A more recent example is the spread of cassava mosaic disease (CMD) in Africa. A severe outbreak emerged in Uganda in the 1980s, with crop losses as high as 80–90%, and cultivation of this vital food crop was abandoned in some areas. CMD, that is now known to be caused by a complex of related Gemini viruses spread by whitefly vectors, has since invaded other countries in sub‐Saharan Africa where it continues to affect food security. Recently, a different virus, cassava brown streak, has spread to East Africa to pose a further threat to this vital staple crop.
Other diseases have had serious economic impacts, such as coffee rust that devastated the industry in Sri Lanka and has now spread worldwide. More recent examples of global pandemics caused by rust fungi include Asian soybean rust, that has now spread to the major producing areas in Brazil and the USA, and black stem rust of wheat, a new variant of which (Ug99) emerged in Africa and is now spreading east, threatening wheat production areas in Asia. Soybean producers in the Americas now have to factor in the cost of fungicide treatments, while there are concerns that Ug99 might decrease wheat production in affected countries.
A further consequence of disease is the impact of measures taken to control pathogens that threaten export markets. Attempts to eradicate the destructive cocoa swollen shoot virus (CSSV) from Ghana by means of statutory removal of infected and surrounding trees not only entailed the most costly eradication campaign ever attempted, but also lead to political unrest. In the USA and South America, spread of the bacterial disease citrus canker has only been contained by burning huge numbers of infected citrus trees and nursery stock, combined with vigilant quarantine measures. In Florida, this included removing asymptomatic citrus trees from private gardens within affected areas, which inevitably brought plant health authorities into conflict with home owners. Outbreaks of crop diseases can also impact on agricultural practices and policy. The major epidemic of southern corn leaf blight in the United States in 1970 (see Chapter 5, Figure 5.1) raised doubts about the wisdom of achieving genetic uniformity in modern cereal crops and forced a reassessment of the breeding methods employed in the production of new cultivars.
Plant pathogens can also have major impacts on both natural and managed forests. Currently, sudden oak death is spreading in the western USA and Europe, while ash dieback has recently invaded the UK. With the expansion of international trade in plants and plant products, the frequency of invasions by exotic pests and pathogens is increasing, with serious implications for many native plant species (see Chapter 5).
Quantifying Losses Due to Disease
Even in areas of high agricultural efficiency, losses due to pathogens, pests, and weeds make constant inroads into production, and hence profits. Such estimates are notoriously difficult to compile but suggest that as much as one‐third of total production is being lost. Table 1.6 shows estimated worldwide losses for six major crops due to weeds, pests, pathogens, and virus diseases during 2001–2003. Furthermore, comparison with data from earlier surveys (Figure 1.9) suggests that the situation has not improved in recent years, despite advances in the science and practice of crop protection. Some of the changes taking place in modern agriculture may in fact have increased vulnerability to disease. The demand for improved agricultural productivity has led to large areas being planted with high‐yielding, genetically identical cultivars. Similarly, the increasing cost of labour and the trend toward mechanization, which involves major capital expenditure, have also contributed to a reduction in the diversity of crop types planted. As a result, long‐established systems of crop rotation have been discontinued in many areas.
Table 1.6 Estimated losses (%) due to weeds, pests, and diseases in six major crops worldwide in 2001–2003
Source: Oerke (2006).
| Crop | Weeds | Pests | Pathogens | Viruses | Total |
| Wheat | 7.7 | 7.9 | 10.2 | 2.4 | 28.2 |
| Rice | 10.2 | 15.1 | 10.8 | 1.4 | 37.4 |
| Maize | 10.5 | 9.6 | 8.5 | 2.7 | 31.2 |
| Potatoes | 8.3 | 10.9 | 14.5 | 6.6 | 40.3 |
| Soybeans | 7.5 | 8.8 | 8.9 | 1.2 | 26.3 |
| Cotton | 8.6 | 12.3 |
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