Greetings, Glenn! Thank you for your question to Ask Extension/Ask a Master Gardener. The photo you included and the detailed explanation regarding plant location and care practices is very helpful. Congratulations on maintaining your beautiful Jade plant for so many decades.

I, too, have kept Jade plants (Crassula ovata) in my houseplant collection for many years. It is one of my favorite plants! In preparing an answer to your question I referenced several research-based sources to be sure that I am providing you with accurate and current information. I have included links to the best of those sites at the end of this response in case you would like to read more.

I will give you my best conclusion as to what might be ailing your jade plant, along with suggestions for treatment, where applicable. I will also provide a resource where you can send samples of your plant for analysis, for a fee of about $20, should you choose to pursue your question further. (BTW, We do not receive any compensation for recommending this service.)

I believe your jade may be suffering from (in order of likelihood) sun scald, injury or pest/pathogen. Here are my thoughts:

Sun Scald You noted that your plants are in a sunny south-facing window, which is perfect for your plants. However, if water is dripped on the leaf, in bright sunlight it can act like a magnifying glass and cause “spot” scalding of the leaf almost immediately. Treatment is to be sure water is not dripped on leaves, or is wiped off immediately.

Injury My research indicates that spots such as these can be caused by accidental injury to the leaves. Other than aesthetics, it does not affect the plant.

Pests/Pathogens Jade plants are rarely the target for pests or pathogens, but there are some exceptions. 1) Powdery mildew can be an issue, but you have eliminated that possibility. 2) Mites (sometimes inaccurately called spider mites) can be an issue. They are notorious for hitchhiking on people, pets, tools and watering cans. My favorite test for mites is to hold a piece of white paper under the plant leaves and gently tap the leaves. The mites, if present, will fall onto the paper, appearing as black dots about the size of the period at the end of this sentence. If you use a magnifying glass, you can sometimes see the legs on the mites. If you find mites, write back to me and I will research best practices for elimination on jade plants. Horticultural soaps are not recommended for jade plants. General insecticides don’t work on mites. 3) Scale insects sometime attack jade. Their signature is the “honeydew”, a sticky substance, they leave behind. If you are not seeing sticky goo, its most likely not scale. 4) Mealy bugs also can attack jade, but they leave a cotton-like fuzz which you did not mention, so my belief is that you do not have mealy bugs.

In conclusion, I believe your Crassula ovata either has sun scald or a simple injury, but I encourage you to check for mites, scale and mealy bugs. If you see any additional symptoms, or would like to discuss further, please feel free to reply to this email and your response will come directly to me. If you would like a more definitive diagnosis, you can send samples of your plant (physical or digital) to the C. Wayne Ellett Plant and Pest Diagnostic Clinic at the Ohio State University. They have the tools to take the analysis to the next level. You can find the information you need here. And, of course, I would be happy to hear from you if you have any questions. (Or if you would like to share a picture of your 50-year-old Jade! I am so impressed … and a little jealous!)

----------------------------------------------

Clemson Home and Garden Information Center https://hgic.clemson.edu/factsheet/jade-plant/

Universiy of Connecticut Home and Garden Information Center https://homegarden.cahnr.uconn.edu/factsheets/jade-plants/

Wisconsin Horticulture, Division of Extension https://hort.extension.wisc.edu/articles/jade-plant-crassula-ovata/

C. Wayne Ellett Plant and Pest Diagnostic Clinic at the Ohio State University https://ppdc.osu.edu/submit-sample