Help! My citrus trees are dying, one after another
I live in Northern California, in a hot, dry climate that is good for growing fruit of all types, including citrus. Recently, a healthy Washington navel orange started to get yellowed leaves -- not yellow with green veins, but solid yellow in the middle of the leaf with green around the outside. Gradually, the green would fade and the leaf would become completely yellow, both sides of the leaf would curl upwards, and the leaf would shrivel, die and fall off. When I noticed this happening, I tried watering more and fertilizing. Nothing worked. Within a few weeks, every leaf had shriveled and the tree was dead. Two trees next to it, a grapefruit and a Valencia orange, are starting to show the same symptoms and leaves have started to die and fall off. The same thing is now happening to a Bearss lime twenty-five feet away. I have searched online in vain to find what might be causing this. It is not insufficient water, because these trees are watered every morning with bubbler irrigation. It is unlikely to be too much water, because I am watering them all the same as I have always done. It is not a lack of fertilizer -- I know what that symptom looks like and how to make it better. It is not gophers, because I don't see any holes in the ground nearby. Could a bug be causing this? If so which one? A fungus? If so, how would I treat it? Would it help to remove all yellowing leaves? To rip out all of the trees that are showing any of these symptoms?
I'm worried that I may lose all of my citrus trees, and I'm mystified that nobody on the Web is reporting these symptoms. Can anyone give me any advice?