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Five Angsana trees return to Esplanade Park

Fungus-resistant angsana trees replace those brought down by massive disease outbreak

Their magnificent crowns give instant shade on the hottest of days to people scurrying on roadside pavements.

Their fan-like seeds scatter to lie unobtrusively on the ground.

Looking at the five Angsana, or Pterocarpus indicus, trees recently re-homed at the Esplanade Park from a field in Bidadari, where they were cultivated, it is hard to imagine the leafy giants that used to stand in their place which were ravaged by a fungal disease that left them for dead, their leaves yellow and branches brittle.

Those five earlier Angsana trees had sheltered a popular dating spot known as "gor zhang chiu kar" in Hokkien, or "under the five trees", from the 1960s to 1980s before they were infected and subsequently removed.

 

For three years in the early 1990s, a team of scientists from the National Parks Board (NParks) studied and experimented with seeds and cuttings from hundreds of Angsana trees in the South-east Asian and Oceania regions, in search of those resistant to the deadly disease fusarium wilt, commonly known as angsana wilt.

The disease killed by obstructing the water transport system of the trees, causing them to wilt and die.

Between 1988 and 1995, more than 800 infected Angsana trees were chopped down all around Singapore to prevent its spread.

Now more than two decades later, those five disease-resistant Angsana trees, weighing an average of 8 tonnes and standing proudly at Esplanade Park, are a testament of the research - bred to be immune to the disease.

Over a month, NParks uprooted them from Bidadari and re-planted them at Esplanade Park. It was the first time mature disease-resistant Angsana trees were transplanted there.

According to the agency, about 200 more disease-resistant trees have been planted, such as those lining Upper Serangoon Road, Toa Payoh Lorong 8 and Lim Chu Kang Road.

At the peak of its spread between 1989 and 1995, the disease would kill an average of 28 Angsana trees on the island in a month.

Trees 30 years of age were wiped out in months, others in as little as just six weeks.

Luckily, some were spared. Currently, some of the Angsana trees lining our roads are about 50 years old and they are still very healthy.

"We were losing some of our biggest, most mature Angsana trees. Some, I recall, then at Alexandra Park had trunk circumferences of 6m to 7m," said Mr Oh Cheow Sheng, director of streetscape at NParks. "It was a major concern."

The disease was first reported in Malacca, on the south-west coast of Malaysia. Between 1870 and 1880, it wiped out a large avenue of trees which adorned the seashore.

In 1914, it appeared on Pulau Brani, an island in the port of Singapore, before moving inland to infect Angsana trees in Connaught Drive and Dhoby Ghaut.

In the years that followed, sporadic infections occurred around Singapore before the massive outbreak began in 1988.

Dr Fong Yok King, assistant director at NParks' Centre for Urban Greenery and Ecology Research, was part of a team doing a survey on the disease when it broke out.

"We were getting too many reports back to our plant health section, 'this Angsana tree is dying, that Angsana tree is dying'," she said.

"Losing 28 trees a month is very high for one single species."

This prompted an in-depth look into the cause, life cycle and spread of the disease to come up with ways to control its spread.

The goal: to identify the weak link in the disease cycle and to take advantage of it to control its spread.

Over the course of the research, they discovered a curious link between the dying trees, lightning strikes and a species of beetle.

The theory was that when lightning struck and damaged an Angsana tree, it attracted ambrosia beetles to the tree, bringing with them the disease-spreading spores that result in angsana wilt.

NParks tried various control methods, including the removal of diseased trees soon after they were diagnosed.

Trees surrounding them were treated with chemicals to reduce the spread of the fungus.

However, this method proved expensive in the long term, and ineffective at times.

Sometimes, the fungicide was injected into the trunks but it would get clogged up by the sap produced by the trees.

So NParks began testing Angsana trees collected from around the region to see if they were resistant to the disease.

The plants were exposed to the fungal pathogen.

Those which survived three inoculations were grown to around 2m to 3m in height before they were transplanted into soil along the roads infested with the disease for a final confirmation of resistance.

Those which survived were considered resistant.

"They have been there since 1997 and none of them has shown symptoms of infection," said Dr Fong.

But even now, the battle with the disease is not completely over as there are still traces of it in the environment.

There are still occasional cases of infection, although the number remains very small and manageable, said Dr Fong.

This is why NParks is still growing its next batch of "soldiers" in its Pasir Panjang nursery.

"There are still some spores here and there, which is why it is good to have some resistant stock for replacement planting," said Dr Fong.

"We want to be prepared."

A version of this article appeared in the print edition of The Straits Times on November 27, 2015, with the headline 'Leafy giants return to Esplanade Park'. Print Edition | Subscribe