark speckling on the underside of the leaves but it looks more like physiological leaf spot - or maybe both types of spotting. I'm just too inexperienced to tell the difference. I did find a couple of tiny light-colored insects on the underside of each leaf I brought in for inspection that might well be lacebugs (if they are tiny enough to be hardly able to make out with a 10X loupe).

karen

[David J Bockman]

As Lacebug damage increases, subsequent opportunistic diseases can also take hold... perhaps you have a multi-front attack on your rhodies!

At this time of year (at least in all but the warmer parts of the US) Lacebugs won't be present, but their eggs will be. As the weather warms, consider using a horticultural oil like Sunspray Ultrafine in order to smother the eggs.

Lacebugs set up shop usually in weakened rhodies that are either getting too much sun, are too dryish, not getting enough nitrogen/chelated iron, or some combination therein.

Dave

[Stephen M. Henning]

Could be either thrips or lace bugs.

Thrips are characterized by a silvery white discoloration on the leaf's upper surface and silvery on the bottom with small black frass specks. The damage is similar to that from lace bugs. Thrips are a problem in warm and dry climates like California and New Zealand. Most of the standard insecticides and insecticidal soaps can be used. Control for thrips whether an insecticide or insecticidal soap is a contact control and must be applied on the adults. This is most likely done in May when they are visible. Locally, biological controls may also be available. It is best to remove infected flowers.

Whitish specks on the upper surface of leaves and dark spots varnish-like on the bottom are symptoms of rhododendron lace bugs . 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. Care must be taken to spray the lower surfaces of the leaves where the lace bugs live.

[Anonny Moose]

Thanks Dave, and everyone. I will look for that oil and consider replanting in shadier locations.

Karen