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Phytophthora

Phytophthora cactorum, P. cambivora, P. cryptogea, P. citicola, P. syringae, P. megasperma, P. drechsleri, etc.

Phytophthora crown, collar, and root rot is the most widely distributed soil-borne disease of apples in the southeastern United States. It is caused by several species of Phytophthora which are common inhabitants of soil in the Southeast. Phytophthora cactorum is the species most often associated with the disease, but P. cambivora, P. cryptogea, P. citricola, P. syringae, P. megasperma, and P. drechsleri have also been associated with it. Phytophthora crown rot is most severe in orchards planted on heavy and poorly drained soils and on trees propagated on size-controlling rootstocks. Losses are usually greatest 3 to 5 years after planting when trees first come into bearing. Losses of 40% or more have been recorded in some orchards 5 years after planting.

Symptoms

Infections on size-controlling rootstocks are most common in the crown where the main roots originate from the trunk. The inner bark of affected tissues is reddish brown and affected tissues are clearly distinguished from healthy ones by a sharp line between healthy and diseased tissues (Fig 1). Collar and root infections (Fig 2) are also characterized by a clear demarcation between diseased and healthy tissues. Roots of trees that have drowned are often streaked with gray and have a sour smell which separates them from Phytophthora root rot.

Fig 1 - Diseased trunk tissue Fig 2 - Diseased root tissue
Fig 1. Diseased trunk tissue Fig 2. Diseased root tissue

Control

Management of the disease is based on cultural and chemical controls. Only healthy, disease-free trees should be planted. Trees should be planted only in well drained sites. On flat or poorly drained sites plant on a raised bed to improve drainage and reduce the likelihood of crown rot. Planting holes should be filled to prevent the soil from settling and forming a depression around the collar of the tree where water can stand. Susceptible rootstocks should be avoided, particularly in heavy, poorly drained soils. Seedling and M.9 rootstocks are generally regarded as most resistant and MM.106 and MM. 104 as most susceptible. MM.111, M.7, M.7a, and M.26 are moderately susceptible. Several fungicides are registered for crown rot control and are most effective when used on a preventive basis.

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Format updated March 13, 2013