Farmers had to stop planting their crops over the weekend as the strongest solar storms since 2003 battered the GPS satellites used by self-driving tractors, according to 404 Media. And the issues struck just days ahead of a crucial date for planting corn, one of the US’s biggest crops.

For parts of the Midwest, planting corn after May 15th can lower crop yields, according to the University of Nebraska-Lincoln, particularly as the end of the month nears. Organic farmer Tom Schwarz told 404 Media he chose to delay planting on his organic farm because of the GPS issues but that bad weather in the forecast may delay things further. He said he uses the centimeter-level accuracy of the GPS system to plant his rows so close to his tractor’s path that a human being can’t “steer fast enough or well enough to not kill the crop.”

LandMark Implement, which owns John Deere dealerships in Kansas and Nebraska, warned farmers on Friday to turn off a feature that uses a fixed receiver to correct tractors’ paths. LandMark updated its post Saturday, saying it expects that when farmers tend crops later, “rows won’t be where the AutoPath lines think they are” and that it would be “difficult - if not impossible” for the self-driving tractor feature to work in fields planted while the GPS systems were hampered.

The recent storms are some of the worst to hit the Earth since 2003, according to the National Oceanic and Atmospheric Administration (NOAA), having reached G5, the NOAA’s highest severity rating. That can mean major power grid and communications disruptions. 

Despite that, the effects have been minimal. We haven’t always been so lucky! In 1989, a solar storm knocked out power for hours in an entire Canadian province, while in 2003, bad space weather was blamed for a power outage in Sweden and forced airlines to reroute flights to avoid elevated radiation.

Update May 13th, 11:01AM ET: Article edited for clarity and length.