Hoya carnosa yellow leaves
I inherited an 80 year old Hoya Carnosa. I have had this plant for the past 4 years and it has been doing fine. For the past 3 months the plant has been yellowing leaves towards the bottom of the plant while the top of the plant has the dark green leaves. I always get 15-20 flowering clusters this time of year and this year was no different. I am very concerned that my plant has some kind of serious problem due to the many leaves that are turning yellow and then falling off. What can be the cause of this? I live in Colorado and the plant is in the northwest facing window. It gets plenty of sun and I water it about once a week. I really do not want my Grandmother's plant to die so I am in need of some serious help here. There are also some brown spots on the yellow leaves. I have been looking on line and get a few different answers to the problem - over watering and too much sun. I am not sure this is the problem as I have had this in the same place for the past 4 years.
Thanks for your help.