Right-Sizing Stem-Rust Research: Follow-up Questions and Answers

John Bakum
Monday, May 13, 2013

Phil Pardey and Jason Beddow, co-authors of a study, Right-Sizing Stem-Rust Research, recently featured in Science, contributed the following Q&A's for the BGRI blog, expanding on their paper's conclusions that maintaining funding of stem rust research is crucial.

What do we know about the global occurrence of stem rust?
Stem rust has long been an important wheat disease throughout many of the world’s major wheat growing areas, often causing significant yield losses. However, since the mid-1960s the disease has not caused substantial losses in the more advanced agricultural systems. Losses decreased in the latter half of the 20th century largely due to successful breeding programs, which established and maintained resistance to stem rust.

The fungus that causes the disease continued to evolve, and new strains, collectively known as “Ug99”, have overcome the resistance that has been bred into modern wheat varieties. To date, these new races have spread from their origin in Uganda throughout southern and eastern Africa, and the Middle East (as far east as western Iran). These new strains have the potential to infect much of the world’s wheat crop.

We found that, based on climate, there are marked differences across regions of the world in susceptibility to stem rust infection. For example, in the United States the disease persists year-round in confined places (namely, the extreme southern areas Texas, Louisiana and California) accounting for less than one percent of the U.S. wheat-growing area. By contrast, the disease can survive year-round in almost two-thirds of Africa’s wheat-growing areas. Overall, 11 percent of the world’s wheat area is suitable for the disease to persist year-round, and 66 percent of the area is climatically suitable for the disease to occur within seasons.

What are the global crop losses associated with stem rust, and how did you estimate these losses?
The Ug99 variants of stem rust have not yet spread to the world’s major wheat growing areas. Thus, much of the world’s wheat is resistant to the strains of stem rust that are present locally. However, much, if not all, of this production is susceptible to the Ug99 races, which could well spread globally.

Because a region is susceptible does not mean that it will necessarily experience a rust-induced reduction in crop yields each and every year. The complex, climate-induced processes of disease development, dispersal and crop yield consequences require a more nuanced, probabilistic approach to estimating potential global losses than has hitherto been reported in the literature.

To do this, we began by creating a global ecological model of the pathogen to determine which of the world’s wheat growing areas are susceptible to the disease. Having sorted out where the disease might occur, we needed to take account of the likely spatio-temporal patterns of stem rust losses—that is, the likely pattern of infection and associated crop losses over time and space assuming Ug99 spread throughout the world. We estimated that if Ug99 were to spread across the globe, average annual losses would be about 6.2 million metric tons per year—enough wheat to almost satisfy the entire annual calorie deficit of sub-Saharan Africa’s undernourished population.

How much should we be spending on stem rust research, and why?
Our probabilistic methods indicate that at least US$51.1 million in annual global stem rust research would be economically justified if Ug99 were to spread worldwide. We estimated this to be around double the amount that is currently being spent annually on research into this particular disease. Moreover, because the pathogen continues to evolve, long-run, sustained funding is needed to keep the disease at bay. Addressing the present Ug99 problem will certainly not put stem rust problems to rest.

What were the biggest challenges in putting your paper together?
The existence of relatively large and widely-cited prior estimates of potential losses necessitated that we make a strong case to reset the status quo. These prior estimates were, in essence, based on a point-maximal approach that essentially scaled up reported losses from certain locations without giving due consideration to either the spatially variable nature of climatic susceptibility to the disease or the complex spatio-temporal processes linking disease occurrence and the associated crop losses.

Reconceiving loss estimates in a probabilistic fashion required access to appropriately scaled long-run crop loss data for this specific disease. Luckily, the USDA’s Cereals Disease Lab has been compiling such data for the United States for most of the 20th century; and notably for the first half of the century, before the era of durable stem rust resistance. We drew on these historical data to calibrate the probabilistic patterns of counterfactual crop losses elsewhere in the world.

What is the most important conclusion that you want readers to take away from your paper?
Since wheat was first domesticated, agriculturalists have been engaged in a Darwinian co-evolutionary dance with stem rust and other plant diseases—through breeding and other efforts, scientists tackle and treat disease problems, but pathogens eventually evolve to overcome resistance. We emphasize that the co-evolutionary nature of crop diseases necessitates sustained research to keep diseases at bay. This requires a long-run vision, supported by corresponding commitments to maintain funding for this (and other forms) of research designed to avert reductions in yields.

Why do you think that investment in stem rust research has leveled off or decreased?
In a nutshell, complacency. First, the successes of scientists in stemming losses to this disease since the 1960s meant that policy attention and, with it public funding, shifted to other seemingly more pressing problems. Unfortunately, as we now know, Mother Nature did not shift her gaze.

Second, the slowdown and cutbacks in funding for stem rust research are part of a broader pattern of a slowdown in funding for productivity-maintaining and -enhancing agricultural R&D in recent decades, especially among the world’s rich countries (and notably the United States). As global commodity prices have soared in recent years, policy attention to agricultural R&D has increased. Whether or not this translates to a sustained revival in agricultural R&D funding in general, and stem rust research in particular, is still an open question.

Can your methods also be used to examine other stresses to wheat, either biotic or abiotic?
Yes. Maintaining past gains in crop yields in light of changing crop pest, climatic and market circumstances requires continued R&D vigilance. Many of the crop loss consequences of these changing circumstances are best conceived through the probabilistic approach we developed as part of this stem rust study. Hopefully others will pick up on these methods; we are actively pursuing similar studies of the crop loss consequences of a range of biotic and abiotic stresses on wheat and other crops.

More information:
Read full paper - Science