The variety, which originated in Malaysia, is grown in the Central Highlands and Mekong Delta and known for its high quality.
Thuc, a farmer in the delta province of Ben Tre, sold a four-kilogram fruit for VND2 million last year but only VND1.2 million now.
Other farmers in the province have also reported a similar fall in prices.
Retailers in HCMC are selling the fruit at VND350,000-450,000 per kilogram, down 30% year-on-year.
Manh Khuong, a trader, said a large supply of black thorn is now available, while Vietnam’s native Ri6 and Thai variety Monthong are also in harvest season.
Many farmers grow Black Thorn on a small scale with around 20 trees in their orchards.
Farmers say Black Thorn durian grown in Vietnam is of the same quality as its Malaysian counterpart.
The variety starts fruiting in the third year while others can take four or five years.