By Julia Symmes Cobb and Luis Jaime Acosta
BOGOTA (Reuters) - Colombian coffee growers still dealing with the consequences of the El Nino drought are now bracing for heavy La Nina rains which could hurt the country's harvest even more, the head of the coffee federation said on Tuesday.
Growers, who are finishing harvesting beans shrunk by lack of water, will soon have to contend with overly humid soil and the possible spread of the roya leaf rust disease, Roberto Velez, the head of the country's Coffee Federation, told Reuters in an interview.
While a strong El Nino phenomenon -- a warming of the Pacific Ocean's surface that causes hot and drier conditions -- is drawing to close, Colombia's meteorologists say there is a 76-percent chance that the opposite La Nina pattern will begin late this year or early in 2017, bringing wetter weather.
The El Nino phenomenon has affected nearly half of the country's growing regions and has caused a lower-quality harvest, he said.
"We are still harvesting coffee with a clear quality problem affected by El Nino," Velez told Reuters at his Bogota office, adding worse damage could be on the cards.
"For coffee-growing La Nina is more damaging than El Nino," Velez said. "La Nina worries us."
Though the potential severity of the phenomenon is not yet known, rain clouds blocking sunlight can slow flowering and wet weather sparks the "devastating" effects of roya, he said.
Farmers in Colombia, the world's top producer of high-quality arabica beans, suffered huge losses between 2009 and 2012, when the combination of La Nina, roya and a renovation program that took thousands of trees out of service caused a sharp fall in output.
Coffee-growers must continue to work to increase the productivity of their trees, which produce an average of 18 bags of beans per hectare (2.47 acres), Velez said.
Brazil produces between 22 to 30 bags per hectare, while Vietnam's output can reach 50 bags, he added.
"We have to make an effort so productivity goes up for the same amount of space," Velez said.
Colombia has long prided itself on exporting only the finest beans, but last year the federation modified export standards to help growers sell lower-quality beans amid the drought.
The country has produced 14.6 million 60-kg bags of washed arabica over the last 12 months, Velez said, above 2015 output figures of 14.17 million bags, which was the highest harvest in 23 years.
(Reporting by Julia Symmes Cobb and Luis Jaime; Editing by Sandra Maler)