Most likely is pigeon pox. Do a search on this site on "pox" . It is not life threatening and can be treated under most circumstances. Many great suggestions for treatment you will find in your search.
Mosquitoes can infect pigeons with pox, so if you have a mosquito problem, you need to get your healthy birds vaccinated, or try to keep down the mosquitoes. It's very contagious, so you would need to separate any sick birds from the others.
Hi: I just saw this post (wow 13 years ago!) and would like to ask as well. Has anyone have had any pigeons that has lesion growth on the beak like in the picture? I've been monitoring my pij for this for almost a month now and it never change color nor the size. It's all clean bright looking not like the usual dark ugly pox lesions. She's about 6 years old.