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Coffee leaf rust, caused by the fungus Hemileia vastatrix, is characterised by small yellow spots on upper leaf surfaces and orange powdery lesions on the underside of leaves. Infected leaves eventually drop off, lowering the yield and quality of the crop.
“Shade also creates a conducive environment for the coffee leaf rust hyper-parasite and we need to maximise this potential.”
Beyene Zewdie, Stockholm University
But a study conducted in Ethiopia considered the origin of Arabica coffee, known scientifically as Coffea arabica, shows that a natural enemy that grows on top of leaves infected by the disease could be key to helping farmers fight it.
“The rust is a global challenge for coffee production,” says Beyene Zewdie, a co-author of the study and post-doctoral researcher at the Department of Ecology, Environment and Plant Sciences at Stockholm University, Sweden, adding that it reduces coffee yields by up to 30 per cent.
According to the study published in Agriculture, Ecosystems and Environment this month (1 May), researchers analysed both the rust and a fungus that attacks it called Lecanicillium lecanii. Observations were carried out from 2017 to 2019 during the wet and dry seasons in Southwestern Ethiopia.
“We found that coffee leaf rust was more severe during the dry season whereas the hyperparasite [parasite whose host is also a parasite or L. lecanii] was more severe during the wet season in two out of three years,” says the study. “The rust incidence increased with management intensity while the hyperparasite was more common under less intensive management.”
“We also found a slight variation in the environmental requirement of the rust and the hyperparasite,” Zewdie tells SciDev.Net. “The rust can thrive in low moisture conditions whereas the hyperparasite favours areas characterised by moist and shaded habitats.”
Zewdie says most farmers in the study area perceive coffee leaf rust as a less important disease because although severe coffee leaf rust infestation leads to leaf-drop, coffee shrubs normally get back the leaves during the next wet season.
But he cautions that the loss of leaves can have a negative effect on the performance of the plant, and says shade would help mitigate the damage.
“Coffee needs shade and growing the crop under shade could buffer the microclimate around the coffee shrubs,” he tells SciDev.Net. “Shade also creates a conducive environment for the coffee leaf rust hyperparasite and we need to maximise this potential to make use of the capacity of the hyperparasite to suppress the rust in areas where the two interacting species co-occur.”
Bernard Mukiri Gichimu, a senior lecturer at Kenya’s University of Embu, Department of Agricultural Resource Management, who was not involved in the study, says that the findings could be important for farmers.
“The discovery of L. lecanii as a hyperparasite against the coffee rust fungus in a natural environment is a major breakthrough that may have a significant contribution in the management of the coffee leaf rust,” says Gichimu. “With climate change, the disease has become even more damaging … even in areas that were hitherto known to be less prone to the disease.”
Mukiri adds that using the natural enemy to fight the disease may be preferable to fungicides which can fail to control the disease either due to poor quality or handling or resistance of the disease-causing agent to fungicide.
“Reduced use of fungicides will also reduce environmental pollution which will be beneficial to the non-target organisms and safe to both the farmers and coffee consumers,” he says.
Beyene Zewdie and others Temporal dynamics and biocontrol potential of a hyperparasite on coffee leaf rust across a landscape in Arabica coffee’s native range (Agriculture, Ecosystems & Environment, 1 May 2021)