Diseases that affect the trunks of apple trees usually result in the decline and death of the trees. Preventative measures are more effective than control measures. Choose a tree with a rootstock and scion that are resistant to disease. Plant the tree with the graft union several inches above the soil line. Plant the tree in well-drained soil and do not overwater. Remove fallen leaves, fruit and limbs from around the tree. Compost healthy plant material and destroy infected plant material. Do not plant another apple tree where one has previously died of a disease.
Burrknot is a genetic disease that affects many popular rootstocks. It is characterized by one or more rough circular growths just above the soil line on the trunk of the tree. The growth contains bumps that are the beginning of new roots. Periods of high humidity and temperatures, with low light levels initiate root growth.
Burrknots can stunt the growth of the tree. If they girdle the trunk, the tree may break with a heavy crop of fruit or in a high wind.
Burrknots may root if the soil is mounded around them. A chemical treatment is available that may reduce the development of small knots, but the surrounding bark may be damaged and the knots may return.
Crown Rot occurs where the roots join the trunk of the tree. The fungus Phytophthora, which lives in wet, poorly drained soil, causes crown rot. The symptoms include poor growth, yellowing foliage and small fruit. The tissue under the infected bark is orangy-red to brown to black, depending on how advanced the disease is. The trees usually decline slowly and die. There is no chemical preventative or treatment for crown rot.
Union necrosis of apple trees results in poor growth, reddish leaves and bark, and abnormally high fruit set. A dark necrotic line and spongy orange tissue appear at the graft union in advanced stages of the disease. Union necrosis is caused by Tomato Ringspot Virus and spread by Dagger nematodes. There is no chemical preventative or treatment for the disease.
Several fungi that enter apple trees through injuries or other diseased areas cause wood rots. They weaken the structure of the trunk, which causes the tree to break. Symptoms of wood rots include fruiting bodies on infected areas and dieback of foliage and limbs. There is no chemical preventative or treatment for wood rots.