Mario Chinchilla, his face shaded by a battered straw hat, worriedly surveyed his field of sickly tomatoes. His hands and jeans were caked with dirt, but no amount of labor would ever turn his puny crop into the plump, unblemished produce the country's main supermarket chain displays in its big stores. For a time, the farmer's cooperative he heads managed to sell vegetables to the chain, part owned by the giant Dutch multinational, Ahold, which counts Stop & Shop among its assets. But the co-op's members lacked the expertise, as well as the money to invest in the modern greenhouses, drip irrigation and pest control that would have helped them meet supermarket specifications. Squatting next to his field, Mr. Chinchilla's rugged face was a portrait of defeat. "They wanted consistent supply without ups and downs," he said, scratching the soil with a stick. "We didn't have the capacity to do it.