Alice Waters, the California chef who helped turn Americans on to seasonal, local and cage-free food, is under attack from some of her own followers who say she has championed fertiliser made of sewage.

Activists from the Organic Consumers Association (OCA) are picketing Waters’s fabled Chez Panisse restaurant in Berkeley today in protest at her failure to condem a fertiliser derived from sewage that, they say, contains industrial chemicals and heavy metals as well as human waste.

The campaign has put Waters in an uncomfortable spot. She is regularly ranked among America’s best chefs, and has worked strenuously to extend good eating from expensive restaurants into daily life. She lobbied hard to get the Obamas to turn over part of the White House garden to growing vegetables, and has encouraged school children to adopt healthy eating habits with her Edible Schoolyard programme in Berkeley.

But those good intentions clashed with activists trying to get greater oversight of a largely unregulated industry producing fertiliser from sewage waste.

The city of San Francisco, which prides itself on promoting greener living, had been giving away the fertiliser to home gardeners and local schools in the name of promoting healthy eating habits, but has now suspended the programme. The fertiliser was made of sewage waste collected from San Francisco and eight other cities. Local authorities labelled the bags as “organic biosolids compost”, although federal government regulations say sewage sludge cannot be used for raising produce that is then labelled as organic.