“My desert roses are dropping their leaves and they have brown to blackish spots on them. I have attached an image.” Question from Charles of Sebastian, Florida
Answer: The yellowing and black blotching and spotting on your desert rose (Adenium obesum) leaves is likely caused by the fungal disease, anthracnose leaf spot (Colletotrichum spp.). Yellowing of the leaves is caused in the early stages, and then they develop black spots and splotches and fall off.
These true desert plants thrive in dry heat and cannot take high moisture and high humidity without eventually succumbing to fungal diseases. I recommend placing them away from any place where they will be subjected to rain and high humidity. An indoor sunroom or south-facing window would be ideal. Bottom water the plants sparingly (to keep water off of the leaves) and hope that this will tackle your leaf spot. In very severe cases, this disease can cause plants to die. Also, be sure to water these plant very little to none in winter to mimic the dry winter period in their native Africa and Arabia. Consider reading this other Ask a Garden Expert Inquiry: What is the Best Soil for Desert Rose?
I also encourage you to watch the video below. It covers desert rose. (Many of the other flowers will also grow well in Florida.)
Black Gold Horticulturist