Q.Spots On Underside Of Leaves And Black Sooty Look To Stems On Crown Of Thorns.
I have several other plants and they don’t seem to be affected.
Black sooty mold is a fungi that latches on to the sweet, sticky honeydew secreted by many insects like aphids and scale. When the insects suck the sap, they excrete a thick gooey sugary substance called honeydew. Determine which insect is causing the problem so you can apply appropriate counter measures. Your local plant nursery can help identify them if you put them in a sealed, transparent plastic bag. If you find aphids or scale, treat your plant with Neem oil. Spray all parts of the plant extremely well but consider not spraying the flowers, which attract pollinator insects (the "good" guys). Follow the directions on the product's label.