What is collar rot?
Collar rot is a fungal disease caused by the Phytophthora fungus, affecting mainly apple trees. It can also affect apricots. It colonizes the union – the swollen part at the base of the trunk marking the join between the rootstock and the upper part of the tree – and can cause the death of the tree by completely girdling the trunk. It can also damage the rootstock just below the soil – crown rot – causing the death of roots. It is often caused by poorly-drained soil. It can be recognized initially from the deterioration of leaves, which may turn red, and shoot die-back. Later, cankers appear just above the rootstock.
How to treat collar rot
Collar rot can be prevented by ensuring that your soil is well drained, with a good structure. The best way of improving soil structure is to add organic matter by means of mulch, manure or compost. This helps maintain soil bacteria and beneficial fungi that can combat the fungi responsible for collar and root rot.
If you suspect collar rot because of slowed growth, small-sized fruit and reddish leaves, examine the base of the tree, and expose the part of the trunk just below the soil. You can recognize collar rot and crown rot by the presence of brownish diseased tissue and red-brown cambium – the living tissue just below the bark that is normally yellow-green – by removing some soil. Scrape the affected area, leaving it exposed to the air. The fungus requires water to survive, and so drying out the trunk may stop progression of the infection.