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Crown rot continues to be a major cause
of tree death in Pennsylvania orchards. It is often observed on
3- to 8-year-old trees. Certain rootstocks are more susceptible
to the pathogen than others. The disease is often observed in low-lying
areas of orchards with heavy, poorly drained soils. The importance
of this disease has increased with the introduction of more dwarfing
rootstocks.
Symptoms
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 (click for larger image)
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.
Disease Cycle
The disease is caused by fungi in the genus Phytophthora,
which belongs to a group of fungi known as the water molds. The
pathogen most commonly found is
P. cactorum.The fungus survives in the soil for several years
as resistant spores. These spores are resistant to drought and are
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 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
Rootstock susceptibility. Of the rootstocks
preferred by growers, none are 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 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.
- Two fungicides are registered to control
crown rot. Refer to the PA Tree Fruit Production Guide
for more information on control.
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