Early detection of oil palm resistance to a devastating fungus

Oil palm is the most widely produced and consumed vegetable oil in developing countries. With around 3.3 tonnes per hectare per year, it is 7 or 8 times more productive than soybean oil. Palm and soybean are the most widely consumed oils worldwide. Indonesia and Malaysia are the world’s leading palm oil producers, with 12 and 14 million tonnes of oil per year respectively. However, a soil fungus, Ganoderma boninense, which causes basal stem rot in oil palms, devastates thousands of hectares of plantings in Southeast Asia every year.

The Ganoderma problem has been known for decades, but the search for solutions has been considerably hampered by a natural constraint: the disease does not cause symptoms until the palms are at least 7 to 15 years old. Until now, the only control method has centred on cropping techniques, notably ploughing before planting and felling of diseased palms during the growing cycle. However, this has had limited results.

In 2001, trials on palms planted between 1974 and 1993 showed that in Indonesia, the mortality rate differed between the main genetic origins used, and also within those origins. The idea was thus to develop an early test of susceptibility to the disease, with a view to selecting sources of resistance as soon as possible.

To this end, it was necessary to find a means of inoculating the fungus artificially, to ensure rapid symptom expression. This was a tricky business, as disease expression is governed by numerous factors: the aggressiveness of the pathogen strain, the form in which the fungus is inoculated, the time taken to prepare the Ganoderma before inoculation, and certain environmental factors (light, temperature, humidity, etc). Moreover, the aim was to obtain disease symptoms on palms as young as possible, in a uniform, reproducible way. It was also vital to be able to correlate the results with field observations.

Three years on, researchers have risen to the challenge: it is now possible to control inoculation of the fungus, and symptoms can be achieved in palms barely three months old.

The technique developed consists in using a rubber log prepared in a predetermined way. The fungus is grown on the log for 12 to 16 weeks. The log is placed in a pot and covered with soil in which a germinated oil palm seed is planted a standard distance from the log. The inoculated seedlings are placed in semi-natural shade until disease symptoms occur. The method is both quick and easy. As in the field, trials have revealed differences in mortality rate between genetic origins and in aggressiveness between Ganoderma strains.

What remains is to validate the inoculation protocol on a large scale so as to test thousands of seedlings per year and develop a reliable, quick selection tool. Once this has been achieved, which should shortly be the case, it will be possible to supply plantations in Southeast Asia with material with a satisfactory level of resistance to Ganoderma.

As the test can induce the disease both quickly and reliably, it also opens the way for research into the diversity of Ganoderma strains and the efficacy of certain antagonistic fungi or fungicides against Ganoderma. In the long term, the tool should be of use in developing an integrated control method combining cropping practices with genetic, biological and chemical factors.