disease on lemon tree
Your photo is not close or sharp enough for me to zoom in on your problem and see it clearly. I do think you have citrus whitefly, which you can verify yourself by watching for these tiny, flying insects. As such, I believe what you believe is a disease is a parasitic fungus, growing on whitefly eggs and nymphs.
Parasitic fungi can provide control of whiteflies on citrus. The problem is that you have to let the whitefly population get to damaging levels to grow the fungus.
It is better to control whitefly problems on citrus with oil sprays. Petroleum oil or clarified neem oil can provide effective control of whiteflies, if used at the right concentration (generally 1 to 2 percent in water) and frequency (monthly as a preventative, or three sprays every 10 days when severe infestations occur).
Consult product labels and appropriate university guides in your growing region for more advice on use of spray oils to control soft-bodied insects on citrus.
Thank you. I'll look into that.