Soybean and corn futures slumped after tweets from Donald Trump that threaten an escalation of the U.S.-China trade war.

The Bloomberg Grains Subindex Total Return tumbled to the lowest since 1977. The prospect of an end to the crippling tariffs that Beijing imposed on U.S. soy, corn and wheat exports was one of the few positives for American agriculture markets. China’s foreign ministry said that officials were still planning to travel to the U.S. for the next round of trade talks -- but was unable to confirm when amid signs that a delay is now being considered.

“A single Trump tweet erases any optimism of a trade deal this week,” Jacob Christy, a trader at The Andersons Inc., said in a video posted online. “We expect volatility to remain quite elevated until we see some type of clarity on the situation.”

Swollen stockpiles and the spread of African swine fever are also hanging over the U.S. grain and soy markets, even as soggy weather impeded plantings in another frustration for American farmers. July soybean futures headed for a seventh straight loss Monday as the contract slumped to a record low.

Even before Trump’s latest missive, investors were ready to throw in the towel on crop prices. The pig-disease outbreak has forced China to cull more than 1 million hogs, denting the outlook for grain use in livestock feed. A measure of combined hedge fund net-short positions across corn, soybeans and wheat is at its most bearish since the data begins in 2006, a U.S. Commodity Futures Trading Commission report on Friday showed.

Soybeans have been one of the hardest hit commodities by the trade war as China, the world’s top consumer, has snubbed American imports. Heavy rains across the U.S. Midwest mean some farmers may switch to plant more of the oilseed, which can be sown later than corn.

“Fears farmers will plant more soybeans this spring are only exacerbated by the dispute with China that leaves demand for the 2019 crop uncertain,” Bryce Knorr, an analyst at Farm Futures, said in a report.