You know what a "big fan" I am of the USDA, but in this case I am quoting them and asking you to look at their website - URL following. From the USDA Website, (https://www.aphis.usda.gov/aphis/resources/pests-diseases/save-our-citrus/soc-citrus-diseases):
I'm quoting from that website: "Citrus greening is one of the most serious citrus plant diseases in the world. It is also known as Huanglongbing (HLB) or yellow dragon disease. Once a tree is infected, there is no cure. While the disease poses no threat to humans or animals, it has devastated millions of acres of citrus crops throughout the United States and abroad.
"Citrus greening disease was first reported to have occurred in Asia during the late 1800s and the disease has already caused devastation in Asia, Africa, the Arabian Peninsula and Brazil.
"Named for its green, misshapen fruit, citrus greening disease has now killed millions of citrus plants in the southeastern United States and is threatening to spread across the entire country. See all of the affected areas.
"Citrus greening is spread by a disease-infected insect, the Asian citrus psyllid, and has put the future of America’s citrus at risk. Infected trees produce fruits that are green, misshapen and bitter, unsuitable for sale as fresh fruit or for juice. Most infected trees die within a few years.
"The Asian citrus psyllid, which spreads citrus greening, is no bigger than the head of a pin. The infected insect spreads the disease as it feeds on the leaves and stems of citrus trees. Once the Asian citrus psyllid picks up the disease, it carries it for the rest of its life. Citrus greening is then spread by moving infected plants and plant materials such as bud wood and even leaves."
California faces some pretty tough challenges in this matter, but has started, appropriately, by first imposing a quarantine around affected trees. People in contact with diseased trees should endeavor to not spread the disease, by carrying infected fruit out of the quarantine area. This is an "organic" approach in that nothing is being sprayed with any chemicals (we have none that seem to affect this critter). This is a good place to start. However, more people have to be informed to NOT spread the psyllid.
These are samples of the disease infecting a citrus. The mottled yellow that is not uniform throughout the plant. Check your citrus for evidence like this. Of course, "yellowed" leaves are also evidence of too much or too little water, but this yellowing is much different - this yellowing has no uniform pattern - with watering problems, you will see brown tips with yellow lower on the leaf. But it will not appear so radically different from other leaves - note these leaves do not show a distinct pattern. That is key.
Keep a lookout and act quickly if you see this problem. I would also contact your local county agent if you suspect a problem.
We all need to do this. Consistently.