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.

There are several reasons for citrus to defoliate and die back. 

• It could be due to inconsistent watering. If a tree dries out at some stage and then has been watered it causes the roots to swell and burst. Frost can do the same thing if the ground has been frozen for a period of time.
• Soil borne fungal diseases such as phytophthora can enter the tree through the roots, the stems may also have a sticky gum on them.
• If it is a root rot fungal disease there is little that can be done for the trees, they are unlikely to bounce back and so will need to be removed as it could spread to other plants.
• Before replanting, the existing soil needs to be removed and disposed of, with fresh potting mix, garden mix or soil added to the planting hole. 
• Citrus defoliate due to stress. It could be water stress, too much, too little; over application of fertiliser which burns the fibrous surface feeding roots; sudden temperature change or a cold snap.
• Weekly applications of Tui Organic Seaweed Plant Tonic may help by stimulating root growth and hopefully help the tree regenerate. Do a knick test on a branch and lift the bark, if it is green underneath the tree is still alive. Mulching around the tree also helps keep the soil consistently moist and prevents drying out.
• Insect pests such as the Lemon Tree Borer can also cause trees to defoliate. They lay their eggs in cuts and open wounds in the tree. Check for small holes in the stem of the tree and sawdust surrounding the tree. Talk to your local garden centre for a suitable control for lemon tree borer which can be squirted into the holes in the tree.

Read the Tui Citrus Growing Guide

More citrus frequently asked questions