Birch will leave in September following the Beautiful South tasting taking place in London.
In a statement, WOSA said: "Su has played a key role in advancing South Africa's reputation as one of the most progressive amongst wine-producing nations in driving eco-sustainable wine production.
"Under the banner variety is in our nature she has actively promoted the country’s competitive positioning in producing a rich variety of wine styles.
"At an international level she has also been one of the key drivers for co-operation amongst normally competing New World wine-producing countries.
"The Beautiful South is an extension of this approach in which Argentina, Chile and South Africa will present their first combined trade exhibition in the UK in September. She has been a prime mover in its conception and organisation."
Birch said the exhibition will be her WOSA "swansong" as she leaves the organisation to pursue a personal project offering "cost-effective marketing solutions for a range of industries".
Birch said: "I shall be leaving the organisation with a sound international team in place at the same time that the country has evolved into a significant player in world wine markets, well on its way to capturing and building on new opportunities."