Bark canker is a debilitating fruit tree disease and is appearing more frequently in orchards and back yards in Queensland. The disease is also prevalent in some areas of S.E. Asia and Papua New Guinea.

Symptoms start with areas of raised cankerous bark growth on the main trunk and gradually it spreads to the outer branches. The affected areas are flaky at first and become nodules of corky bark in the more advanced stages, and may cover most of the bark surface.

The tree loses vigour and cropping is markedly reduced. The areas of bark canker are infested with fungi and even small larvae, when conditions are right.

The species most susceptible to bark canker are: Duku and Langsat varieties (Lansium domesticum), Pulasan (Nephelium mutabile), Rambutan (Nephelium lappaceum) and some ornamentals such as hibiscus.

Amazingly, researchers have spent years trying to determine whether the cankerous growth is caused by the fungi or the larva, when in fact it is caused by excessive fertilizer. Even organic fertilizers such as fowl manure and blood and bone will bring on the symptoms, if applied excessively - particularly in hot, humid weather conditions.

The underlying cause is due to growers trying to make their trees or plants grow faster than nature intended by applying extra fertilizer. There may be good economic reasons for faster growth and larger crop yields, however, most plants can't tolerate the higher chemical levels, and within months, some tissue break-down occurs, and disease symptoms appear. Only a few plants I have encountered can tolerate high levels of applied nutrients without serious effect.

The cure for bark canker was found quite accidentally when one of our duku-langsat plants - still in a pot - developed cankers on the trunk. It was put into an area where it was neglected, and received no fertilizer for two years. miraculously, the symptoms disappeared.

There is a good example of a langsat tree in the suburb of Edge Hill in Cairns, which is now recovering from a severe case of bark canker. The cure is simply to withhold all fertilizer.

When bark canker occurs, the treatment is to withhold all forms of fertilizer for at least two years. The cankers will clear from the trunk first and eventually from the outer branches. After the cankers and flaky bark have completely cleared up, fertilizing may be recommenced, but only light applications, not more than twice per year.

Over the years I have seen some interesting results from excessive fertilizer application. Some trees have developed Phytophthora root rot and some plants have developed Erwinia and yet others have succumbed to Fusarium wilt. Some of the more tolerant species only develop rust or leaf spot.

John Robert Marshall, Whitfield, Cairns

DATE: February 1998

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