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Phytophthora root, crown, and collar rot

Phytophthora cactorum (Lebert & Cohn) J. Schröt.

Crown and collar rot are often and mistakenly used interchangeably. Collar rot refers to infection that affects the bark tissue of the scion portion of the tree at or just below the soil line, whereas crown rot affects the bark tissue of the rootstock portion of the tree. Infected trees often have a normal bloom, but developing fruits tend to be small, the leaves wilt and drop, and the tree shows a general decline and eventually dies (A). Symptoms on apple usually develop over several seasons, becoming progressively worse. Apple, cherry, peach, and apricot trees are more susceptible to infection than pear and plum trees. Phytophthora-infected tissue often shows a characteristic reddish-brown discoloration of the inner bark several inches below the soil line with a characteristic, clear-cut margin of diseased from healthy tissue (B, C).

Distribution

Common to all fruit-growing regions in eastern North America.

Similar Species

Any disease or disorder affecting the root system produces similar above-ground symptoms. Rootstock blight (caused by fire blight), "wet feet" (root asphyxiation), borers in burr knots, winter injury and apple union necrosis (tomato ringspot virus) are often misdiagnosed as Phytophthora root and crown rot. The reddening and stark delineation of infected tissue from healthy tissue is often diagnostic. Winter injury and rootstock blight can nearly be ruled out if the reddening is located predominantly below the soil line. Also, Phytohphthora-infected trees are likely to be localized in low-lying, historically wet areas of the orchard or in areas with heavy soils.

Management

The fungus requires standing water or saturated soils to infect; thus, avoid sites that drain poorly, are slow to dry, or experience periodic flooding. Planting trees on berms or ridges is recommended because it raises the crowns of the tree above the portion of soil where pathogen activity is the greatest. The use of resistant rootstocks is necessary on historically wet sites. Fungicides may also be used to manage disease and should be applied either in early spring or in the fall when most infections occur.

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