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The impact of plant disease depends on agricultural, biological, socioeconomic, and historical factors. In developed agricultural systems, impacts are usually measured in terms of reduced crop yield and quality, and overall effects on farm profitability. In less developed systems, the consequences of disease can be much more serious, affecting food security, regional and national economies, and social stability. Invasive pests and pathogens can also destroy native trees and alter natural ecosystems, as well as impacting on biodiversity (Table 1.4).

Some examples of these impacts and the pathogens responsible are given in Table 1.5. Prior to understanding of the germ theory of disease, and discovery of chemicals and other means of controlling epidemics, disease outbreaks could devastate crops and consequently cause famine and social disruption. It is believed that a plant disease, most likely a virus, contributed to the collapse of the ancient Mayan civilization in the ninth century. But the most notorious case is the potato late blight outbreak of 1845–1846, caused by the oomycete pathogen Phytophthora infestans, that spread rapidly across Europe and had particularly disastrous consequences in Ireland, where many communities were dependent on potatoes as their sole source of food. Around 1 million people died of starvation and countless others were displaced, many emigrating to the USA. Fortunately, with diversification of food sources, and improved crop protection, this scenario is now much less likely to be repeated in developed countries, but in subsistence agriculture is still a constant threat.

Table 1.4 Some impacts of plant disease

Developed agriculture

Reduced crop yield

Reduced crop quality

Compromised product safety, e.g., mycotoxin contamination

Reduced profitability

Developing agriculture

Food security – malnutrition and famine

Impact on communities or national economies

Social instability

The natural environment

Loss of key species or natural communities

Damage to landscapes and leisure amenities

Table 1.5 Some examples of the impacts of specific plant diseases

Type of impact	Disease	Causal agent	Country/region affected
Famine	Late blight of potato	Phytophthora infestans	Europe 1845–1846
Brown spot of rice	Helminthosporium oryzae	India 1942–1943
Failure of maize crop	Maize mosaic virus?	Guatemala, ninth‐century Mayan civilization
Cassava mosaic disease	Cassava mosaic Gemini viruses	East Africa 1980s to present
Economic	Coffee rust	Hemiliea vastatrix	Sri Lanka 1870, now worldwide
Cocoa swollen shoot	Cocoa swollen shoot virus	Ghana/Nigeria 1930–present
Citrus canker	Xanthomonas axonopodis pv. citri	Florida 1912, 1986, 1995–present
Agricultural	Southern corn leaf blight	Bipolaris maydis	USA 1970
Asian soybean rust	Phakopsora pachyrhizi	Asia 1900s, Africa 1995, Brazil 2001, USA 2004
Black stem rust	Puccinia graminis f.sp. tritici	USA 1900s, new race Ug99 in Africa 1999, now Middle East and potentially Asia
Ecological	Dutch elm disease	Ophiostoma novo‐ulmi	Northern hemisphere 1930, 1970–present
Jarrah dieback	Phytophthora cinnamomi	Western Australia 1920–present
Sudden oak death	Phytophthora ramorum	California 1995, UK 2002
Ash dieback	Hymenoscyphus fraxineus	Poland 1990s, western Europe, UK 2012

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).