Anybody know what this is?

I have this grayish looking growth on the foliage of one of my Hoya plants and don't have a clue what it is or how to treat it. I'm hoping someone might recognize it and have some advice. It's only affecting the reverse side of the leaves and only on three leaves. I've quarantined the plant because I sure don't want it to spread to my other plants. I hate the thought of having to destroy the plant so I'm hoping someone will be able to tell me what this is, some kind of scale, canker, fungal infection ...

Thanks in advance for any advice and help.

Lin

that almost looks like cold damage to me. some of my succulents got that this year even though they were in the garage during the worst weather. if you scrape it, what happens? i supposed it could be a scale of some kind but i don't think so. i'd be inclined to take a q-tip with alcohol and wipe the spots just to see if there it will come off and if there is green underneath.

Hey Trackin: I posted this over on the Hoya forum too: http://davesgarden.com/community/forums/t/996612/#top also and a few other folks have the same thing on a few hoya's. AlohaHoya had us e-mail photo's to her and she took them to a plant pathologist at her local ag. center who thinks it looks like slime mold, which wouldn't be contagious. I have isolated my one plant that has this stuff, just in case. My Hoya's were all inside the house during the winter months so I don't think it's from cold damage. This particular plant has been out on the deck for the past few months and was hiding in amongst other plants in full shade and with all the rain we've had, (it rained for 7 striaght days here, 21" in that one week, and then showers and storms a few more days here and there) so I'm hoping the problem was caused by wet conditions. None of the other plants have whatever this hard scale type stuff is, but this one plant was not getting much air circulation or light.

hey lin. since you have a diagnosis, if it were me, i'd take a little baking soda and water and wipe the leaves. i get slime mold in the garden beds once in a while and a squirt of baking soda water clears it right up.

glad for you that it's not worse!

Thanks Track! I've been wiping the leaves down with alcohol which evaporates really quickly. I am going to try your baking soda/water idea, it might help, and sure won't hurt!

I'm hoping whatever it is on this plant will not spread. I guess I think of slime mold as something slimy that you could wash or wipe off ... this looks like the back of the leaves are turning a grayish white color. I don't know much about plant diseases or maladies and I keep thinking this looks like a scab is growing on the backs of the leaves and if it takes over and covers them, they won't be able to breathe. I hate to lose the plant but would really hate if it spread to others and killed them. I'm still keeping the plant isolated, just in case ... a few other folks on the Hoya forum said they have more than one plant with it.

can you cut the bad leaves off?

Yes, that's what I plan to do.

Just saw this, and noted how much it looks like the algal leaf spot that you see on Magnolias. Your description sounds like it too - hard, scabby. You can't flick algal leaf spot off with your fingernails either. You're right, slime mold is just that - slimy. And you can wipe it off. I Googled hoya and algal leaf spot and didn't get any hits, so I don't know if it can affect hoyas. But the conditions you describe are also the conditions that algal leaf spot thrives under.

For Magnolias, the treatment is a copper fungicide, sprayed to cover the leaves, top and bottom. I don't know how hoyas would react to that, so if you want to try it, I would just treat one leaf and see what happens to it. Wait a week or so, and if there is no detrimental reaction, go ahead and spray the whole plant.