Mounting fears of September bringing this autumn's first frosts in the US have sent Chicago crop prices sharply higher, with corn jumping 9% to fractionally below its daily limit.
The prospect of a cold front hitting major Midwest growing districts later next week sent traders rushing to cover short positions in the major crops, reversing a slide which sent corn to a 2009 low last week, and wheat to levels not seen since April 2007.
World Weather said that frost could affect parts of Illinois, Indiana, Iowa and Nebraska, although many other services were less specific.
Meteorlogix, in its six-to-10 day outlook, said that Midwest temperatures would cool to "near to below normal".
Vic Lespinasse, GrainAnalyst.com's Chicago marketwatcher, said: "Corn and beans are so far behind normal in development that even a freeze in late September will cause damage to these crops.
"This prospect is causing shortcovering as well as keeping some would be sellers out of the market, enabling prices to rally."
However, he added: "Some forecasters still doubt it will get cold enough to do significant damage to the still developing corn and bean crops. I think [prices] are overdoing it to the upside."
Temperatures are viewed as needing to fall to about minus 2 degrees centigrade, or 28 degrees Fahrenheit, for two hours to cause much harm.
December corn, in its first day as the near-term contract following the expiry of the September lot, ended 28.25 cents, or 9.1%, higher at $3.46 a bushel, earlier rising the full 30 cents allowed by exchange rules in a day.
This was corn's highest finish since early August, when concerns of benign weather and huge yields were taking their toll.
November soybeans - which earlier dropped to $9.01 ½ a bushel, their lowest since the end of March – rebounded to closed up 5.6% at $9.60 ½ a bushel.
These performances helped December wheat finish up 3.6% at $4.70 ¼ a bushel, its highest close this month, after hitting $4.81 ½ a bushel at one stage.
And this despite the potentially bearish news of a rise in Australia's wheat crop estimate, and Egypt giving Russia a clean sweep in a tender for 240,000 tonnes, which had earlier helped Paris milling wheat to its lowest in three years.
Paris's November contract recovered to close up E2.00 at E121.025 a tonne, with London wheat for November ending up £1.25 at £93.00 a tonne.