Our garden is looking lush - except for our lemon and lime trees, planted a decade ago right next to each other, which have both suddenly curled up and died. Is it a root fungus problem? They are both planted in well draining soil above a retainer wall, and no other trees (including a third citrus tree) appear to be affected.
It could be due to watering. If a tree has dried out then been watered this causes the roots to swell and burst. Fungal diseases (phytophthora) can enter the tree through the roots. The stems can have a sticky gum on them as well.
Unfortunately if it is a fungal disease there is little that can be done for the trees, they need to be removed as it could spread to other plants. Feel free to send a photo to firstname.lastname@example.org and we might be able to get a better idea. Sometimes citrus when stressed due to hot weather can defoliate. Also is there any sign of borer in the trees? If so there will be a sawdust and holes in the stems where the borer has entered.