Armillaria root rot

Introduction

Armillaria root rot is a disease of trees found throughout the world caused by a number of species of Armillaria (Fr.:Fr.) Staude.. Armillaria root rot has not been recorded in crops in Vietnam, but causes disease in fruit trees and in crops such as coffee, rubber and tea in other countries.

Symptoms

Symptoms begin as yellowing, wilting and shedding of foliage and may lead to the eventual death of the tree. The base of diseased tree can exhibit cracking of the bark with exudation of gum. In rubber, splitting of the bark of the roots causes exudation of latex ahead of the infection. The collar of the tree may turn black and the outer root tissues soften and thicken. When the bark is removed, creamy white fans of fungal mycelium are present on the surface of the wood. Sporophores, the mushrooms or fruiting bodies of the fungus appear in clusters at the collar region of the affected trees in the wet season. They are yellowish-brown, 5-6 cm tall and 5-10 cm in diameter, and may only appear in the advanced stages of the disease; sometimes they may not appear at all. In some species, the fungus can spread through the soil by means of rhizomorphs, long thread-like aggregations of mycelium, which are produced from colonised food bases such as the remains of old trees. They are 1-3mm in diameter, and can be several meters in length. Young rhizomorphs are cream coloured but soon become brown or black.

The Pathogen

Armillaria is a basidiomycete fungus in the order Agaricales. There are many species (more than 55 species) throughout the world, which are differentiated by the nature of the mushroom and mating tests. Common names for the fungus include the honey fungus, honey Agaric and bootstrap fungus.

Distribution within Vietnam

This pathogen, although found worldwide has not yet been documented in crops in Vietnam. The three general geographic groups are from North America, Europe and the Asia/Pacific region.

Host Range

The host range of this pathogen is large and poorly defined, but it mainly includes the woody perennials, including ornamental trees, fruit and industrial crops and native forest trees. All species of stone fruits may be affected by Armillaria root rot and it has been documented as severe and widely distributed on rubber, coffee and tea in cooler mountaneous areas, and occasionally on cocoa and palm.

Epidemiology

The disease frequently occurs in patches, as adjacent trees are affected before the original source of infection becomes apparent. Transplanting infected trees or the introduction of infected mulches can also disseminate the pathogen. The fungus may exist in an orchard site prior to planting, as the mycelium and rhizomorphs persist in the soil on infected roots for many years. The fungus spreads either as rhizomorphs in the soil or across roots where they come into contact with one another.

Control

Good horticultural management and hygiene is essential to minimising the spread of the pathogen into new areas in mulches, contaminated soil or transplanted trees.
Various stone fruit rootstocks may vary in their susceptibility to this disease, and may be useful in new plantings.
When planting, avoid newly cleared forest land or old orchard sites with a known history of Armillaria.
If clearing land, the trees should be killed by ringbarking so that saprophytic organisms outcompete any Armillaria if present so as to prevent the dead tissue from becoming an inoculum source.
If replanting in old orchard sites, the soil must be cleared of as many old roots possible.
When a diseased tree is diagnosed, it should be removed and destroyed.
There is no effective chemical control for the disease.

Typical Armillaria fruiting bodies

Botanic Gardens Trust, Sydney, Australia

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