The number of Asian soybean rust fungus cases in Brazil has nearly doubled from the previous year due to heavy rainfall in the south triggered by El Niño, according to industry data.

From June until Thursday, there have been 73 incidents of the fungus that attacks plants in humid conditions, mostly in the southern states of Rio Grande do Sul and Parana, according to the public-private anti-fungus consortium that monitors the sector.

A year earlier, there were 43 cases of fungus at this time.

"El Niño causes a higher average rainfall in the south, favoring rust (fungus)," said Claudia Godoy, a plant pathologist at agricultural research body Embrapa Soy, referring to the climate phenomenon that tends to bring rain to southern Brazil and drought to the northeast.

Rust detected early in the season that officially started in September is not a particularly large risk for production as the fungus can be controlled, but it should alert farmers, Godoy said. Brazil is forecast to produce a record 2015/16 soy crop that could surpass 100 million tonnes.

El Niño, the first to influence Brazil's grain harvest since the 2009/10 crop year, will likely strengthen before the end of the year and become one of the strongest on record, the U.N. weather agency said this month.

"There is a very high tendency for rust all season," said agrometeorologist Marco Antonio dos Santos, of Somar Meteorologists. He said rains could continue into the harvest period, which runs from January to April.

According to Godoy, El Niño caused warmer than usual temperatures in southern Brazil, meaning some of the previous crops soy lingered in fields in July and August - a period when fields should be cleared to prevent crop pests.

Soybeans are Brazil's main cash crop and the South American country is the world's top exporter of the oilseed. In recent years fungus, once a huge threat to Brazil's emergence as an agricultural powerhouse, has ceased to affect output as farmers learned to use fungicides effectively.

In a year with high costs of inputted goods like pesticides and fertilizers due to a strong dollar, crop margins are expected to be tight and farmers may be tempted to apply less or opt for cheaper fungicides, said Mario Lucio Melo, technical coordinator of the Coopavel cooperative in western Parana state.

"This can't happen - farmers must use the best products available," he said.