The flat, fertile fields of the Orange Free State that produce the bulk of South Africa's wheat this year gave forth only stunted, shriveled stalks, resulting in only half the normal yield as this country experienced its worst drought in 20 years.

The unusually dry and sunny skies since February have dealt South Africa's white commerical farming industry, which makes South Africa a food exporter, a severe blow.

This is not comforting news to its black-ruled neighbors to the north, many of whom secretly purchase South African food and may need to buy more than usual because of dought in their own countries this year.

While the South African government likes to remind its critics of vital need it fills on a continent where hunger is common, the drought here has brought into focus a serious problem that South Africa shares with its northern neighbors despite its vast wealth; widespread malnutrition and hunger among its rural, subsistence-farming black population. Many of these blacks have been brought critically near to starvation by the drought.

World Vision, a religious international aid organization, estimates that each year in South Africa 100,000 children under the age of 5 die of malnutrition and related diseases. Most of them are rural black children. The South African Institute of Race Relations computes that in the black rural homeland of Transkei the infant mortality rate is 240 per 1,000 against 69 for urban blacks and 12 whites.

About one baby in four born in the homelands faces death during the first year, usually from hunger or associated diseases, said Trudi Thomas, a white pediatrician working here in the black homeland of Ciskei.

Vast disparity of wealth is a problem found in many African countries. Here, however, it is compounded by the government's policy of apartheid, or racial segregation, that rigidly divides wealth along racial lines and has forced about half the country's black population of 20 million into jobless, crowded "homelands" that make up only 14 percent of the country's land.

The government increasingly recognizes the seriousness of rural poverty that causes malnutrition. Last month, the govenment-established Bureau for Economic Research publicly stated for the first time that per capita income in the homelands is less than in all but 10 African countries.

Prime Minister Pieter W. Botha talks frequently of integrating the homelands' economies with the white-dominated economy, but so far the government has not come to grips with the problem with any significant rural development programs.

Wheat, in which South Africa is self-sufficient, has been the crop most severely hit by the drought so far, diminishing the upcoming harvest to 1.5 million tons from last year's 2.2 million, according to official figures. If the drought continues much longer, South Africa may have to curtail next year's wheat exports, which go to countries such as Malawi and Lesotho, or even import wheat, officials say.

A final decision, however, will only be made after some months.

Fortunately, corn, Africa's main staple food, has not yet been affected by the drought.

For South Africa's sugar industry in Natal, "it's been the worst drought since 1900," said Merlin Morgan of the South African Sugar Association. Output has been cut from 2.07 million tons in 1979 to 1.65 million tons this year. However, this will not cause a shortage or affect exports that go to such countries as Canada or Japan, Morgan said.

The most devastating effects of the drought are seen in the two black homelands of Ciskei and Kwazulu in Natal. There, families are forced to walk miles to rivers for water because their village wells have dried up. Fields have gone barren, depriving villagers of food, and cattle are dying in great numbers.

The Kwazulu national herd of 1.5 million has lost 91,500 head since April this year, almost equal to the loss of 97,000 in the whole year prior to April, authorities said.

Newspaper publicity about the precarious situation of these rural people means that "people are learning for the first time about the real conditions [in the homelands] and they are shocked," said Inka Mars, chairman of a private drought relief effort in Kwazulu headed by the Red Cross. "The drought has dramatized the situation for them," Mars said.

The South African government gave more than $9 million for drought relief to Ciskei, which used it to provide temporary work for six months to 12,000 unemployed villagers to buy food they otherwise would have grown, The South African Army has also been delivering emergency water rations to the rural areas of Ciskei.

But when the temporary jobs run out, private relief efforts ebb and rains end the dryness, widespread malnutrition will still remain, because its roots are in the poverty and the migrant labor system on which the homelands survive.