Abstract
Wheat rusts are the key biological constraint to wheat production in Ethiopia—one of Africa’s largest wheat producing countries. The fungal diseases cause economic losses and threaten livelihoods of smallholder farmers. While it is known that wheat rust epidemics have occurred in Ethiopia, to date no systematic long-term analysis of past outbreaks has been available. We present results from one of the most comprehensive surveillance campaigns of wheat rusts in Africa. More than 13,000 fields have been surveyed during the last 13 years. Using a combination of spatial data-analysis and visualization, statistical tools, and empirical modelling, we identify trends in the distribution of wheat stem rust (Sr), stripe rust (Yr) and leaf rust (Lr). Results show very high infection levels (mean incidence for Yr: 44%; Sr: 34%; Lr: 18%). These recurrent rust outbreaks lead to substantial economic losses, which we estimate to be of the order of 10s of millions of US-D annually. On the widely adopted wheat variety, Digalu, there is a marked increase in disease prevalence following the incursion of new rust races into Ethiopia, which indicates a pronounced boom-and-bust cycle of major gene resistance. Using spatial analyses, we identify hotspots of disease risk for all three rusts, show a linear correlation between altitude and disease prevalence, and find a pronounced north-south trend in stem rust prevalence. Temporal analyses show a sigmoidal increase in disease levels during the wheat season and strong inter-annual variations. While a simple logistic curve performs satisfactorily in predicting stem rust in some years, it cannot account for the complex outbreak patterns in other years and fails to predict the occurrence of stripe and leaf rust. The empirical insights into wheat rust epidemiology in Ethiopia presented here provide a basis for improving future surveillance and to inform the development of mechanistic models to predict disease spread.
Highlights
Ethiopia is the largest wheat-producing country in sub-Saharan Africa [1]
Our analyses adapt the following sequence: aggregated wheat rust disease levels in all surveys from years 2010–2019; analysis of spatial trends; disease prevalence on different wheat varieties; temporal trends; spatiotemporal trends; estimation of financial losses caused by wheat rusts in Ethiopia; testing the performance of two simple logistic models for predicting wheat rusts on national scales in Ethiopia
Our analyses indicate that there were repeated wheat rust disease outbreaks in Ethiopia during the last decade, despite ongoing efforts for surveillance and control
Summary
Ethiopia is the largest wheat-producing country in sub-Saharan Africa [1]. In most of Ethiopia, rainfed wheat is grown in the highlands by smallholder farmers with an average field size of about 1 hectare. The timing of the wheat growing season varies amongst wheat producing regions. In most parts of the country, the main wheat season follows the long rainy season (meher), which starts around June and lasts until December. Wheat is grown in the preceding short rainy (belg) season, from March or April to June or July. In the main wheat producing areas of Ethiopia (southeastern and central-western parts of the Ethiopian highlands), the environment is suitable for infection with wheat rusts almost year around [3]
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