I have a pachypodium lamerei, about 120cm high, which has begun putting out new leaves and spines with the start of the spring season. But almost as soon as the new leaves emerge, they begin turning black and soft, then fall off. They only get about two or three cm long before beginning to turn black. The blackness begins forming at the tips of the leaves, then moves toward the stem.
Little amber nodules are also found on the underside of the new leaves. I am almost certain this is dried sap, though in some forums some people people describing similar problems seem to think they may be eggs or something insect related. Again, I doubt this in this case -- it really does seem to be sap, and in some cases is sticky, though usually dried.
The blackness I'm seeing on the leaves may also be visible in some of the new spines, though for the most part those seem to be emerging safely and hardening as I'd expect.
Last year's leaves, which are the large ones visible in the photos below, are doing just fine -- though the lowermost leaves continue to turn yellow and fall off, just as I'd expect.
The plant is in a southwest facing window, and gets sun about half the day. I have probably been watering it too infrequently -- every three or four weeks. I've had pachypodiums for years, and tend to err on the side of underwatering, but maybe I'm overdoing (underdoing?) it in this case. I've owned this plant for about 10 months, and this is the first sign of trouble. One other final note is that the water in our area may be rather hard -- not sure what effects this may have, but thought it worth mentioning. (For what it's worth, it is right next to a P. Saundersii that is about 60 cm tall and seems quite healthy, and has been putting out a lot of new green growth.)
I'd be grateful for any help anyone can provide! The plant has seemed so healthy, and I've searched the internet but found nothing. Thanks so much.