Apple Crown and Collar Rot
The first symptoms to appear in the spring are delayed bud break, leaf discoloration, and twig dieback. These symptoms indicate that crown infection is advanced. Although infected trees might survive the growing season, they show symptoms of leaf and bark discoloration and premature leaf drop in the fall.
The most obvious symptom found on affected trees is a partial or complete girdling of the trunk. Infected bark becomes brown and is often slimy when wet. Close examination of the roots often reveals reddish-brown, water-soaked areas of necrotic tissue located at the base of the root where it attaches to the rootstock. The entire underground portion of the stem is usually water soaked and brown, and the necrotic area usually extends upward to the graft union.
The disease is caused by fungi in the genus Phytophthora, which belongs to a group of fungi known as the water molds. The fungus survives in the soil for several years as spores that are resistant to drought and somewhat resistant to chemicals. The fungus requires high levels of moisture and cool temperatures for growth and reproduction, and grows best at temperatures around 56°F. Trees, therefore, are attacked at about blossom time (April) and during the onset of dormancy (September). The fungus can infect apple trees in the following ways:
- collar rot--infection above the tree union,
- crown rot--infection of the lower trunk and root bases, and
- root rot--infection of the lateral and fibrous root system.
The following techniques are useful in managing apple crown and collar rot:
- Rootstock selection--Of the rootstocks preferred by growers, none is completely resistant to crown rot. The rootstocks M.7 and MM.106 have appeared to be the most susceptible. The most resistant rootstock is M.9.
- Orchard site selection--Avoid planting orchards in heavy, poorly drained soils. These sites favor fungal growth and development. Crown rot prevention is difficult, and eradication almost impossible in low-lying, poorly drained sites.
- Horticultural--If the tree has not been girdled completely, remove the soil from the base of the tree; scrape the surface of the discolored area and leave it exposed to dry. Drying often stops crown rot from progressing further.