We have. 50 year old miniature orange tree that all of a sudden seems to have either a decease or a bug infection. There seems to be a small brown very hard round spot developing on the underside of some leaves as well as on the stem where some oranges are developing. Small oranges start to ripen then fall off. Also a sticky substance. It has produced wonderfully year after year. I will Attach photos of the tree last year and also a photo of what i spoke about. Please send me some ideas of what is happening and what we can do to cure the tree. Thank you. I have received some answers but I'm unable to see them....where do they show up??

Possibly galls. You can check out different treatments for them. Neem oil seems to work. They aren't very damaging but they are unsightly and can cause leaf drop. I'm not fully awake yet or I would check out more treatments.

It looks like scale insects to me. The insects live under a hard little shell so most treatments don't work. If its a small tree, use some rubbing alcohol on cotton balls to wipe them off then wash the tree with warm soapy water to get rid of the alcohol residue. The good news is that they spread very slowly but you don't want to leave them there because they will spread and they are sucking the life out of your tree.

Daisy