Phytophthora Root and Crown Rot

Symptom expression depends upon how much of the root or crown tissues are affected and how quickly they are destroyed. Generally, crown rots advance rapidly and trees collapse and die soon after the first warm weather of spring. Leaves of such trees wilt, dry, and remain attached to the tree. Chronic infections, usually in the roots, cause reduction in growth and early senescence and leaf fall; leaf tissue or veins often turn red. These trees may be unthrifty for several years before succumbing to the disease. Phytophthora infections typically kill young trees because their root systems and crown areas are small compared to those of mature trees.

Periods of 24 hours or more of saturated soil favor Phytophthora infections. Conversely, good soil drainage and more frequent but shorter irrigations reduce the risk of root and crown rot. Rootstocks vary in susceptibility to the different Phytophthora species; none are resistant to all pathogenic species of the fungus. Thus, the success of a rootstock may depend in part upon the species of Phytophthora present in the orchard.

The most effective ways to manage Phytophthora root and crown rot are to select a good planting site, select an appropriate rootstock, and properly manage irrigation water. Planting on raised berms can help in managing this disease. Avoid over irrigating, especially in spring and fall when soil temperatures are most conducive to disease development and water use by the tree is low.

Fungicides are available to treat soil around newly planted trees. If there is a history of Phytophthora root rot in the orchards and problems are anticipated, treatments may be warranted.