Coffee Growers Take Note: Fungi Have Sex, Too

Coffee rust

Image: Howard Schwartz, Colorado State University

Here’s news to researchers studying coffee rust, a fungal disease that has devastated coffee crops around the world for more than a hundred years: It was always assumed the coffee fungus reproduced asexually—meaning its cells split instead of fused with other cells from another host. But new research confirms they also reproduce sexually.

Coffee rust is the most economically damaging disease affecting coffee crops worldwide (estimated to cost the global coffee industry up to $3 billion a year). This new insight into the personal life of coffee fungus could help control the spread of the deadly disease in coffee and other plant life as well, including wheat grain and pine trees, which also suffer from different forms of rust fungi.

Coffee growers wage a never-ending war with fungi that escalates periodically as the disease constantly adapts to fungicides, developing increasing genetic immunity to every control the growers throw at them.

“This is the principle example of the arms race in fungi, or rust, versus their hosts,” says Shawn Kenaley, a post-doctoral associate in the plant pathology and plant-microbe biology department of Cornell University. It’s Darwin’s theory of evolution by natural selection at work: “Eventually there will be a selection for genes which allow for these fungi to be undetected by the coffee or able to thwart the genetic defense of coffee,” he adds.

Understanding how the fungi reproduce is vital to geneticists who are trying to stay ahead in the arms race.

Among nearly 8,000 species of rust fungi, coffee rust fungi have proven especially mysterious. Originally discovered on wild coffee in East Africa in 1861 and on cultivated coffee in Sri Lanka in 1869, coffee rust has been responsible for halting entire national exports and is considered the reason why tea became the social drink of the British in the 19th century when the disease drove the price of coffee imports too high.

“With coffee rust, it’s always been perplexing because there is this undercurrent of genetic variability that’s been there,” says Kenaley. If the coffee rust fungus only reproduced asexually (giving rise to genetically similar populations), researchers wouldn’t expect to see the amount of genetic variability in the DNA of the fungi as they observe in the lab. Yet, sexual reproduction in fungi was thought to require an alternate host—one that has never been found.

The breakthrough came when researchers determined an alternate host wasn’t actually necessary for sexual reproduction in coffee rust. Instead, one of the spores normally thought to function asexually appears to contain hidden sexual reproduction. Researchers call this cryptosexuality. In other words, these fungi are having sex and have been hiding it from us all along.

Harry Evans, plant pathologist at CABI Biosciences in Great Britain and co-author of the study that confirmed sexual reproduction of coffee rust, says his team determined the presence of sexual reproduction in the life cycle by measuring the amount of DNA in the fungi cells.

He says they observe in coffee rust fungi that “the DNA content is halved. If it were mitosis (asexual reproduction), it would be the same amount of DNA. With meiosis (sexual reproduction), because the chromosomes are splitting and going to the poles, the amount of DNA is halved. The only way of confirming this is when you measure the amount of chromatin and that can be nothing else but meiosis.”

Researchers hope to apply this new knowledge of the coffee rust life cycle to new methods for biological control of plant diseases, which sometimes employ rust fungi as a weapon to combat other types of diseases.

Evans, who has been helping control invasive weeds around the world for fifteen years, cites the 1990’s rubber-vine weed invasion in Australia as an example of successful control using rust fungi.

Originally projected to affect 600,000 square kilometers of trees and bushes throughout the country, the rubber-vine weed had already covered one-tenth of that area in 1995 when scientists introduced rust fungi by dumping massive amounts on affected plants from airplanes.

“It stopped the advance of the weed and is estimated to have saved over $250 million Australian dollars,” says Evans. “And it’s actually saved a lot of the national parks, too.”