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Collar rot caused by various species of Phytophthora are not diagnosed positively without laboratory culturing. However, scouting can help identify trees which may be infected, and so that samples may be collected for laboratory analysis.
SYMPTOMS: The symptoms of collar rot are often similar to those caused by any agent which interferes with part or all of the tree's root or trunk system (rodents, soil compaction, mechanical injury, canker diseases, fire blight, etc.). Affected trees may exhibit poor terminal growth; have small, chlorotic leaves; and/or wilt and eventually die. Trees collapsing as a result of collar rot will be largely or completely girdled by a zone of dead inner bark just beneath the soil line. Bark discoloration sometimes extends above the soil line as well. Collar rot is most common on MM.104 and MM.106 clonal rootstocks, although all rootstocks are at least partially susceptible.
DISEASE CYCLE: The collar rot fungus persists for long periods in diseased host tissue or as resting spores in the soil. When the soil becomes saturated with water, the fungus produces small swimming spores (zoospores), which are chemically attracted to plant roots. Spores may also be carried by water as it drains down a slope and through the soil profile. Upon contacting a root or the trunk, the spores may germinate and infect. Both of these processes depend upon the susceptibility of the rootstock, and how long the soil remains excessively wet. Spore production and infection is most common in the spring and the fall.
SCOUTING: Observe and note the location of any weakly-growing or collapsing trees. Note these trees especially if they appear to be growing in a low-lying or poorly-drained area. Gently dig the dirt away from a portion of the crown (trunk/root area) and check to see if the inner bark is still alive. If so, it will appear green and the wood just inside the bark will be firm and white with no discoloration being evident.
For sampling, dig a shallow hole within the drip line of the tree, and put some moist soil and segments of roots into a plastic bag. If the crown or lower trunk appears infected, remove a few pieces of inner bark from near the margin of the infected region, and place them in a plastic bag with moist soil. It is important to KEEP SAMPLES COOL and do not place bags in direct sunlight before they are delivered.
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