A reinvestment in agricultural R&D is critical to ensuring sufficient food for the world in the coming decades.

Breeding new crop varieties with resistance to the biotic stresses that undermine crop yields is tantamount to increasing the amount and quality of biological capital in agriculture. However, the success of genes that confer resistance to pests induces a co-evolutionary response that depreciates the biological capital embodied in the crop, as pests evolve the capacity to overcome the crop's new defences. Thus, simply maintaining this biological capital, and the beneficial production and economic outcomes it bestows, requires continual reinvestment in new crop defences. Here we use observed and modelled data on stripe rust occurrence to gauge changes in the geographic spread of the disease over recent decades. We document a significant increase in the spread of stripe rust since 1960, with 88% of the world's wheat production now susceptible to infection. Using a probabilistic Monte Carlo simulation model we estimate that 5.47 million tonnes of wheat are lost to the pathogen each year, equivalent to a loss of US$979 million per year. Comparing the cost of developing stripe-rust-resistant varieties of wheat with the cost of stripe-rust-induced yield losses, we estimate that a sustained annual research investment of at least US$32 million into stripe rust resistance is economically justified.

Helicoverpa armigera has recently invaded South and Central America, and appears to be spreading rapidly. We update a previously developed potential distribution model to highlight the global invasion threat, with emphasis on the risks to the United States. The continued range expansion of H. armigera in Central America is likely to change the invasion threat it poses to North America qualitatively, making natural dispersal from either the Caribbean islands or Mexico feasible. To characterise the threat posed by H. armigera, we collated the value of the major host crops in the United States growing within its modelled potential range, including that area where it could expand its range during favourable seasons. We found that the annual value of crops that would be exposed to H. armigera totalled approximately US$78 billion p.a., with US$843 million p.a. worth growing in climates that are optimal for the pest. Elsewhere, H. armigera has developed broad-spectrum pesticide resistance; meaning that if it invades the United States, protecting these crops from significant production impacts could be challenging. It may be cost-effective to undertake pre-emptive biosecurity activities such as slowing the spread of H. armigera throughout the Americas, improving the system for detecting H. armigera, and methods for rapid identification, especially distinguishing between H. armigera, H. zea and potential H. armigera x H. zea hybrids. Developing biological control programs, especially using inundative techniques with entomopathogens and parasitoids could slow the spread of H. armigera, and reduce selective pressure for pesticide resistance. The rapid spread of H. armigera through South America into Central America suggests that its spread into North America is a matter of time. The likely natural dispersal routes preclude aggressive incursion responses, emphasizing the value of preparatory communication with agricultural producers in areas suitable for invasion by H. armigera.

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Is increased support needed for wheat disease research to avert crop losses from current and future strains?

The notion that global agricultural output needs to double by 2050 is oft repeated. Using a new International Agricultural Prospects (iAP) Model, to project global agricultural consumption and production, we find in favour of a future where aggregate agricultural consumption (in tonnes) increases more modestly, by around 69 per cent (1.3 per cent per year) from 2010 to 2050. The principal driver of this result is a deceleration in population growth in the decades ahead. Per capita income growth and changing demographics (generally ageing population) have significant but secondary roles in spurring growth in agricultural consumption, as does our projected growth in the use of agricultural feedstocks to meet the growth we envisage in biofuel demand. Worldwide (but not equally everywhere), crop yield growth has generally slowed over the past decade or so. Notwithstanding a projected continuance of this slowdown, the prospective improvements in crop productivity are still sufficient to reduce per capita cropland use, such that land devoted to crops would need to increase by less than 10 per cent. Even in our upper-bound (highconsumption) scenario, we estimate that there remains sufficient productive agricultural land to more than meet the demand without ploughing-in additional forestdominated lands.

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This study provides a bio-economic assessment of the global climate suitability and probabilistic crop-loss estimates attributable to wheat leaf rust. We draw on a purpose-built, spatially-explicit, eco-climatic suitability model for wheat leaf rust to estimate that 94.4% of global wheat production is vulnerable to the disease. To reflect the spatio-temporal variation in leaf rust losses, we used a probabilistic approach to estimate a representative rust loss distribution based on long-term, state-level annual U.S. loss estimates. Applying variants of this representative loss distribution to selected wheat production areas in 15 epidemiological zones throughout the world, we project global annual average losses of 8.6 million metric tons of grain for the period 2000-2050 based on a conservative, base-line scenario, and 18.3 million metric tons based on a high-loss scenario; equivalent to economic losses ranging from US$1.5 to US$3.3 billion per year (2016 U.S. prices). Even the more conservative base-line estimate implies that a sustained, worldwide investment of US$50.5 million per year in leaf rust research is economically justified.