IZU, Japan >> For three decades, Mitsu­yasu Asada has proudly tended the same lush mountainside terraces where his father and grandfather grew wasabi, the horseradish-like plant with a fluorescent-green hue and head-clearing pungency that unmistakably connotes Japanese cuisine.

Yet at the age of just 56, Asada is already thinking about retiring, worn down by the many threats facing this indispensable condiment that graces plates of sushi and bowls of soba.

Rising temperatures have rendered his crops more susceptible to mold and rot. He worries about unpredictable rainfall, deluging floods and more intense typhoons. The thick cedar forest that blankets the mountain overlooking his paddies — a result of postwar timber policy — has degraded the quality of the spring water that the wasabi needs to grow. Wild boar and deer increasingly attack his fields, driven down the mountains by lack of nutrition at higher altitudes.

And his two adult daughters have shown no interest in succeeding him on his 1.5 acres in Izu, a city in Shizuoka prefecture, about 90 miles southwest of Tokyo.

“If no one will take it over,” Asada said, “it will end.”

Asada is just one of many growers in Shizuoka, one of Japan’s largest wasabi-growing regions, who must confront rising challenges from global warming, untended forests and demographic decline.

Already, these hazards have chipped away at the centuries-old culture of wasabi in the area and imperiled the future of one of the prefecture’s most important agricultural products, and a pillar of its tourism business.

Over the past decade, the volume of wasabi produced in Shizuoka has declined by close to 55%, according to the Ministry of Agriculture, Forestry and Fisheries.

“I have a sense of crisis,” said Hiro­yuki Mochizuki, president of Tamaruya, a 147-year-old company in Shizuoka that processes wasabi to sell in tubes, salad dressings, flavored salts, pickles and even nostril-tickling chocolate.

“In order to protect Japanese food culture,” he added, “it is important to protect wasabi.”

Wasabi plants sprout in spring water that flows down from the mountains, helping to foster gradations of pungency and hints of sweetness. The most well-known Shizuoka variety, called mazuma, tends to sell for 50% more than wasabi from other parts of Japan.

Over time, local growers say, the spring water has deteriorated in quality, compromised by an abundance of cedar and cypress trees.

In an effort to supply Japan with a fast-growing source of lumber to rebuild after World War II, government planners seeded mountain tracts exclusively with the cedar or cypress. But as cheap wood imports supplanted Japan’s lumber in the 1960s, the trees were left to grow, crowding out other plants that would better contain and nourish the mountain springs that wasabi needs to thrive.

“People talk about climate change and how there is less water,” said David Hulme, a retired Australian journalist who grows wasabi in Okutama, about 50 miles from Tokyo. “But the real problem is that the hills are not holding the water long enough.”

Global warming has upset the balance even further. The delicate wasabi plants, which take more than a year to mature, do best in conditions no hotter than about 70 degrees Fahrenheit. In recent years, heat waves have regularly pushed temperatures into the 90s and beyond, causing more stalks to rot.

On a recent afternoon, Masahide Watanabe, 66, a fourth-­generation grower, stepped into one of his paddies. With a small hoe, he dug a wasabi plant from the mud, unearthing a pockmarked green rhizome sprouting leaves shaped like water lilies.

He rinsed the plant in flowing spring water, chopped off the leaves and a tangle of roots, and inspected the body for blemishes.

“Sometimes the plant will be missing the stems that grow out of the top,” he said. “We call it ‘headless syndrome.’” Such diseases have grown more frequent with rising temperatures.

Government researchers and local growers are experimenting with crossbreeding to develop hardy varieties that will thrive even in the rising heat.

The challenge is that, unlike other crops, extracting seeds and growing seedlings from wasabi require sophisticated technology. Most growers rely on specialized companies to clone seedlings. Crossbreeding entails complicated pollination efforts and, most of all, time.

Hope could yet come from people such as Haruhiko Sugiyama, 44, who recently started his own wasabi-growing operation in Izu. He leases a half-acre of paddies from a retired grower.

A dozen years ago Sugiyama, the son of grocery store owners, decided he wanted to work outside. A middle school friend who descended from a long line of wasabi growers connected him to a farmer who needed help.

Yet to reach the point where he could start his own operation, Sugiyama had to prove his worth to the local growers association, which controls access to wasabi fields. In 12 years working for another grower, Sugiyama said, he never took a day off while learning every step of local growing techniques.

“In a way it is a closed society, made up of people who have grown wasabi for generations,” said Sugiyama, who was ultimately granted approval to take over abandoned paddies. “If I were not recognized by the association, they would not help me or allow me to grow on favorable land.”