WASHINGTON — Neofusicoccum ribis, one of more than 90 fungi in the genus Fusicoccum, is causing leaf fall disease among Hevea brasiliensis trees in Indonesia.
How much of a problem this is creating, however, depends on whom you ask.
"The future of the Indonesian Hevea remains at risk," said Kavickumar s/o Muruganathan, vice president, environment, health & safety at Halcyon Agri Corp. Ltd., in an October 2018 article titled, "The Sorry State of the Indonesian Hevea."
"Vast swathes of rubber trees have already been lost to the Fusicoccum pathogen, and more losses are due until a concrete solution is found to combat this dilapidating disease," Mr. Muruganathan wrote.
The March 2019 issue of Natural Rubber Trends & Statistics, the monthly publication of the Association of Natural Rubber Producing Countries, suggested the Fusicoccum fungus was a problem affecting natural rubber (NR) supplies in Indonesia.
In that issue, ANRPC Secretary General Nguyen Ngoc Bich noted that world NR production fell 5.2% in the first quarter of 2019, with sagging prices and a long wintering season partly to blame.
"The new leaf fall disease in South Sumatra might also have contributed to the drop," Mr. Nguyen wrote.
But other sources within the NR industry said they doubted that the Fusicoccum fungus was spreading quickly or having much of an effect.
"Yes, there is a leaf blight in Indonesia, but it's been contained in the area where it is," said one industry source who asked to remain anonymous.
The information available on Indonesian leaf fall disease in the West is fragmentary and often contradictory, according to Alan L. Grant Polymer.
According to the estimate now circulating, about 100,000 hectares (slightly less than 250,000 acres) of NR-growing land in Indonesia has been affected by the fungus, said the Norfolk, Va.-based rubber supplier.
"This represents 3% of Indonesia's mature growing area, meaning impact on Indonesian production is only 3%," the company said. "(But) other sources are claiming Indonesian production is down 10-30%."
Neofusicoccum ribis is not related to the Microcyclus ulei ascomycete that caused leaf blight in Brazil, effectively ending that country's NR producing industry.
It is a member of the Botryosphaeriaceae family of sac fungi, which are common parasites on a large variety of plants.
Fusicoccum species were isolated from infected leaves collected from rubber plantations in three widely separated locations in Malaysia in 2010, according to a paper published in The Plant Pathology Journal in 2013. The fungus was in Malaysia as early as 1987, according to various reports.
Leaf fall disease first occurred in North Sumatra in 2016 and had spread to South Sumatra by the following year, according to Mr. Muruganathan.
It then spread to areas in South Kalimantan, Java, Lampung and Central Sulawesi, he said.
"Fusicoccum leaf fall disease usually attacks young leaves," he wrote. "The first symptom is brown spots on the leaves. The small spots will change from light to dark brown."
Severely infected leaves will dry and fall or become abnormal on the branches of the tree, according to Mr. Muruganathan. Infected leaves fall about four months after infection, he said.
"The occurrence of Fusicoccum leaf fall disease can also have far-reaching effects as a result of international trade, as they may be spread undetected from one area to another, causing potentially serious damage to hosts that might have no co-evolved resistance," Mr. Muruganathan wrote.
As of October 2018, research into Fusicoccum was limited, and there were no studies considering its potential economic impact, according to Mr. Muruganathan.
Applying fertilizer to Hevea trees to balance their nutrients is generally the best way to prevent leaf fall disease, according to various sources. However, the current low prices for NR have discouraged careful upkeep of rubber trees.
"People aren't fertilizing the trees enough to fight the disease," the anonymous source said.
Halcyon Agri did not respond to an email requesting further information on leaf blight disease.