Agriculture watchdogs in Brazil fear foreigners arriving for the World Cup could bring up to 350 new plant parasites into the country.
American tourists pose the biggest threat, since the United States has 225 agricultural parasites that do not currently exist in Brazil, said the National Plant Defense Association (Andef), an agro-industry umbrella group.
Italian fans, who could bring in up to 126 new parasites, and French fans, who could bring in up to 120, are also making farmers nervous.
A recent study found foreign parasites are more and more frequent in Brazil, such as the cotton bollworm, or Helicoverpa armigera, which arrived from Australia late last year.
Major sports events raise the risk of invasions of foreign species, said Andef.
During the 2008 Beijing Olympics, 35 new parasites arrived in China.
The Brazilian authorities have moved to crack down on unwanted pests by keeping a tight control on imports.
On June 9 they impounded 39 kilograms (86 pounds) of the Uruguayan national team's dulce de leche, a caramel-like confection made from milk that is a national favorite in Uruguay.
The team lacked the necessary document from the authorities, said a spokesman for the federal agriculture regulators' union.
Andef said the most exposed of Brazil's 12 host cities was Recife in the northeast, a region known for its sugar cane, cotton and cocoa farms.
With matches featuring Ivory Coast, Croatia, Italy, Costa Rica, Japan, Mexico, Germany and the United States, Recife could be invaded by no less than 323 new parasites.
About 600,000 tourists are expected to attend the World Cup. The United States leads the pack with 187,000, followed by Germany with 57,000 and Argentina with 56,000.