This picture is the top side of the leaf.
This pic is the underside of the leaf.
It looks to me to be a blight. This would be fungal. Is it all of the leaves or just a few? Make sure to take off all of the leaves effected and put in garbage.
Blights can be treated, just see if someone else checks in
It starts out just a few and then eventually most of the leaves get it. I have daconil and copper-manzate mix. I think both should work against blight. I will try those. Thanks!
When I saw the top side of the leaf I didn't think it was anything to be concerned about but the underside is weird! The only type of blight I'm familiar with is tomato blight and have no idea what all the other blights look like. I agree with rcharles, make sure to dispose of any affected foliage in a sealed bag in the trash and most definitely don't compost it. You might want to post your pics on the 'Pests & Disease Forum'. Good luck.
Thanks, just posted there.
I sprayed with copper-manzate mix, which also is bacteriostatic if I am remembering correctly. Then I sprinkled some granular propriconazole on the soil. I will try the daconil and I think I have some other granular fungicide in the garage so I will try that, too.
Don't throw everything at it at once. Try one thing and see how it responds.
I would do as karyn stated and only do one thing at a time so that if there is a problem you will know what the cause was/is.
There was an article I read about many people using 'cinnamon' for something, now I am going to have to check it out again.
I thought looking at the top also that it may be something more environmental until I saw the underside.
The Rhododendrons can get a fungal/blight of sorts and it does not always mean the lose of a plant.
I am always trying to find environmental ways to control problems to the best of my ability.
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I picked the copper-manzate mix because it has both fungistatic and bacteriostatic properties. So if it works, I won't know whether it is fungus or bacteria anyhow.
And the granular propriconazole has practically cured the rust problem with my canna lilies so I thought it worth a try. It is supposedly taken up systemically by the plant so the fungus can't take hold to begin with. It may help to deplete the fungus in the soil, too, not sure.
Living here in south florida, it is a never ending battle against the bugs and fungus, and this is the worst time of year for both here. It is nothing like farther up north in Florida. This is drained swamp land and it wasn't drained that well to begin with. And there are mangroves that are not too far to the west of me, which are breeding grounds for all kinds of bugs. The mosquitos here are especially horrendous.
I might have to try the granular propriconazole on one of my plumerias that has a terrible case of rust. The copper has done a less then satisfactory job of getting rid of it. As for cinnamon it's supposed to have antifungal, antibacterial properties. I've tried it a few times and am less then impressed. I find that sulfur works better as a fungicide. We also have a terrible time in this area with powdery mildew and black spot which is why I refuse to grow roses. Even the supposedly resistent varieties aren't. During the summer the humidity here is as bad as FL but at least we don't have those freaky giant cockroaches, Palmetto bugs. I hate those things and always find a few dead ones when I go to our FL house in the winter. Gross!!!
LOL, karyn about the palmetto bugs. They aren't cockroaches, though. They come for water, not food.
For canna rust, I use chlorothalonil (Ortho Garden Disease Control) and the granular propriconazole as well as a copper-manzate mix. Adding the granules has made the biggest difference.
I don't think this is a pathogen at all - it looks cultural/physiological, like a case of too much water/not enough air in the rhizosphere (root zone). The small corky patches on the underside of the leaf are OEDEMA (click me), and the larger necrotic areas on leaf tips/margins are related to the plant's inability to move water to distal parts due to impaired root function or root rot.
Wow, Giant cockroaches, Palmetto bugs, fungus, mozzies, bugs and excessive humidity...thank goodness for the dear 'ole UK!!! That's not to say we don't have our own set of specific probs...such as dire weather with too much rain, not enough sun....each to their own I guess!! lol! Karyn, black-spot is also prevalent in my area and roses struggle, especially white ones, my one is always leafless. Told it's more common in areas with clean air??
Gill from the wet and windy UK!