From the photo you supplied, the wart-like growths on your apple tree are burr knots. These are clusters of root initials that can develop on the trunk or branches of some apple trees, particularly on certain rootstocks and varieties such as columnar apples.

Burr knots are quite common and usually appear as rough, knobbly growths on the bark. They are not caused by pests or disease, and unfortunately there is no chemical or non-chemical treatment that will remove them once they have formed.

Some apple varieties and rootstocks are simply more prone to developing burr knots than others. Conditions that increase humidity around the trunk or reduce air movement can also encourage them to form.

How to help reduce the risk of burr knots worsening

• Keep the base of the tree clear of weeds and vegetation. This improves air circulation and helps the trunk dry more quickly after rain.
• Avoid using tree guards on young trees, as they can trap moisture and create humid conditions around the trunk.
• Do not cut or remove burr knots. Cutting them out can damage the bark and may allow disease or canker to enter through the wound.
• If the burr knots are becoming more noticeable, it’s a good idea to let the garden centre where the tree was purchased know, so they can pass the feedback on to the grower.