Employees dig up sweet potatoes in a farm in Brits, near Pretoria, South Africa.
Employees dig up sweet potatoes in a farm in Brits, near Pretoria, South Africa. - 
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Steve Chiotakis: You may find yourself out in the garden in these dog days of summer, fighting a relentless battle against weeds in your tomato or cucumber patch. So pity the poor subsistence farmer. He or she faces huge losses from weed infestation, according to data out this week from the United Nation's Food and Agriculture Organization. Marketplace's Mitchell Hartman has more.

Mitchell Hartman: A familiar sound in suburbia -- the electric weed-whacker. But it's not going to cut it out in the fields, where farmers face an overwhelming task getting rid of weeds that can destroy entire crops.

The U.N. says farmers worldwide lose $95 billion a year to weeds. And it's worst in the rice, sorghum, and millet fields of the developing world. Alan Watson is a weed expert at McGill University in Montreal.

Alan WATSON: Subsistence farmers are left with trying to deal with weeds with their hands.

In Africa, for instance, farmers spend more than half their field-time weeding. Watson says sophisticated herbicides, and crops that are bred to be immune to them, are an option for farmers in rich nations. But not in much of Asia, Africa, and South America.

WATSON: Very often, if they were going to use herbicides they're not available or they can't afford them.

The U.N. reports that weeds cost farmers even more than plant diseases. And they cancel out about twice as much food production as insects do.

I'm Mitchell Hartman for Marketplace.

Follow Mitchell Hartman at @entrepreneurguy