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Phytophthora Crown and Root Rot

Fungus

Phytophthora spp.


In a Nutshell

  • Plant stunting and small leaves. A brown discoloration can be seen in the crown vascular tissue or throughout the crown. Infection of the roots causes a brown to black root rot.

Symptoms

Symptoms are typical of many root diseases. Small and slightly chlorotic leaves, poor fruit set and rotting of internal tissues are the main signs of crown and root rot. Infected vines or trees may be stunted and the sparse foliage may fall off prematurely. The bark may show signs of rot and in some cases, drops of gum are visible. Cross sections of the base of the trunk show reddish-brown necrotic cankers extending from the crown to the roots. Cankers eventually girdle the trunk, limiting the transport of water and nutrients to the aerial parts, eventually killing the branches, starting from the tips. The vines or trees gradually die and can easily be pulled out of the ground.

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Hosts

Trigger

The symptoms are cause by several species of fungus of the genus Phytophthora. Once introduced into a field, they can survive in the soil for many years and their eradication is not possible. These fungi depend on high soil moisture levels and humid, hot weather for their development. Diseased vines or trees are often found singly or in small groups in the field, associated with poorly drained areas, periodic flooding or excessive irrigation. In vineyards or orchards equipped with drip irrigation, the symptoms can appear occasionally on trunks that were directly under a valve, where the water had been running off directly. Another way to propagate the disease is through the transport of infected material, for example during grafting.

Organic Control

Anti-fungal biological treatments can be carried out, for example by painting the lesions and pruning cuts with Bordeaux mixture. A preventive treatment with same concoction also reduces the attacks. When trees are widely affected, the disease is incurable and only preventive measures can help to reduce it s severity.

Chemical Control

Always consider an integrated approach that combines preventive measures together with biological treatments if available. An effective way to treat trees and vines is through the application of fungicides through irrigation. At first symptoms, irrigate well the base of the trees with fungicide based on phosetyl aluminum, metalaxyl or methyl thiophanate-methyl. To prevent the spread of the disease, agricultural tools should be disinfected with bleach after use.

Preventive Measures

  • Use Phytophthora resistant varieties if available.
  • Do not transport infected plant material between different fields, areas.
  • Do not plant in areas prone to flooding.
  • Correct any soil drainage problem that could favor accumulation of water around the trunk.
  • Avoid repeated and prolonged flooding of soil.
  • Maintain a organic mulch over the root zone.
  • Keep the orchard or vineyard clear of plant residues.
  • Plan a balanced fertilization.
  • Remove infected plant parts or dead trees or vines and burn them.
  • Do not use intercrops that could spread the disease.
  • Make sure to clean thoroughly the farm equipment before moving from infested sites to clean areas.
  • Make sure to adjust drip irrigation so that water do not directly runs down the trunks.
  • When grafting, make sure to keep unions well above the soil.

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