Altitude is the main driver of coffee leaf rust epidemics: a large-scale survey in Ethiopia

Arabica coffee (Coffea arabicaL.) is a native from Africa, particularly from Ethiopia. Previously of little concern to Ethiopian farmers, coffee leaf rust (CLR), caused by the fungusHemileia 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 complimentary log-log model was developed to predict mean field severity from mean field incidence. Altitude, a known surrogate variable 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 and improved varieties grown under the shade in forest systems dominate the scenario. Our results contribute to increase awareness of a growing problem threatening coffee fields that are still under lower risk, such as at the highlands, particularly if 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.