With Halloween around the corner, any costume shop can tell you what a witch’s broom is supposed to look like – brown, brittle, and gnarly. But as farmers in Vietnam know, when a fruit tree looks that way, it’s usually a bad sign.

Longan – a small, sweet, gelatinous stonefruit protected by a thin shell – is one of Vietnam’s more important fruit crops. It is also an important component of the country’s $2.4 billion fruit and vegetable exports. But longan trees have long been plagued by a syndrome known as “witches’ broom.” Before the Virginia Tech-led Feed the Future Innovation Lab for Integrated Pest Management began working on the problem this year, no one knew why.

Virginia Tech researchers helped solve the mystery by working with a partner in Vietnam. They predict their work may cut the problem in half.

The syndrome makes the trees’ leaves look more like a claw than a leaf and causes malformed flowers that do not develop into fruit. Eventually, affected shoots and flowers dry up and die, leaving remains that inspired the colorful shorthand term. In southern Vietnam, with almost 99,000 acres devoted to growing longan, witches’ broom currently causes roughly a 50 percent annual economic loss.

For more than 25 years, scientists sought to identify the syndrome’s cause. Some thought it was a virus, while others believed it to be the work of other parasites called phytoplasma. Without full knowledge, farmers struggled to institute controls.

This past winter, Virginia Tech’s Vietnamese partner – the Southern Horticultural Research Institute – began working with experts from Virginia Tech to help solve the mystery as part of a project in Vietnam funded by USAID. The project focuses on controlling pests and diseases for four high-value fruit crops, including longan.

Hoa Nguyen Van, who serves as the institute’s director, and Hahn Tran, an entomologist at the institute, worked with Virginia Tech’s Muni Muniappan, director of the Innovation Lab, to investigate the source of the deformation. The team found the culprit: the eriophyid mite. Another research group in Vietnam, the Plant Protection Research Institute, also came to the same conclusion independently.

The eriophyid mite’s saliva is toxic to the tree’s shoots. Thousands of the microscopic pests can live on one longan tree, feeding on its leaves. When the mites attack the fully formed leaves, their gnawing doesn’t cause any distinct symptom. When mites feed on new shoots, however, the witches’ broom syndrome kicks in. Then, new leaves come out brown, malformed, and crinkled instead of flat and green. Eventually the leaves die.  

With the mite identified, Virginia Tech researchers and the institute in Vietnam are working together on a strategy to control the mite and stop its spread. Muniappan said, “Now that we know this syndrome is caused by the eriophyid mite, we can recommend using safe miticides, such as sulphur, in addition to pruning infected areas. Once a strategy is fully developed, we should see a roughly 50 percent increase in longan production in Vietnam.”

The Integrated Pest Management Innovation Lab is a project of the Office of International Research, Education, and Development, part of Outreach and International Affairs.

Written by Stephanie Parker and Dana Cruikshank