PhytophthoraCollar, Crown and Root Rots are often observed on 3- to 8-year-old trees grown on Malling Merton (MM) 104, MM.106, M.7, and to a lesser degree MM.111 rootstocks. The disease is often observed in low-lying areas of orchards with heavy, poorly drained soils, as well as more problematic during rainy seasons.
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. While infected trees may 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. 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 the fungus Phytophthora cactorum, which belongs to a group of fungi known as the water molds. Phytophthora spp. can survive in the soil for 1-2 year, which is important to note if replacing an older orchard. Old trees can tolerate this microorganism whereas younger trees cannot. The fungus requires high levels of moisture and cool temperatures for growth and reproduction, and grows best at temperatures around 56°F. Trees are therefore attacked at about blossom time (April) and during the onset of dormancy (September). The fungus can infect apple trees in the following ways: (1) collar rot--infection above the tree union, (2) crown rot--infection of the lower trunk and root bases, and (3) root rot--infection of the lateral and fibrous root system.
Disease management, cultural
- Rootstock susceptibility--Of the rootstocks preferred by growers none are completely resistant to crown rot. The rootstocks M.7, MM.104, and MM.106 have appeared to be the most susceptible. Although less susceptible, M.2 and MM.111 can be infected by crown rot under favorable conditions. The most resistant rootstocks are M.4 and 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. Be mindful of locations that were previously an orchard. Fumigation may be necessary.
- Horticultural--If the tree has not been completely girdled, remove the soil from the base of the tree; then scrape the surface of the discolored area and leave exposed to dry. Drying often stops crown rot from progressing further. In-arch grafting may also be used to bridge the damaged area.
Disease management, chemical
Alliette and Ridomil Gold EC are presently registered for control of crown rot on apple and stone fruits. Refer to the label for specific use recommendations.