Evidence That Simulated Climate Change Affects Plant Disease In Experiments

Next we consider two types of evidence for effects of changes in climate on plant disease. The first is evidence that simulated climate change affects plant disease in experimental settings. The effect of simulated climate change has been studied in experiments with altered heat treatments, altered precipitation treatments and carbon enrichment treatments. Where there are apparent effects from these treatments, this implies that, to the extent that the simulations do effectively represent future climate scenarios, plant disease will respond. The second type of evidence is for changes in patterns of plant disease in agricultural or wildland systems that can be attributed to climate change with some level of confidence, discussed in Section 4. In this case, the changes in plant disease might be taken as fingerprints of climate change. We also discuss what types of plant disease scenarios might qualify as fingerprints of climate change in this sense.

The range of possibilities for climate change simulations can be characterised in terms of the scale of the effect being considered [29]. For many well-studied pathogens and vectors, the temperature ranges that support single infection events or survival are fairly well characterised. The effects of plant water stress and relief from water stress on disease risk have also been studied in controlled experiments for some pathogens, and may be quite relevant to scenarios where patterns of drought occurrence are changing. Advances in the development of technologies such as microarrays make it possible to study drought effects on plant gene expression in the field, including genes that may be important for disease resistance [30]. Drawing conclusions about larger-scale processes from plot-level experiments may be challenging, however, since additional forms of interactions are important at larger scales.

Field experiments that incorporate simulations of changes in temperature and/or precipitation are becoming increasingly common in both agricultural and natural systems, often associated with long-term study systems such as the US National Science Foundation's Long-term Ecological Research sites. For example, in Montane prairie Roy et al. [31] studied the impact of heating treatments on a suite of plant diseases. They found that higher temperatures favoured some diseases but not others. This type of 'winners and losers' scenario is likely to be common as more systems are evaluated; the overall level of disease under climate change may be buffered in some environments as some diseases become less common and others become more common.

The impact of elevated CO2 on plant disease has been evaluated in the context of several free-air CO2 enrichment (FACE) experiments (reviewed in Ref. [32]). Compared to studies in experimental chambers, FACE experiments allow more realistic evaluations of the effects of elevated CO2 levels in agricultural fields or natural systems such as forests. Higher CO2 levels may favour disease through denser more humid plant canopies and increased pathogen reproduction but may reduce disease risk by enhancing host disease resistance [33], so the outcome for any given host-pathogen interaction is not readily predictable. Elevated ozone levels can also affect plant disease risk (reviewed in Ref. [32]).

In addition to the more direct influences of the abiotic environment on plant disease, climate change may also affect plant disease through its impact on other microbes that interact with pathogens. While certain microbes affect plant pathogens strongly enough to be used as biocontrol agents, a number of microbial interactions probably also have more subtle effects. As the effect of climate change on microbial communities is better understood [34], this additional form of environmental interaction can be included in models of climate and disease risk.

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