When the yellowing is that fast and concerns many leaves , most of the times its rot. Too cool conditions, not enough light and too much water (ie water that is not lost in evaporation nor transpiration and just sits there) is the cause. I would suggest taking cuttings, and very fast. Fungi (ie rot) tend to move very fast.
If you have a systemic, use it.
If you want to try Hydrogen peroxide on roots, here's what I used on a goner palm tree (that I saved from the garbage, it was sitting in water with lots of dead leaves/fans) and on my orchids : my H2O2 is 10% and I diluted it 10 X so it was 1% when I used it. It makes a psshttt noise when you pour it in the soil ,but I wouldnt worry

Good luck and let us know what you decide to do and what happens.

my tanna, diversifolia, and a few others are doing this too, and there is no sign of rot. i think it might have something to do with the cold drafts i am getting through my 100+ yr old windows (they are origional to the building) that paired with the awful gloomy weather we are having cannot be good for them. they will be moved after christmas so hopefully they will hang on till then. but i am only getting 1 or 2 leaves dropping every week, and mine are from new growth. hmmmm, maybe it is new york lol :) jennifer

Not that this will be helpful, or even scientific... I find that leaves that were grown in full sun, or high light will actually fail when light levels drop off. You see this most often with much older leaves, often the bottom growth. Seems when light levels drop considerably, the plant jettisons these leaves as too hard to maintain, and just too old to be of much use.

An explanation, but not helpful.

Have you had any bitterly cold nights, and are these plants closest in to the glass? If no, than I am stumped. I agree, it sounds worrying, and odd that it would happen so suddenly to multiple plants...

So perplexing .... We did have one kinda cold night, noticeably so & maybe the heat was off. I had thought that caused a Tsangii bud to blast (but happily, that spur is budding back up).

But both afflicated plants are further from the window, Diversifolia is halfway across the room. Well I got brave & unpotted it. Current roots look/smelled OK, but it must have lost some roots, as for its curent rootball, the remaining soil volume was too big, seemingly overpotted. It's my own potting, so I doubt I overpotted it so. Got rid of a couple of wobbly leaves, potted it down smaller w/ bottom third of mix as mix, top 2 thirds of mix pure pumice.

This is something a succulent friend taught me, esp. for re-rooting Crassulas, Aloes, some others. Just a little bit of mix in the bottom & the rest pumice, rocks, or the like. Supposedly encourages roots to stretch down deep to get to the mix.

So, embolded by this idea, I turned out the Coronaria too (was on a hoop, so really a pain). All looked good, but it too was in too much mix. So I potted that down too, w/ a stake to hold it up. I did this w/ abt 1-2" of mix & the balance just pumice.

Jennie, I thought it might have been the gray weather too, but these 2 Hoyas are the only ones of abt 30 that are having trouble. So I thought maybe it's NOT the gray skies.

So Naz, what abt the H Peroxide pls? I have 3%, should I use it 1 pt HP to 3 pts water, to get the 1% you mentioned? I haven't watered yet as both of these had rather wet mix. I shook off all the soil I could when potting down.

Thanks for trying gang, cross yr. fingers pls.!!

Pirate, if you're pretty sure its rot, go ahead and dilute in 2parts water (1part H2O2 and 2 H2O).
Just in case though since you mentionned about the potting mix, it couldnt be underwatering causing this to happen is it?

Hey Naz,

"Just in case though since you mentionned about the potting mix, it couldnt be underwatering causing this to happen is it?"

Not sure what you mean? Can you clarify this pls?

What I meant was this sentence : "Just a little bit of mix in the bottom & the rest pumice, rocks, or the like" that can indicate that hte mix is getting dry too fast..in winter with the electric heating and all the air can get pretty dry.
I know you know, but just in case you haven't thought about it.

Karen...I have had this same thing happen to me over the last couple winters. Some hoyas thrive and others just slowly yellow until there are no leaves left. I think some are just more sensitive to changing conditions. I also have had some rot issues, but not necessarily on the same plants as the ones who are yellowing. So far this year the only one to yellow was my australis, who was right by the front door. I treated with a fungicide and moved it away from the drafty door and no more leaves have yellowed. Last winter I lowered the thermostat during the christmas break (thinking I would save on my heating bill) and came back to several dead or dying plants. So I am convinced my main issue is cool damp conditions. I have been keeping an eye on the dampness of my soil, but mainly I have been keeping my thermostat a little higher. I also applied weather stripping to all my windows. So far so good although we are just now starting to experience cold temps here in Ohio. I did lose my little diptera, it just turned mushy but it was by the window before the stripping was applied. It also had a hard time during the summer months. I have all of my hoyas either in my plant room (east facing but three light stands so a lot of indirect light) or in my kitchen in front of the south window. I also have been running humidifiers in both rooms (the one in the plant room has a fan). Now that christmas break is again approaching I am once again nervous to leave for fear of what I will discover upon my return. To prepare I have turned off the humidifiers this week to get them ready for the lower humidity (can't run them while I am gone). I also have been avidly treating them with Safer soap since low humidity seems to equal "hello spider mites". I will also be keeping my thermostat up. I will be crossing my fingers for you and me both!

Kim

Hi Again Gang,

Well Naz, maybe I was unclear but

"Just a little bit of mix in the bottom & the rest pumice"

is what I just potted it into, AFTER having found them ailing. Prior to that they were in my regular Hoya mix which seems fine for them all.

Tonite I made the H2O2 mix, hope it helps 'em, so thx. Diversifolia continues to worsen. Cornonaria does too, all except for 1 separate branch which seems unaffected. I may separate that branch for safety's sake tho' don't want to keep disturbing them, I'm on the fence here.

Kim,

Thx, that may in fact be the case, these 2 just are more sensitive to cold, tho' here in my top floor, heated aptmt which tends to get a bit too much heat, it also gets kinda dry but thankfully never invites spider mites. So at least it's not getting cold & wet at the same time.

My own piece of Diptera has done nothing for the entire year I've had it (can't make out the month on my label, but I think it's just abt 1 yr old, my tag says __/03). (I'm discovering pencil writing fades when it's under the level of the mix.) I'm concluding it's just a slow grower or hasn't decided it likes me yet & that it'll act better in Spring!!

So thx again for trying!

i am having the same problem with a few of my hoyas and i had decided to repot them in smaller pots this week. they are still in the plastic hanging baskets. should i put them in clay pots. i know i have not overwatered but i didn't know if clay would help if it were some kind of rot or fungus. what did you repot yours into. and are you going to increase lighting.

I had left mine in the same pots they'd been in before. The Diversifolia which is looking like a goner is in plastic. The Australis (larger, heavier vines) is in clay. I think I'm going to pull out the Australis' one remaining healthy looking stem, to a tall, narrow plastic cup.

I don't grow w/ lights, so don't have any way to increase the lights. Hum .........