Dimocarpus longan, or longan, is the third most cultivated and second most exported fruit crop in Vietnam. In 2017, the tropical fruit’s export value was $62.13 million, a significant contribution to Vietnam’s economy. (Photo credit: Sara Hendery)

By Sara Hendery

Dragons are known to have copious enemies who threaten their territory; longan, translated to mean “dragon eye,” is no different.

Sara Hendery

Circular, gelatinous, and with a shell that can be cracked like a hard-cooked egg, Dimocarpus longan, or longan, is the third most cultivated and second most exported fruit crop in Vietnam. In 2017, the tropical fruit’s export value was $62.13 million, a significant contribution to Vietnam’s economy. However, longan pests and diseases can cause up to 100 percent crop loss. Shipments of longan from Vietnam, in fact, have been intercepted in U.S. ports due to the presence of crop threats like Drepanococcus chiton, a pest that leaves mold behind.

In an article published this week in the open-access Journal of Integrated Pest Management, researchers profile longan’s most aggressive pests and diseases in Vietnam. The article is one of the first to outline comprehensive data on the fruit outside of the South Asian context.

“Longan is not a well-known fruit and there is not a lot of literature available on it in the U.S.,” says Muni Muniappan, Ph.D., director of the Feed the Future Innovation Lab for Integrated Pest Management and one of the article’s authors. “Since longan mostly grows in Vietnam, China, Taiwan, and Thailand, the literature available is written for those contexts and languages.”

Hanh Tran (standing) speaks to a group of longan farmers in Vietnam about integrated pest management techniques for the fruit. They were joined by Muni Muniappan, Ph.D. (foreground), director of the Feed the Future Innovation Lab for Integrated Pest Management. (Photo credit: Sara Hendery)

In recent years, longan’s popularity has risen in U.S. and European markets. This emergence underpins a collaboration between the IPM Innovation Lab and the Southern Horticultural Research Institute (SOFRI) in Vietnam to improve the export of certain high-value tropical fruits, longan included.

In the article, authors from varying institutions—the IPM Innovation Lab, SOFRI, West Virginia University, Washington State University, University of California Davis, and University of California Cooperative Extension—cover 10 pests and three diseases that threaten longan.

Some of the pests profiled in the article include moths such as Conopomorpha sinensis or the oriental fruit fly (Bactrocera dorsalis), but chiefly highlighted is a crop affliction SOFRI and the IPM Innovation Lab have long worked to resolve: longan witches’ broom (LgWB).

After many years assuming the causative organism of LgWB was phytoplasma or a virus, the two teams uncovered it was in fact a small white mite, Eriophyes dimocarpi. The mite’s saliva, toxic to the tree’s shoots, causes the curled, malformed leaves harkened in the witches’ broom name.

“Failing to control longan witches’ broom in the Mekong Delta region could spread it to rambutan, another important tropical fruit,” says Hanh Tran, Ph.D., a co-author on the study and whose work was also instrumental in identifying the mite.

The saliva of the mite Eriophyes dimocarpi is toxic to the the longan tree and causes the signature curled, malformed leaves of the affliction called witches’ broom. (Photo credit: Feed the Future Innovation Lab for Integrated Pest Management)

After many years assuming the causative organism of longan witches’ broom was phytoplasma or a virus, researchers uncovered it was in fact a small white mite, Eriophyes dimocarpi. The mite’s saliva, toxic to the tree’s shoots, causes the curled, malformed leaves harkened in the witches’ broom name. (Photo credit: Naidu A. Rayapati)

One technique for protecting longan from pests is bagging the fruit as it grows on the tree. (Photo credit: Hanh Tran)

In Vietnam, the Mekong Delta region is a vital economic center that produces nearly half of the country’s tropical fruits. With the mite now identified, SOFRI and the IPM Innovation Lab are working to develop Integrated Pest Management (IPM) strategies for controlling its spread, predicting to cut damage by 50 percent.

The article highlights several IPM methods, like fruit bagging, to combat longan threats. Longan bagging not only abates fruit borers, but it also limits the need for synthetic chemicals that can threaten natural enemies beneficial to crop health.

Some additional IPM best practices listed in the article include:

• Fertilize the trees with compost inoculated with the antagonistic Trichoderma fungus.
• Set up methyl eugenol traps and protein bait for controlling fruit flies.
• Apply Beauveria bassiana, Paecilomyces sp.,orMetarhizium sp. for controlling stink bug.
• Spray sulfur, neem oil, petroleum oil, or Paecilomyces sp. to control longan eriophyid mite.

With mango now being exported from Vietnam to the U.S. for the first time after years of planning and deliberation, prospects for exports of other fruits is optimistic. Just as long, of course, as all the fruits’ enemies are kept at bay.

Sara Hendery is the communications coordinator for the Feed the Future Innovation Lab for Integrated Pest Management at Virginia Tech. She holds an MFA in nonfiction from Columbia College Chicago. Email: saraeh91@vt.edu