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Phytopthora Root Rot

Phytopthora Root Rot


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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved

FRUIT PEST UPDATE

Phytophthora Crown and Collar Rot of Fruit Trees

A disease known as crown rot, collar rot, or Phytophthora collar rot affects most fruit and nut species. It is a serious disease; frequently killing trees 5-7 years old, just as they begin to bear fruit.

Collar rot shows as dead areas on the trunk called cankers between the soil line and crown roots. Cankers are difficult to detect when young, but as they enlarge, they turn dark, sink, and girdle the tree.

This disease is not easily diagnosed from symptoms in the top of the tree. Scattered yellow leaves occur on trees in mid-season. Infected trees usually have sparse foliage and low vigor. Foliar symptoms may appear only on branches directly above the canker, while the remainder of the tree appears normal and continues to bear fruit. Some trees remain alive for 2 to 3 years, while others die very quickly.

Overwatering encourages the disease since the fungus is a water mold. It is more serious in heavy, poorly drained soils than in light, well-drained soils, although it can occur in almost any improperly irrigated soil.

Collar rot is difficult to control because it is an erratic disease. When infections are discovered, it is usually too late to eradicate the disease and save the tree. Use the following control practices to prevent or decrease collar rot:

Choose resistant varieties and rootstocks. The most susceptible apple rootstocks are Lodi, Grimes Golden and Duchess. Golden Delicious, Jonathan, McIntosh and Rome Beauty are moderately resistant. Red Delicious, Wealthy and Winesap have good resistance. Seedling Rootstocks appear somewhat tolerant of collar rot.

Do not form a basin around the trunk or place trickle irrigation outlets so that the tree base is continually wet. Always channel water away from the trunk. Keep irrigation periods to less than 8-hour runs. Plant trees on raised beds.

Do not plant trees so that the graft union is below the soil line. This results in increased collar rot and may allow the scion to root thus resulting in loss of dwarfing. Reduce frequency or length of irrigation if used in excess. Soil saturated for more than 36 hours is conducive to disease.

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