Azalea Pests and Diseases

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I have been wanting to get back to talking about pests and diseases for quite awhile – since the first couple of articles last year. Mostly because I was only able to hit on a few highlights the first go around. We introduced some of the broad issues and a short list of the chemical solutions to fight them. But going forward, I would like to addresses pests and diseases more specifically and hopefully give you the tools to both identify and treat them on your trees. As my own challenges with these problems occur, I have been trying to keep a photographic record so that I could share them as the need arises. While documenting such an outbreak of leaf gall on my azaleas, it occurred to me that I had a nice little package on azaleas, with some good treatment strategies, so here goes. Be forewarned, I only have one azalea bonsai in my collection – a Kozan variety Satsuki which is blooming right now. But I just counted and I have 38 azaleas, plus about a dozen rhododendrons in the landscape around our house. Most are very old and mature, and nearly all are summer blooming, putting them in a class with the Satsuki breed that we usually see as bonsai. It has provided a great learning ground, and may also help you with your landscape plants as well as your bonsai. 

Azalea Leaf Gall
This is a very unsightly disease and the cause of my latest round of spraying. I first saw this disease on a bonsai many years ago. It causes the leaves to thicken like a jade leaf and then distort into grotesque forms. It first starts out a light lime green. They usually occur randomly across a plant. The easiest solution is to just prune them out and that may be all that is actually needed to control it. The galls won’t kill the plant, but they will weaken the branch. But they just plain look bad. If you don’t get them pruned, they will turn white and fuzzy as they start to fruit and give off spores. You can see both stages in the photos, but don’t let them get that far. I discovered a relative epidemic once I started trimming the bushes around the house and found them thriving just under the new green leaves. They start to appear at, or just after flowering. Once they are removed, you can spray with the fungicides propiconazole (Bannermax, Fertilome Liquid Systemic) or chlorothalonil (Daconil) to prevent future outbreaks. I am just trying this spray treatment now, so I cannot verify the effectiveness yet.  

Azalea Leaf Gall

Grrrhhhh. I hate these guys. My wife was at Portland Nursery last week and they had a sign up that basically said, give up on azaleas and don’t plant them, all because of these guys. They are really becoming a nuisance the last few years in Portland. The nursery also said there are some resistant varieties out there. But as I said, I have about fifty mature plants to protect and they are my favorite landscape plants, pretty much ever, short of a Japanese maple. So they are here to stay and I am digging in. I am including photos of these buggers in three different stages. You can find better photos on the internet if you like. I am sharing these just to let you know the threat is real and can be dealt with, and that you may have to recognize them in different forms. The first signs are the stippled leaves. This may remind you of spider mites, but if you turn the leaf over, you may see adult or juvenile lace bugs running around. And they do have legs! I put this one guy on my barbecue and it was hard to keep up with him. Note also the black dots of fecal excrement. The lack of webs also tells you that it is not spider mites. I have also included a photo of spent lacebug carcasses to further indentify their presence. 

The short solution is to use a product with Imidacloprid in it and they go away. This is a systemic treatment and affects most bugs that want to suck on leaves. The problem being that if not used properly, it may affect good insects like bees. I really don’t want to enter into that discussion and don’t have strong enough information to properly guide you. I am just making you aware of what you are dealing with, both from a diagnostic and treatment point of view. There are also other direct contact sprays that you can use. The challenge will be getting the spray to them. I suggest a tank sprayer that you can get a high pressure on and stick the nozzle up under the branches so that you get all the undersides of the leaves. These guys should be fairly easy to kill, similar to aphids.   

Juvenile Lacebugs


Lacebug carcasses


Lacebug damage

Azalea Bark Scales
You may have never heard of these critters, and I hope that you never do. But, I have had todeal with them at both my old and new house, on azaleas and also Andromeda. These guys look like wooly aphids, or a wooly scale. They like to hang out in the crotches of branches or in the grooves of bark where they get more protection. Most sucking insects feed on the sap in the leaves, which is traveling up from the roots, through the xylem and out to the leaves. These bark scales suck on the sap that is flowing down through the phloem. That means that they are resistant to most systemic treatments like imidacloprid, which only works on the sap flowing up. Very few systemics are able to work both directions, but there are some that do, and of course are expensive and elusive. What is so insidious about these guys is that they hide out under the canopy, out of sight, where they go unobserved until permanent damage occurs. They tend to colonize on single branches of a plant, sucking it dry until you see it start to slowly fade and then eventually die. Pruning back the leaves reveals many tiny cottony tufts along the stems of the plant. But take heart, there is an effective solution. It’s work, but it can be done. Immediately prune out the infected branch. Pretty much it’s toast down to the roots. That means that you will have to grow a new branch to fill in, but in the landscape this is not a problem, just a patience issue. On bonsai, if you are paying attention, it should never get this far. But once you have pruned out the dead branches, it opens the tree up to spraying. Since the purpose of the cottony white shell is to protect the bug once it is stationary, the only time you can effectively treat them is in the spring – April-May, while they are out crawling around to a new home and have not yet developed their shell. I leave it to you to select your favorite poison. Some have suggested using a spray oil to suffocate the bugs in other seasons, but I have not tried this. I have had full recovery of plants using this method though, and am in the process of treating a few plants at our new home. 

That’s it for azaleas. Scott Elser