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News Navigator: What is the sweet-potato rotting disease spreading in Japan?

Sweet potatoes infected with foot rot disease are seen in this photo, with the lower potato's black rot inside shown. (Photo courtesy of the National Agriculture and Food Research Organization)

The Mainichi Shimbun answers some common questions readers may have about a contagious disease spreading among Japan's sweet potatoes.

    Question: What's this I hear of a contagious disease causing sweet potatoes to rot?

    Answer: It's called foot rot disease, and has since November 2018 been confirmed in Japan's southwestern prefectures of Okinawa, Kagoshima and Miyazaki. It then emerged in the eastern prefectures of Ibaraki and Chiba at the start of 2021. As of early September, cases have been confirmed in 20 prefectures including Tokyo.

    Q: What kind of disease is it?

    A: It is caused by a filamentous fungus. When the disease develops, plant leaves go red or yellow, and the stem's base turns black and dies. Rain or other factors can spread the disease to other plants, making entire fields not harvestable.

    Q: That's scary. Why is it spreading?

    A: The disease has been confirmed in Taiwan, China and the United States, but it is unknown how it entered Japan. Here it seems to have spread across the country through seed potato and seedling distribution. With the exception of the three prefectures where it was first confirmed, so far infections have been contained to a few areas. Still, there's no room for complacency.

    Q: Is sweet potato production stable?

    A: In Kagoshima Prefecture, the nation's top sweet potato producer, the 2020 harvest was down nearly 20% from the previous year. Sweet potato "shochu" brewers are increasingly concerned. Japan's 2020 harvest as a whole was a record low 687,000 metric tons, and the average price per kilogram rose from 490 yen (about $4.40) in April 2018 to 608 yen (about $5.50) in April 2021.

    Q: Is there any way to prevent the disease?

    A: It is difficult to detect if the fungus remains in seed potatoes and seedlings, and at this point, there is no established way to completely eliminate the fungus. Shigetou Namba, a project professor of plant pathology at the University of Tokyo's Graduate School of Agricultural and Life Sciences, said, "The government must share a sense of crisis with producers and take comprehensive, nationwide measures, such as thorough inspections and rotating crops by planting other produce in fields where the disease has been confirmed."

    (Japanese original by Yu Yoshizumi, Kyushu News Department)

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