Brown root rot

Phellinus noxius

Asia, Africa, Central America, the Caribbean, Oceania. It is recorded from cocoa in American Samoa, Federated States of Micronesia, Papua New Guinea, Samoa, Solomon Islands, and Vanuatu.

The fungus has a wide host range, attacking native forest trees and plantation crops, including cocoa (and Leucaena shade trees), oil palm (upper stem rot), rubber, coffee and Cordia (especially in Vanuatu). It also infects breadfruit, and may be associated with Pingelap disease that occurs in atoll countries. Ornamental trees in urban spaces or along roads are also hosts, e.g., poinciana, Delonix regia. In Papua New Guinea it is recorded from coconut.

The fungus attacks the roots, and this causes the leaves to yellow and wilt, beginning at the tips of the branches. Very quickly, all the leaves fall. The fungus forms a dark brown to black crust on the trunk, up to 1 m high, with a white margin, often with clear drops of liquid. It grows as a crust along roots, too, and often soil and small stones can be seen within it.

When a forest is cleared, stumps remain, and if infected fruit bodies are formed 3-4 years later. One type is a bracket type, dark brown to black on top and grey below (Photos 1&2). The other type is grey, flat against the lower trunk, and less obvious than the bracket type. The life cycle is illustrated (Diagram).

In the soil, the fungus moves from diseased to healthy trees by root-to-root contact with those nearby. When the fungus reaches the base of the newly colonised tree it grows up the trunk as a sterile crust, often referred to as a 'stocking' (Photos 3&4). The crust does not produce spores. Cracking of the bark and gum formation may occur, and when the crust has girdled the trunk, the tree wilts suddenly.

Spread is by spores infecting the surfaces of cut trees. If left unattended, the fungus will spread outwards from the stumps along the roots to healthy trees, infecting them and spreading the disease. In a short time, a large number of trees are killed in an ever-widening circle. The fungus remains in the soil for many years.

In forests, the natural habitat for Phellinus, it causes relatively little harm. However, when the forest is cleared for cocoa, oil palm or a plantation of timber trees, condition becomes ideal for its spread. In new plantation, all the trees are alike and, if susceptible to Phellinus, spread can be rapid. The impact of the disease on cocoa depends if these early infections are managed or not.

In urban areas, there is the danger from falling dead branches or the entire tree.

Look for the sterile crust that grows up the trunk from the base of infected trees; look for the crust on the roots, too. Look for brackets of the fungus on old stumps of forest trees. In cocoa plantations, look for brackets on shade trees, for example, on Leucaena.

CULTURAL CONTROL

Before planting:

• After clearing the forest, it is important to survey stumps for brackets of Phellinus. Remove the stumps together with all roots greater than 2.5 cm diameter. The surveys need to be done on a regular basis, every 3 months at least.

During growth:

• Inspect trees of cocoa and other trees of high value every 6 months. If diseased trees are found, they should be removed immediately, taking out as much of the larger roots as possible. Carefully remove the soil from around trees nearby and inspect for the sterile crust at the base of the trunk and on the larger roots. If seen, remove the trees.
• Chip trees, and then compost for at least 16 weeks before using as a compost. Ensure piles reach 75^oC, and turn compost regularly.

CHEMICAL CONTROL
There is no chemical control option for this disease.