Emerging plant pathogens (EPP) are the cause of new plant diseases. There is recent increased frequency of emergence of such diseases. This increase appears to be manmade and results from extensive growth of the same crop (monoculture), which promotes rapid spread of plant diseases. Availability of the host plant year-round to the pathogen aids spread of EPPs. International trade allows for the spread of such plants as fungi around the world.
In the 1970’s, Dutch Elm disease caused by fungus Ophiostoma novo-ulmi destroyed millions of elm trees. Currently, fungi are threatening ash trees in Europe, Hawaiian plants Ohi’a (Metrosideros fimbriata), as well as agricultural barley, oats, and wheat. Fungal diseases are even emerging in amphibians, bats, and bees. Other diseases have been observed to jump to new plant species, and this is expected to increase with further climate changes.
With climate change, temperatures are expected to increase around the world one to two degrees centigrade by 2100. An increase in temperature extremes is projected, with alternations of warm storms and drought. Climate change alters the invasiveness of plant fungal diseases and the susceptibility of host plants to disease.
Historical records show that EPPs have been moving toward the poles at the rate of 2.5 to 3 km. per year since the 1960s. The rate is 6-7 km. per year for some fungi. Others say that predicting the future of plant diseases is difficult because of many variables, and that we should prepare for a number of possibilities. “Climate change is not simply gradual increases or decreases in temperature or rainfall, but produces unpredictable short-term changes in weather and extreme weather events.”
Carbon dioxide (CO2) is the major greenhouse gas (GHG) pollutant resulting from the present man-made climate change. CO2 is a major cause of global warming. Causes of increased CO2 are increased deforestation, intensification of agriculture, and urbanization. The growing worldwide demand for meat is a major problem, as are increased use of fertilizers and urbanization.
GHG pollutants since pre-industrial times are the following: CO2, CH4 (methane), N20 (nitrous oxide). Gases such as NO (nitric oxide) and NO2 (nitrogen dioxide) are from fossil fuel production and GHG pollution.
Nitrogen fertilizers have had important and increasing use since the early 1900s and are considered essential to feed the world. However, 90% of the nitrogen applied to crops is lost in GHGs, such as NH3 (ammonia), N2O, and NO. These gases have the ability to change Earth’s atmosphere, thereby altering both the resistance of plants and the virulence of crop infectious diseases. Nitrates are lost in water runoff and eventually become atmospheric nitrogen.
Nitrogen levels are increasing in soil and atmosphere. Plants which are high in nitrogen can be attractive to some pathogens. Increased plant NO and NO2 can cause plants to have impaired growth. Reactive nitrogen species, such as ONOO- (nitrosamine), are produced which have negative effects on plant growth.
CONCLUSION: Climate change with severe and difficult to predict changes in host-pathogen relationships result from increased atmospheric levels of CO2 and nitrogen species. They are likely to result in severe changes to plants and agriculture by 2050 at the present rate of pollution. For example, changing this prediction depends on willingness to alter use of fossil fuels and fertilizers.
NOTES: *Reactive oxygen species are chemicals derived from oxygen, which can cause oxidative damage in plants.
**Reactive nitrogen species are chemicals derived from nitrogen, which can cause plant oxidation damage.
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