Pls. show us the whole plant, it's difficult to say the problem from just seeing the leaves.

Showing us the plant shows the plant, how it's growing & hopefully the type of mix too, all those factors & more matter.

Hi Mayyan,

I see from your page that you're in Israel - I don't know how hot it's been there, but it's been ungodly hot here for a few weeks. We're better now, down into the low 90's, but when it's very hot, it's hard for me to water right. I'm always afraid they will dry out too much and suffer, and the top soil DOES dry out very fast, but I think sometimes, they're still pretty damp below the top soil. So I find when it's very hot that some get overwatered. I had two Hoyas that started developing yellow leaves and dropping them, so I backed off on the water and the yellowing stopped. So my first thought is overwatering. Please do tell us how you care for your Hoya as PG suggests.

Denise in Omaha

From those photos it does look like overwatering/poor drainage.

Thank you all for your replays.
In the summer the average temperature where I live varies between 20-30 degrees Celsius. So far the relative humidity this summer varies between 40%-70%. The potting mix I am using is about 2/3 Baltic sphagnum peat moss and 1/3 perlite. I am watering about 2 time a week in the summer and misting them every few hours in the hot hours. Most of my hoyas are doing fine but I lost a hoya vitellinoides to the same problem.

The plant ( As you can see it already lost many leafs):

Another view of the plant:

new seams to be healthy growth:

The potting mix

Thanks for the additional pix & info. We here often tend to avoid using peat 'cause it can cause problems.

When you water, does the water come out the bottom quickly? Or could the mix be hardened & not allowing water into the center of the rootball? Perhaps stop misting in the meantime?

Handsome Hoya, whatever variety, I hope others here can be more helpful.

Looks like verticillta varagata to me.

Denise in Omaha

Peat moss has a pH of about 4.5 which is too acidic for most plants. Generally speaking most plants enjoy a pH of around 5-6.5 and every time you go up or down the logerithmic pH scale one point you change the pH X100. Only true acid loving plants can deal with a pH as low as 4.5 and the yellowing you are seeing is most likely a nutrient deficiency due to nutrients being made unavailable to the plant due to the acidity of the potting mix.
I would reccomended using potting soil as your main component and then adding perlite and some peat if you wish. Orchid bark is a good a choice because it helps keep the mix open but will not cause the pH to dip so low.
You may have to start your plants over as cuttings or perhaps take a cutting of each and try them in a potting soil, orchid bark, and perlite mix for comparison.

Mike