Paper Reviewed
Bebber, D.P., Castillo, A.D. and Gurr, S.J. 2017. Modelling coffee leaf rust risk in Columbia with climate reanalysis data. Philosophical Transactions of the Royal Society B 371: 20150458.

Each year, damage from fungal pathogens reduce global crop production by approximately one-fourth, losses that -- if prevented and recovered -- could feed hundreds of millions (Fisher et al., 2012). Despite efforts to reduce such damages, some have expressed the concern that they will only increase in the future in consequence of CO2-induced climate change, where it is predicted that rising temperatures and changes in moisture content will favor pathogen expansion and virulence (Anderson et al., 2014; Bebber, 2015).

But is there any evidence to support such predictions? A recent study by Bebber et al. (2017) suggests there is not, at least when testing the hypothesis "that climate change increased the likelihood of the 2008-2011 outbreak of coffee leaf rust in Columbia."

Coffee leaf rust (CLR) is caused by the fungus Hemileia vastatrix and results in an estimated average annual reduction of global coffee yield of around fifteen percent. On a regional scale the reductions can be much higher, such as the approximate 40 percent decline that was observed in Columbia from 2008 to 2011, which was hypothesized by at least one research team to have been caused by climate change (Avelino et al., 2015). Investigating this possibility, Bebber et al. developed a model of CLR germination and infection risk using estimates of leaf wetness duration and canopy temperature from the Japanese 55-Year climate reanalysis dataset. And what did their investigation reveal?

According to the three United Kingdom researchers, they say they "found no compelling evidence for a large increase in predicted infection risk over the period in which the CLR outbreak is reported to have been most severe, and no long-term trend in risk from 1990 to 2015" (see figure below). In light of these findings, Bebber et al. conclude that "long-term climate change is unlikely to have increased disease risk," adding "we reject the climate change hypothesis."

Thus, it would appear that coffee growers (and drinkers!) have little reason to be concerned about climate change-induced fungal damage on future coffee yields. And that's something they might want to raise a glass and toast to!

Figure 1. Mean daily risk of coffee leaf rust infection in Columbia over the period 1990-2015 (see authors' text for details on calculations and the region under study). Source: Bebber et al. (2017).

References
Anderson, P.K., Cunningham, A.A., Patel, N.G., Morales, F.J., Epstein, P.R. and Daszak, P. 2004. Emerging infectious diseases of plants: pathogen pollution, climate change and agrotechnology drivers. Trends in Ecology and Evolution 19: 92-100.

Avelino, J., Cristancho, M., Georgiou, S., Imbach, P., Aguilar, L., Bornemann, G., Läderach, P., Anzueto, F., Hruska, A.J. and Morales, C. 2015. The coffee rust crises in Columbia and Central America (2008-2013): Impacts, plausible causes and proposed solutions. Food Security 7: 303-321.

Bebber, D.P. 2015. Range-expanding pests and pathogens in a warming world. Annual Review of Phytopathology 53: 335-356.

Fisher, M.C., Henk, D.A., Briggs, C.J., Brownstein, J.S., Madoff, L.C., McCraw, S.L. and Gurr, S.J. 2012. Emerging fungal threats to animal, plant and ecosystem health. Nature 484: 186-194.