Denise to me it sounds like a fungal disease called leaf scab. Do the infected spots resemble the brown scabs sometimes seen on apples? I am unsure if this is a disease of Hoyas or not but it does sound fungal in nature. You might want to try some garden fungicide and if the problem persists maybe a systemic fungicide.

Mike

Hey Denise,

Karen here, hi. While I don't know anything about this problem, if an antifungal is what you need, what about applying some cinnamon?

I sometimes dab it on roots of things, Aloe roots, Sans. roots, re-rooting Haworthias. Perhaps applying it to the leaves might be a place to experiment?

Denise I did a little searching and it appears that the leaf spotting in Hoyas is often caused by Botrytis and Rhizoctonia fungi. These fungi are very common Botrytis is most often seen in grapes, if a grape is rotten you can bet that it is Botrytis. Rhizoctonia (Rhiza = root) is a common fungi that causes damping off in seedlings and root rot in mature plants.

If you scroll down you can find the entry for Hoya carnosa. The problem still sounds more like a rust or scab but there is info on treating the problems.

Mike

Here is a link that might be useful: Fungal infections Shield Soap directory

Thanks for all the info, Mike. I don't know much about mildews and fungii except that they tend to appear when the humidity is too high or not enough air flow, which certainly isn't a problem where I keep this plant. The "rust" sounds about right, though I assume it's still a form of fungus? Are you suggesting a powder fungicide on the leaves? I'd like to nip it in the bud before it gets worse!

Denise in Omaha

P.S. Karen, I love the smell of cinnamon - I may be tempted to "nibble" my plant using it!

Denise the way most fungal infections work is that a spore germinates on the surface of the plant or in a wound and then sends out hyphae which are little root like structures that can form hyphal mats or colonies that resemble a spiders web when seen on the surface of soil etc. When the infection is not superficial and instead is internal applying topical treatments like cinnamon or sulfur will often not cure the infection. When you see the rust spot or a mushroom what you are seeing is the reproductive body of the fungi, this body will eventually release spores. Topical treatments can help prevent the reproductive phase of the fungi but to actually cure the infection you would need to use a systemic fungicide. Some systemic insecticides also contain a fungicide but ones that do not can often be mixed with an insecticide or used alone. Many fungicides are quite toxic so you need to be careful just like when using insecticides. You could always try dusting the affected areas with garden sulfur and if the problem persists then move on to a systemic.

Mike