UH COLLEGE OF TROPICAL AGRICULTURE AND HUMAN RESOURCES A team of UH researchers has discovered a rare pathogen spread by an insect alien to Hawaii that is ruining watercress crops on Oahu. Above, Cooperative Extension Service agent Randy Hamasaki recently collected samples from a ruined crop.

Pest threatens
Oahu watercress

UH scientists discover an insect
spreading a crop-killing disease

---

Star-Bulletin staff

Oahu consumers may find locally grown watercress in short supply because crops are being destroyed by a rare pathogen spread by an insect alien to Hawaii.

The mysterious crop-killer was identified in a complex detective effort involving a team of scientists at the University of Hawaii-Manoa College of Tropical Agriculture and Human Resources.

"It's like a new disease here," said Steve Fukuda, Cooperative Extension Service agent, who led researchers in hunting for the watercress foe after seeing failing plants at one farm.

Scientists pursued various suspects before Wayne Borth, working in virologist John Hu's lab, confirmed aster yellows phytoplasma in the sick watercress and some adjacent weeds.

Fukuda and Randy Hamasaki, also an Oahu Cooperative Extension agent, knew an insect had to be spreading the disease, and began scouring the fields. Hamasaki found an unfamiliar leafhopper in the margins of a watercress paddy.

UH COLLEGE OF TROPICAL AGRICULTURE AND HUMAN RESOURCES Agent Steve Fukuda, tracked the problem from its beginning, while researcher Wayne Borth provided analysis.

Dick Tsuda of the college's Agricultural Diagnostic Service Center made a preliminary identification, and colleagues at the Department of Agriculture obtained confirmation from a mainland expert that the insect was the aster leafhopper.

Both the pathogen and insect are newcomers to Hawaii.

Fukuda said about half of the watercress crops on Oahu have been affected, and "it's getting worse. We expect this disease to continue to spread."

He said it is a bacterialike organism that acts like a virus. Like ringspot disease on papaya trees, once watercress has the organism, "nothing can be done in a practical way," he said.

Watercress growers have been advised to destroy infected crops and weeds and control the leafhoppers.

WATERCRESS FACTS >> About 45 acres of watercress were harvested statewide in 2000, according to the Hawaii Agricultural Statistics Service Branch. >> A breakdown was not available for the number of acres per island, but most of the crops are on Oahu. Production totaled 970,000 pounds, with a farm value of $1,067,000 and an average farm price of $1.10 per pound. >> Multiple watercress crops can be grown annually. Crops are harvested, regenerated and harvested again, with about five to six weeks between harvests, depending on the time of year.

Desmond Ogata, pathologist in the agricultural diagnostic center, said Borth still is conducting tests, and crops are under surveillance by the UH and state Department of Agriculture researchers.

Oahu has about a dozen watercress farms, mostly behind the Leeward Community College area, where the disease is spreading, Ogata said.

Borth also found signs of the disease in the Sumida watercress farm in Pearl City, although the crops so far have escaped damage, Fukuda said.

There is no danger in eating watercress that might be infected because aster yellows phytoplasma is a plant disease, the scientists emphasized.

The first farm infected lost most of its crop, and the disease spread to a second farm, with a 50 percent crop loss within two months, the scientists reported.

The outbreak, confined to Oahu thus far, occurred about a year ago, Ogata said.

Many faculty members were consulted for their expertise in water quality, plant nutrient management, soil chemistry, salinity and other areas. Ray Uchida, manager of CTAHR's Agricultural Diagnostic Service Center, coordinated the teamwork.

Despite extensive research to find an answer to the watercress plight, Ogata said: "Our hands were up in the air. We couldn't really pinpoint it."

The scientists eventually sought help from Borth, who was studying a phytoplasma that infects a native Hawaiian plant.

Tests still are going on, Ogata said, pointing out at least 300 different plant species can be infected by the unusual pathogen. "County agents are alerting vegetable growers to keep an eye out for abnormal growth where issues take on a more yellow color and a more distorted shape."

Fukuda expects all watercress fields eventually will suffer the disease.

If it is similar to the disease known on the mainland, he said, it could affect economically important vegetable crops here.

"Talk to us maybe next month, and they may be in chrysanthemums or tomatoes," he said. "We haven't identified any yet but there is potential."