Plant Pathology and Plant Pathogens. John A. Lucas
Disease is a Dynamic Phenomenon
The intensification of agriculture through plant breeding, widespread use of fertilizers, pesticides and plant growth regulators, with larger fields and shorter rotations, has brought in its wake new and sometimes severe disease problems. Changes in the types of crops grown, or in crop management practices, almost invariably lead to new challenges. Irrigation has opened up whole regions to agriculture, but the same water which brings life to the crop can also nourish and spread its microbial enemies. The extension of crops into new geographical areas has exposed them to novel disease agents; for instance, tropical plant species such as cocoa and cassava, which originated in South America, are grown extensively in Africa where they have succumbed to virus diseases spreading from native species. While breeding programs and chemical control measures have won notable victories in the campaign against plant disease, the situation is never static. If anything, in recent years the threat posed by pests and pathogens has increased, rather than decreased (Figure 1.10).
Figure 1.10 A convergence of forces increasing the threat of plant disease.
Source: Lucas (2017b).
The age we are living in has been described as the “Anthropocene,” as no part of the planet is now unaffected by human activity. Environmental change has taken place on a massive scale, altering ecosystems, modifying the distribution of species, and reducing biodiversity. The globalization of trade and travel has redistributed crops and plant products and inadvertently introduced their enemies into new regions. There is now conclusive evidence that the global climate is changing as a result of human actions. Altogether, the pace of such change is accelerating the adaptation and evolution of biological systems, including the pathogens causing animal and plant diseases. There is a need therefore for constant vigilance to ensure that the plants we grow remain healthy and productive.
Further Reading
Books
1 Ainsworth, G.C. (1981). An Introduction to the History of Plant Pathology. Cambridge: Cambridge University Press.
2 Buczacki, S. and Harris, K. (2014). Pests, Diseases and Disorders of Garden Plants, 4e. London: Harper Collins.
3 Chakraborty, U. and Chakraborty, V. (eds.) (2015). Abiotic Stresses in Crop Plants. Wallingford: CABI.
4 Holliday, P. (1989). A Dictionary of Plant Pathology. Cambridge: Cambridge University Press.
5 Ingram, D.S. and Robertson, N. (1999). Plant Disease: A Natural History. London: Collins.
6 Perry, R.N. and Moens, M. (eds.) (2013). Plant Nematology, 2e. Wallingford: CABI.
7 Press, M.C. and Graves, J.D. (eds.) (1995). Parasitic Plants. London: Chapman & Hall.
8 Putnam, R.J. (ed.) (1989). Mammals as Pests. London: Chapman & Hall.
9 Radosevich, S.R., Holt, J.S., and Ghersa, C.M. (2007). Ecology of Weeds and Invasive Plants: Relationship to Agriculture and Natural Resource Management. Chichester: Wiley.
10 Robinson, J.B.D. (ed.) (1987). Diagnosis of Mineral Disorders in Plants, vol. 1–3. London: HMSO.
11 Schumann, G.L. (1991). Plant Diseases: Their Biology and Social Impact. St Paul, Minnesota: APS Press.
12 Smith, I.M., Dunez, J., Phillips, D.H. et al. (eds.) (1988). European Handbook of Plant Diseases. Oxford: Blackwell Scientific Publications.
13 Van Embden, H.F. and Harrington, R. (eds.) (2017). Aphids as Crop Pests, 2e. Wallingford: CABI.
14 Zimdahl, R.L. (2018). Fundamentals of Weed Science, 5e. San Diego, California: Academic Press.
Reviews and Papers
1 Anderson, P.K., Cunningham, A.A., Patel, N.G. et al. (2004). Emerging infectious diseases of plants: pathogen pollution, climate change and agrotechnology drivers. Trends in Ecology & Evolution 19 (10): 535–544.
2 Bos, L. and Parlevliet, J.E. (1995). Concepts and terminology on plant/pest relationships: toward consensus in plant pathology and crop protection. Annual Review of Phytopathology 33: 269–102.
3 Fears, R., Aro, E.‐M., Pais, M.S., and ter Meulen, V. (2014). How should we tackle the global risks to plant health? Trends in Plant Science 19 (4): 206–208.
4 Fisher, M.C., Henk, D.A., Briggs, C.J. et al. (2012). Emerging fungal threats to animal, plant and ecosystem health. Nature 484 (7393): 186–194.
5 Hodges, R.J., Buzby, J.C., and Bennett, B. (2011). Foresight project on global food and farming futures. Postharvest losses and waste in developed and less developed countries: opportunities to improve resource use. Journal of Agricultural Science 149: 37–45.
6 Jarosz, A.M. and Davelos, A.L. (1995). Effects of disease in wild plant populations and the evolution of pathogen aggressiveness. New Phytologist 129: 371–387.
7 Oerke, E.C. (2006). Crop losses to pests. Journal of Agricultural Science 144: 31–43.
8 Rottstock, T., Joshi, J., Kummer, V., and Fischer, M. (2014). Higher plant diversity promotes higher diversity of fungal pathogens, while it decreases pathogen infection per plant. Ecology 95 (7): 1907–1917.
9 Savary, S., Bregaglio, S., Willocquet, L. et al. (2017). Crop health and its global impacts on the components of food security. Food Security 9: 311–327.
10 Vurro, M., Bonciani, B., and Vannacci, G. (2010). Emerging infectious diseases of crop plants in developing countries: impact on agriculture and socio‐economic consequences. Food Security 2: 113–132.
2 The Microbial Pathogens
It was first necessary to determine if characteristic elements occurred in diseased parts of the body, which do not belong to the characteristics of the body, and which have not arisen from body characteristics.
(Robert Koch, 1843–1910)
Heterotrophic microorganisms, unlike autotrophs, are entirely dependent upon an external supply of organic carbon compounds. The ultimate source of most carbon compounds is green plants, but there are a variety of routes by which microbes can obtain these nutrients.
A large number of microorganisms are decomposers. These organisms utilize substrates in dead tissues and their activities eventually lead to the disappearance of plant and animal remains. Such decomposers play a key role in the ecosystem by releasing nutrients which would otherwise remain locked up in plant litter. Some