Altitude is the Main Driver of Coffee Leaf Rust Epidemics: a Large-scale Survey in Ethiopia

Arabica coffee (Coffea arabica L.) is native from Africa and Ethiopia is often regarded as its birthplace. Previously of little concern to Ethiopian farmers, coffee leaf rust (CLR) caused by the fungus Hemileia vastatrix is an emergent disease globally. To update the status of CLR, a large survey was conducted in 405 coffee fields across nine production zones of Oromia and Southern Nations Nationalities and Peoples (SNNP) regions. The disease, evaluated one time during the month of the peak for CLR intensity for each region, was present in every single field; mean incidence and severity ranged from 5 to 86.7% (mean = 35.3%) and 0.22 to 55.5 (mean = 9.09), respectively. A complementary log-log model predicted mean field severity from mean field incidence. Altitude, a known surrogate for temperature, was the main driver of the epidemics. Incidence and severity were highest at the lowland fields, where poorly managed plantations of local varieties grown under open sun were also more dominant. CLR intensity decreased with the increase in altitude at the highlands where well-managed improved varieties grown under the shade in forest systems dominate the scenario. This survey contributes to increase awareness of a growing problem in Ethiopia, especially for coffee fields at the highlands should temperatures rise and farmers cut forest to grow plantations of susceptible cultivars. Improving genetic resistance and adoption of best management practices are urgent to prevent the rapid surge of new races and mitigate crop losses currently overlooked by coffee farmers at the lowlands.