WHEN PRESIDENT REAGAN arrived in Brazil yesterday at the start of a five-day trip to Latin America, he found a country in dire economic straits. Each subsequent stop during the journey--in Colombia. Costa Rica, and Honduras--will present Reagan with a similarly gloomy picture. The central dilemma for all of these countries is the same--they depend on exports to the United States and other developed nations for economic solvency. But the industrialized world, in the midst of a recession, cannot continue to gobble up Latin American goods and spit out cash or other products in return. Instead, it is erecting more and more exchange barriers--despite all the free trade rhetoric--in order to protect its own products. So the coffee, sugar, fruit and vegetables from the South have nowhere to go.

A quick rundown of the situation lends itself to pessimism. Brazil has a foreign debt of about $70 billion and an inflation rate approaching 100 percent. Colombia and Costa Rica are barely surviving the sharp drop in world coffee prices, their principal export. And Honduras has the distinction of being the second poorest Latin American nation after Haiti.

The President has few viable policy options. He seems likely to reaffirm U.S. commitment to the Carribean Basin plan the White House unveiled last spring. That program had a handful of promising long-range features designed to shore up Latin American economies. For example, most basin exports to the U.S. were to be duty-free, technology transfers were to increase and Washington was going to help bail out financially strapped countries like Honduras and Costa Rica with additional non-military aid. But most of these measures have bogged down in Congress, where senators and representatives give what gifts they can to constituents, not foreigners. The White House itself, in setting up new tariffs on subsidized goods, has gone against the spirit of the Basin Plan.

Ironically, one problem Reagan probably won't spend too much time addressing might hold the key to averting economic disaster. While Latin America has been newsworthy of late for the turmoil of El Salvador, Nicaragua and Guatemala, the President will visit none of those countries, though he will meet briefly with Salvadoran President Alvaro Magana and Guatemalan leader Gen. Efrain Rios Montt in Honduras. The U.S. is spending several hundred million dollars a year in military assistance to prop up governments in El Salvador and Guatemala and to topple the Sand inista regime in Nicaragua. Costa Rica and Honduras, concerned by the Nicaraguan arms buildup, are diverting more and more funds to the military. Most of this money could be put to more productive use as a stimulus for Latin American economies.

IF REAGAN TRIED to negotiate the problems in El Salvador, Nicaragua and Guatemala instead of confronting them head on with muscle, military aid could be cut significantly. Then Congress would have funds to finance the positive measures of the Carribean Basin plan. Similarly, if the U.S. reaches an agreement with Nicaragua, both Costa Rica and Honduras would likely spend less on defense and hence more on immediate economic concerns.

But Reagan has shown little inclination to treat the Salvadoran, Nicaraguan and Guatemalan dilemmas as more than incidents in a larger East-West conflict. Last week, one official in Washington aid that troubles in Latin America came from "leftists, communists, and other subversives." this off-repeated White House line obscures a striking reality about Latin America: The economic chaos Reagan will find during his trop sums up the recent history of El Salvador, Nicaragua and Guatemala as well. But in those countries, the resulting economic, social and political inequities--not a bunch of revolutionary communists--led to upheaval. Reagan hopes to avert a similar mess in the nations he will visit this week. Such a worthy goal might be fostered by paying a little more attention to the problems already on the board.