Angsana wilt disease complex

Introduction

Wilt of Angsana trees Pterocarpus indicus, while not a disease problem in Vietnam is a good example of a disease complex where insects have a key role in the disease cycle. It has caused serious problems in Singapore and occurs in Malaysia.

Angsana trees are commonly grown as amenity trees along avenues and in parks and gardens. Studies have indicated that this disease is a complex between lightning strike injury, ambrosia beetles, Platypus prallelus and the vascular wilt fungus Fusarium oxysporum. F. solani has also been associated with the diseased trees but is not thought to be the primary cause of the wilt.

Symptoms

When lightning damages a tree, the leaves dry and turn brown. Areas of the outer bark at the site of the strike peel off (30 x 100cm). Thin vertical cracks appear down the trunk (10cm long). Small areas of bark explode to appear stranded. Large areas of bark peel off, perhaps down half of a branch.

Within days of the strike, trees may be colonised by the ambrosia beetles. The gallery system that the beetles create consists of a radial entrance tunnel, then a turn, with only one entrance and vertically arranged chambers in rows. The tunnels are often stained black. Four to six weeks after the first successful boring, adult beetles begin to emerge. Each tunnel holds one adult pair that can produce 50-80 eggs.

The beetles may carry the pathogen from diseased trees to other trees affected by lightning strike, establishing new infections leading to wilt. The leaves of diseased trees gradually yellow, wilt and the tree leaves defoliate. Vascular tissue is discoloured. The pathogen can be isolated from all tissue.

Bark rot occurs as the bark and cambium layers collapse and F. solani can also be isolated from the leaves. Neighbouring trees become infected through local dispersal of the pathogen in soil or water or root grafting.

Distribution

This disease complex is not known to be present in Vietnam.

Angsana trees are known to grow in Malaysia, Philippines, Myanmar, Solomon Islands, Papua New Guinea, Java, Sulawesi Islands and Molucca Islands. Angsana Wilt has caused significant losses of amenity trees in Singapore and is the origin of much of the research into the disease complex.

Host Range

Angsana trees, Pterocarpus indicus

Epidemiology

The biology of the ambrosia beetles makes it almost inevitable that they would disseminate spores of F. oxysporum into trees that they colonise. Ambrosia beetles have features such as hair tufts or chitinous pockets that can carry fungal spores.

Secondary spread from the primary infection site occurs with F. oxysporum colonising the next tree through the soil. This may be aided by water movement, especially if roots lie in stormwater drains below city streets, or root contact and grafting with other closely planted trees.

Control

Ideally control measures should include the immediate removal of trees affected by lightning strike before beetles have the opportunity to colonise the damaged trees. The rapid removal of trees already infected will reduce the population of beetles and remove the primary source of inoculum before it establishes itself in the soil for secondary spread to take place.

Healthy trees in the immediate vicinity of infected trees have a high risk of infection by this soil-borne pathogen. The healthy trees can be treated with injections of suitable fungicides to reduce the risk.

Monitoring the disposal of infected tree parts is also important to minimise the dispersal of the beetle to uninfested areas. The beetle itself only has the ability to migrate one mile in a direct flight.

Early symptoms of disease