Crops are "being damaged nationwide" by lockdown walkers avoiding mud, a rural business organisation has said.
The Country Land and Business Association (CLA) said an influx of people walking over planted crops was affecting farmers' businesses.
One farmer said there had been a tenfold increase in walkers during lockdown, while another had seen a 5ft (1.5m) path widen to 36ft (11m) across.
Walking charity Ramblers said people must "stick to marked paths".
CLA president Mark Bridgeman said while he did not want to discourage people from using the countryside, "crops are being damaged nationwide" by those avoiding quagmires.
"[People are] circumnavigating the mud and walking over planted crops, damaging food crops and impacting farmers' businesses," he said.
He said the organisation, which represents 30,000 rural businesses across England and Wales, advised people to get "a decent pair of wellies... and stick to the route of the footpath".
"Farmers are working hard to feed the nation, so let's help them by sticking to the public right of way and following the Countryside Code," he added.
Merseyside farmer Olly Harrison said the number of walkers had increased tenfold since the current lockdown began.
"It's great they're here, but stick to the paths," he said.
"It may look like grass, but it's corn, barley, and wheat [and] once it's damaged, it won't recover."
Buckinghamshire tenant farmer Daniel Hares said he had lost the wheat equivalent of 9,000 loaves of bread, worth about £1,000 and seen the busiest footpath on his farm widen dramatically.
He said he believed the damage was "entirely accidental", caused by "city-dwellers" who just needed an "education" in what was happening.
Oxfordshire farm manager Simon Beddows said it was "extremely understandable" that walkers would want to "take their exercise in the beautiful British countryside", but the combination of heavy footfall and wet weather was causing "lasting damage across the whole country".
He said was also concerned it would "damage the soil below, which will lead to more problems in the future".
"The crops are our livelihood and your food," he said.
"If we get a poor harvest as a result of all this damage, we will all end up paying more in the shops."
The National Farmers' Union deputy president Stuart Roberts said it was a "serious issue".
"Virtually every farmer I talk to who has rights of way on their land is seeing footpaths widening," he said.
However, he said his members were "also really enjoying seeing people appreciating the countryside".
"It's a new population and many of them haven't traditionally walked out here, so they have less experience," he added.
"It is a working environment, so it's important to take care and be mindful of your surroundings."
Gemma Cantelo from the walking charity Ramblers said it was "fantastic" to see so many people discover "the joy of walking", said they must "stick to marked paths on farmland".
"With everyone having to stay local, it can be difficult to avoid the crowds when out walking," she said.
"If you can, head for less popular spots or avoid the busiest times of day if possible, so that narrow paths don't become overcrowded."