15211Emerging Plant Diseases: What Are Our Best Strategies for Management?Karen A. Garrett, Ari Jumpponen, and Lorena Gomez MontanoThe impact of plant disease can be stark. Famine can result if no sys-tems are in place to replace lost crops. Notorious famines precipitated by plant disease include the Irish potato famine of the 1840s due to potato late blight, the Bengal famine of 1943 due to rice brown spot, and famines in Uganda in the 1990s due to cassava mosaic disease. Whole ecosystems can be altered when ecologically important plant species are decimated by invasive pathogens with which they have not coevolved. For example, the generalist pathogen Phytophthora cinna-momi has wreaked havoc in Australian natural areas, its relative Phy-tophthora ramorum threatens temperate forests worldwide, and the eastern United States has been altered by the removal of native chest-nut stands by Cryphonectria parasitica. There are also many less dramatic diseases that produce a cumulative 10% reduction in crop yield (Strange and Scott 2005; Savary, et al. 2006). While some parts of the world take food availability for granted, the recent jump in food prices has brought new attention to food security and the factors that threaten it.Our objective in this essay is to make readers more aware of the factors that drive emerging plant pathogens as a risk factor for foodEmerging Plant Diseases 153security and for the biological integrity of natural areas. We would also like to share some of the excitement of applied plant pathology, a fi eld in which scientists work with fascinating microbes, “worthy adversaries” in work to protect agriculture and wildlands. Consider the range of strategies that a country’s plant pathologists are respon-sible for developing. Some diseases are already established in areas where they potentially cause yield loss every year. They may be costly to manage, but management strategies are available. For agricultural species, re sis tance genes may be available for incorporation in breed-ing programs. The Red Queen from Alice in Wonderland has often been invoked as a symbol of the coevolution of plants and pathogens, where plants need to keep running just to stay in place in terms of evolving new forms of re sis tance to pathogens that are also continu-ously evolving. In agricultural systems, plant breeders developing re-sistant crop varieties take the side of the crop species in the “arms race” with pathogens. Re sis tance strategies may include partial re sis tance, where the direct selection pressure for new pathogen types is lower. In cases where re sis tance genes are not available or not suffi cient for disease management, strategies for the use of other cultural methods, such as changing planting times to avoid pathogens, or pesticides may be used. Plant pathologists work to optimize these systems by, for ex-ample, minimizing unnecessary pesticide use through weather- based disease forecasting to guide when pesticides would and would not be useful. But diseases that are new to a region are generally the most problematic, because neither plants nor plant pathologists have had suffi cient opportunity to adapt to them.Decisions about plant- disease management, just like decisions about medical treatment, consist of a series of compromises. Plant- disease risk is a complicated function of the characteristics of the host, pathogen, and environment. Some controversies in plant pathology arise when there is not enough information to know how to weight dif-ferent risk components. There is an old joke about risk managers using license- plate numbers in the parking lot outside their offi ce as a source of pa ram e ters for their risk analyses. The fact is that decisions about how to manage plant diseases generally need to be made with a level of information that is unsatisfactory to scientists. For example, most plant pathologists would agree that having a homogeneous planting of a par tic u lar crop species could pose a risk if the genotypes deployed are unexpectedly susceptible to a disease, or if the acreage exerts se-lection pressure for new pathogen types to overcome re sis tance. Butestimating the magnitude of these two types of risk is diffi cult, so if there is a cost to avoiding the homogeneity, it is diffi cult to know how to balance the different types of costs. Similarly, it is clear that inter-national trade in plant species provides many benefi ts but also an in-creased risk of introduction of a new pathogen. Because it is diffi cult to estimate the risk precisely, it is diffi cult to determine how much to restrict plant movement. The new science of characterizing ecosystem ser vices and their economic value offers to help with such decision- making pro cesses (Cheatham, et al. 2009). For example, if tillage can be used to reduce disease risk but it also increases the risk of soil loss, it would be useful to be able to compare these two risks from an eco-nomic standpoint. But for the moment, the ecosystem- services con-cept only offers a framework for evaluating the costs and benefi ts of strategies. For now, in most cases, the actual economic value of more complex ser vices can only be loosely defi ned.What Are Plant Pathogens and What Causes Them to “Emerge”?Plant diseases can be caused by a combination of biotic and abi-otic agents. Unfavorable environmental conditions, such as an excess or defi ciency of nutrients, humidity, light, toxic chemicals, and so on, can damage plants. Infectious plant diseases can be caused by nema-todes, viruses, bacteria, fungi, and protozoa (Agrios 2005). The clas-sic plant- disease triangle of host, pathogen, and environment is often used to illustrate how the interaction of these components determines the occurrence and impact of plant disease in time and space (Mad-den, et al. 2007).There are many reasons why a plant disease may “emerge,” or in-crease in importance: (1) The introduction of a new pathogen species or a new type within a species may be the most obvious source of emergent pathogens. An example of the latter is race Ug99 of the wheat stem rust pathogen, discovered in East Africa and moving north through important wheat- growing regions. Wheat stem rust has caused little yield loss in the United States in recent years because of effective re sis tance in common wheat varieties. But U.S. varieties do not generally have re sis tance to Ug99, so there is great urgency to de-velop effective re sis