Research in Vietnam finds declining resistance to CLR and spread to higher elevations.
Coffee leaf rust, or Hemileia vastatrix, is a fungal disease that afflicts coffee trees throughout much of the world. During the five-year period from 2012 to 2017, CLR resulted in about $3 billion in damage, forcing 2 million farmers to switch to other crops or other work, according to The Atlantic.
A study published in Frontiers in Plant Science in April examined the genetics and spread of CLR fungus in Vietnam. The authors, researchers from the University of Tsukuba and Ibaraki University in Japan, focused on the incidence and genetics of CLR in coffee-producing areas in the northwest, central highlands, and southeast regions of Vietnam. The results of the research shed light on the genetic similarities between CLR fungus populations in Vietnam and beyond, as well as the migration of CLR spores.
CLR fungus, named for the rust-like appearance of its orange spores, can be found in coffee-producing regions around the world. The telltale spores mar the previously green, healthy leaves in orange while causing them to brown and shrivel. Once a plant is infected, the spread is typically swift. “In a matter of two to three weeks, most of the leaves present visible lesions,” explained farm consultant Arturo Ballar of Bio Soils Hawaii.
CLR can eventually cause leaves to fall off the plant. With no leaves, the tree cannot produce the energy it needs to feed the coffee cherries. The fungus might not kill the plants, but infestation typically results in lower-quality beans and reduced yields.
The researchers behind the Frontiers in Plant Science study collected samples from 41 coffee plantations in Vietnam. They traced the spread of CLR from northwestern Vietnam to the southern region. Through genetic sequencing, they found that the CLR fungus in Vietnam is closely related to fungus populations in countries like Mexico and Brazil.
Rust-resistant coffee plants have been the best weapon in the fight against CLR, but the researchers discovered that in Vietnam even these varieties have become susceptible. For example, Coffea arabica had been considered CLR-resistant, but the authors discovered rust on these plants. Other researchers, too, have found evidence that CLR is infecting rust-resistant coffee plants, raising concerns that susceptibility is rising.
"To control this disease, we need to understand rust population diversity. We must also identify the genetic variations that underpin it and anticipate potential future variations,” wrote Professor Izumi Okane, one of the study’s senior authors.
The authors are also interested in using genomic population data to better understand how CLR migrates. While natural factors, like wind, can aid the spread of the fungus, the researchers also want to understand how human activities play a role.
Climate change adds another layer of complexity. The fungus flourishes in warm, wet conditions. Other research has shown that coffee plants grown at higher altitudes are typically less susceptible to CLR, but temperatures are rising. Coffee farms planted in areas that were once considered high enough and cool enough to avoid CLR are now at risk. And, alas, the study in Vietnam found CLR in all coffee-growing regions there, including both high and low-altitude.
Understanding the genetics of this disease and the factors behind its spread can help coffee industry stakeholders prevent CLR and protect their valuable crops. The study authors intend to continue their work with further population genomic studies on CLR.