A contagious disease is spreading among Japan's sweet potatoes. It's called foot rot disease, and since November 2018 it has 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.
The disease 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.
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.
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 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.