II History of Sweetpotatoes
(1) Sweetpotato is considered to have originated in the tropical Americas, most probably Mexico, where there are wild species of the Convolvulaceae family closely related to sweetpotatoes. Japanese researchers have determined, through morphological variation, genetic properties and chromosome numbers, that the modern sweetpotato derives from Ipomoea trifida (H.B.K.) G. Don.
(2) Ipomoea trifida is thought to have produced numerous variations through mutations and mutual crosses over time. After subsequent artificial selection, the modern sweetpotato was born. Today, sweetpotatoes are cultivated extensively around the world, including in tropical, subtropical, and temperate regions.
(1) Sweetpotatoes may have been introduced to Japan through any of several routes. They are known to have been brought to Polynesia by sea from South America around 1,000 B.C. Historical records indicate that sweetpotatoes were introduced into India, Malaysia, Indonesia, and the Philippines in the 16th century, reaching Fujian, China in 1584.
(2) Sweetpotatoes first reached Japan at Miyakojima Island, in 1597. In Ryukyu, sweetpotatoes were brought from Fujian, and began to be cultivated there in 1605. Starting in 1609, they were brought to Satsuma, old name of Kagoshima Prefecture, ollowing its rule by the Satsuma clan, and were cultivated mainly in Kyushu. Sweetpotatoes gained a reputation as an emergency crop during the repeated famines of the early and middle Edo era, and became widely planted across Japan, centering on the nation's warm southwestern area.
(3) The ancient Bansho-Ko text describes a cultivation method used for sweetpotatoes in the middle Edo era, when Konyou Aoki introduced them to the Kanto area. According to this text, the optimal cultivation method was unknown, so sweetpotatoes were cut like yamanoimo (yams) and the cuttings were planted. This method, however, led to problems with rot, and low yields, so there was a gradual shift toward the method used currently: sprout transplantation.
(4) In the late Edo era, sprouts were already being raised in nursery beds. Modern practices such as fertilization, ridging, sprout collection and transplanting, and storage were also employed. In fact, yields were a fairly high 1500 kg/10 a., not so different from current levels.
3. Current cultivation practices
(1) According to 1998 data, about 8.87 million ha of sweetpotatoes are planted worldwide. Total production is 130 million tons, or about half that of potatoes. Asia accounts for 90% of world production, with China alone producing some 110 million tons, or 86% of world production. Other major producing countries include (in order) Indonesia, Uganda, Vietnam, India, Japan and Rwanda.
(2) In Japan, sweetpotato production rose sharply after World War II (peaking at 7 million tons in 1955), but subsequently declined as the Japanese diet became more stable. At present, less than 45,000 ha in Japan are dedicated to sweetpotato cultivation; annual production is about 1.0-1.1 million tons. Despite a decline in production, however, sweetpotatoes are used in a wide range of markets, including fresh produce, processed foods (including confectioneries), and starch-related products. In upland areas of the Kanto district and southern Kyushu, sweetpotatoes remain an important crop essential for the promotion of local industries.