The US President says the countries' weak currencies and cheap exports are harming US farmers.Read more
The former campaign strategist Alberto Fernández won nearly half the vote.
The Argentine peso has fallen more than half a percent today, with roughly 60 pesos to the US dollar.
In an effort to boost the peso, the country's Central Bank has said they will sell US dollars to the tune of $800m (£624m).
Argentines will go to the polls this Sunday with a swing to the left and change of government predicted.
Before a blackout period began this morning, current centre-right President Mauricio Macri finished his campaign in Cordoba.
He has presided over a period of serious economic crisis in the South American country with polls placing opposition leader Alberto Fernandez 20% higher.
BBC Radio 5 Live
Political turmoil in Argentina battered the country's stock markets and its currency on Monday.
Why such a reaction after conservative Argentine President Mauricio Macri lost in primary elections to centre-left rival, Alberto Fernández?
Jane Sydenham, investment director at Rathbones, says: "Argentina has defaulted on its debts more times than almost any other country.
"It has had a pretty chequered history but since 2015, the current president has been quite reform-oriented."
She says: "He managed to get a loan from the International Monetary Fund, there have been capital controls...the fact that his opposition leader has then suddenly done so well in this poll means that perhaps these reforms are going to stop and Argentina goes back to its old ways."
Edward Glossop, Latin America economist at Capital Economics, has been looking at the situation in Argentina.
He says Mauricio Macri's government would probably pull out all the stops to try to shore up popular support.
This could include easing the pace of economic austerity imposed as part of Argentina's agreement with the International Monetary Fund.
"An outright loosening of the purse strings is possible. The IMF would probably turn a blind eye to this, since it is in its interest for President Macri to secure re-election," he said, but added: "We doubt that these efforts would be enough to change voter perception."
There's more here.
Back to Argentina, where as mentioned earlier, currency and bond prices are plunging after the unexpected defeat of Conservative Argentine President Mauricio Macri in primary elections.
There have been big drops in companies such as cement producer Loma Negra - down around 50% - and electricity distributor Pampa Energia which is off around 30%.
The stock market is 11% lower, as measured by the Merval index.
This BBC story explains that voting in the primaries is compulsory and is not restricted to party members but open to all those eligible to cast their ballot in the presidential polls. The vote was won by the president's left-wing rival, Alberto Fernández (pictured).