WASHINGTON — Despite some economic gains by developing nations during the past two decades, the lot of rural women - the backbone of many third-world economies - has worsened."It is the women who suffer the greatest impact from current economic conditions," says Idriss Jazairy, president of the International Fund for Agricultural Development (IFAD), a Rome-based UN agency that is trying to assist them. The number of rural women living in poverty - 550 million - has increased by 50 percent since the early 1970s, according to the findings of an IFAD investigation released yesterday. The vast majority are farmers who produce most of their family's food supply. Ironically, women have fallen into increasingly distressed circumstances even as the premium on their labor has grown. As rural poverty has increased, larger numbers of rural men have migrated to cities or abroad to find work. The women left behind have to raise both families and food. Despite the increasing feminization of rural third-world agriculture, most governments, banks, and even foreign donors have made it harder for women to succeed, IFAD says. Land tenure and inheritance laws, for example, discriminate against women. Since they cannot own land, women are unable to gain access to credit. Meanwhile, agricultural and marketing assistance usually goes to men or is geared to large, capital-intensive farms. Mr. Jazairy says rural development efforts have faltered because they treat women as an isolated problem rather than an integral part of the economic system. The way to help women and to increase food production at the same time is to give women equal opportunity to succeed. IFAD, which funds rural development projects in over 90 countries, is improving access for women to credit, technology, and marketing services.