Even in China’s remotest places, relentless overproduction—here it is mushrooms and cement trucks—is clouding the country’s path to prosperity and jolting the global economy. When 48-year-old farmer Yang Qun began trading at Suizhou’s bustling morning mushroom market a half decade ago, the fungus industry was expanding, even attracting a rural lending arm of British financial giant HSBC Holdings HSBC 1.47 % PLC. Ms. Yang saved enough to buy a minivan. When wet snow fell last month, she was settling for closeout prices to unload six bags of dried mushrooms that took a half year to cultivate. For Xu Song, a clanging sound was once a welcome reminder that his 200 colleagues were busy pounding steel into the giant barrels used on cement trucks. On a recent day, he sat alone watching videos in an unheated office, the only worker left on an abandoned factory floor where dozens of […]