Epidemiology

The socio-economic effects of disease epidemics and the consequent crop losses have been well documented. There are some iconic invasive diseases, often exemplified due to their large demographic impacts on communities that are dependent on a single staple crop, resulting into epidemics. Some emerging infectious diseases cause famine and favour human diseases, and technical crises for the management of whole agricultural communities. Frequently cited examples include the Irish potato famine caused by Phytophthora infestans, the oomycete plant pathogen, with one million deaths and two million emigrations from 1845 to 1847 in Europe [7]. The high dependence of large Irish population on potato for sustenance, the lack of resistance in the plant to the pathogen, and wetness of the environment caused Phytophthora to take an epidemic form. Its most notorious species, costing annually on a global basis in excess of $5 billion in terms of losses of the potato crop and control measures [8].

Great Bengal Famine (Rice brown spot) of 1943 and the southern corn leaf blight epidemic of 1970-1971 in the USA were the two another big disasters caused by fungal pathogens of the genus Cochliobolus. The former one was caused by C. miyabeanus, an estimated two million people died owing to the high dependence of most of the population on a single crop, rice. Pathogen's spread was favoured by the environmental conditions pertaining at that time [9]. In the USA, the corn (maize) crop was completely destroyed by C. heterostrophus, named race T, which was specifically virulent for maize containing a cytoplasmically inherited gene for male sterility (Tcms). It had been incorporated into about 85% of the American crop by 1970 due to self-fertilization and favourable climatic conditions. Alternative sources of nutrition were plentiful, so no one died and the endemic brought to an end by the withdrawal of susceptible varieties and the establishment of new hybrids [10]. Corn Leaf Blight is renowned for having set a record in terms of economic losses produced on a single agricultural crop in a single season with estimated historic losses of $1 billion [11].

During the first 50 years of the nineteenth century, in Ceylon (now Sri Lanka), there was a massive increase of coffee cultivation by British planters. In 1868, there was total elimination of coffee trees by a rust fungus Hemileia vastatrix, which was likely to have spread from Ethiopia, the center of origin of both the plant and its rust [12]. By 1905, the coffee cultivation area in Ceylon had shrunk from 275,000 acres in 1878 to around 3,500 acres in 1905 [13]. Because of the epidemic, coffee had to be replaced, fortunately with success, by tea.

The threat of epidemics occurring with catastrophic consequences has been sharply reduced in developed countries compared to developing countries, due to technological advances such as, diagnostics, agronomic practices and the use of specific disease management strategies [14]. Re-emergence of a disease is the coincidence of a number of unfortunate events, including many anthropogenic activities such as introduction of plant species into new area. But many such introduced species, like corn, wheat, rice, domestic chicken, cattle, and others are beneficial and now provide more than 98% of the world food supply with a value of more than US$ 5 trillion per year [15]. However, alien plant species (introduced plant species) are also known to cause major economic losses in agriculture, forestry, and several other segments of the world economy [16, 17]. Some pathogen communities are introduced together with a newly introduced plant species and resulted in an emerging disease to that new area. Besides trading of whole living plants, alien pathogens can be introduced through vegetables, germplasm, and grafts or via international seed trading. For example, it has been estimated that at least 2,400 different plant pathogens were contained in the seeds of 380 plant genera [18], and that up to one third of the plant pathogenic viruses are transmissible through seeds to at least one of their hosts [19]. Many factors affect the dissemination and infection by an introduced pathogen like in Pierce's Disease of grapevine, caused by the bacterium Xylella fastidiosa. It was first reported in California as not being serious for more than a century, but in 1997 a new vector, Graphocephala atropunctata, was introduced in California. This allowed the rapid development of the disease in the vineyards, with estimated damage of 6 million dollars in 1999 [20]. Lacking the elements favoring their further dissemination, some pathogens may remain restricted to their area of introduction, making very limited impact. Another example of epidemic occurred due to some introduced variety is vine downy mildew disease caused by Plasmopara viticola in France from 1868 to 1882. This disease was first observed in America in 1834, and then the pathogen was carried to Europe on American stock, where it was first recorded in France. From France, the mildew-pathogen spread throughout Europe, where it is now a very notorious pathogen. Losses in Europe have been enormous due to this disease. The greatest losses to American viticulturists from this disease are incurred in Northern United States; where in some localities it is estimated that 25-75% of the crop is destroyed [21].

There are some more emerging infectious diseases of crops that are challenging the current preventive measures of farmers, such as Cassava Mosaic Virus (CMV), Banana Xanthomonas Wilt (BXW), stem rust of wheat, Citrus Huanglongbing etc. Among them the effects of CMV disease on the farming communities in Uganda became apparent in the early 1990s. The initial impact was greatest in the north-eastern areas of the country, because the particular cultivars were susceptible to the virus. Here, cassava production between 1990 and 1993 was reduced by 80-90% and many farmers stopped its cultivation [22]. The cultivation of other crops, mainly sweet potatoes were preferred at that time to overcome the situation. Several attempts have been made to quantify the losses due to the virus, the most reliable estimate being around 600 thousand tonnes per year valued at 60 million dollars [23]. CMV is the most important disease of cassava in Africa, Sri Lanka and Southern India [24].

The disease caused by the bacterium Xanthomonas campestris pv. musacearum to banana plantations, known as BXW is one of the most important emerging risks. This disease was initially reported in Ethiopia about 40 years ago on Ensete ventricosum, a genus closely related to Musa [25]. It was reported in Uganda in 2001 on banana and from there it has spread rapidly to all regions of Africa where the crop is grown. No varieties of banana have complete genetic resistance, but they differ in degree of susceptibility [26]. It has been estimated that, if not controlled, the pathogen can increase the area infected at a rate of 8% per year [27]. The damage caused by the disease each year is estimated at $2 billion. A recent study estimated 53% loss in yield of banana production in Uganda in last 10 years. Production losses caused by the disease threaten the food security of about 100 million people and the income of millions of farmers in the Great Lakes region of Central and Eastern Africa [28].

One of major epidemics occurred in the 1940s and 1950s in Australia and the United States is stem rust or black rust of wheat caused by microscopic fungus, Puccinia graminis f. sp. Tritici [29]. It took more than a decade to find out cause of re-emergence of stem rust due to its complex life cycle that requires barberry (Berberis vulgaris) as well as a cereal species. Another recently worldwide occurring most destructive disease of all citrus pathosystems is Huanglongbing, the yellow shoot disease. The disease is associated with three bacteria: Candidatus liberibacter asiaticus (Las), C.L. africancus (Laf), and C.L. americanus (Lam). To date, there has been a decline in all commercial citrus industries that have faced the disease [30].

Nowadays epidemiological models are constructed to increase understanding of the complex interactions between vectors, pathogen, host plants, and the environment. If these are accurate and validation with field data is demonstrable for a range of epidemiological scenarios, it can be used for decision support over targeted control of epidemics [31, 32].

Food crisis in developed countries due to failure of a crop can be overcome as the impact of plant disease is mostly an economic issue but in developing countries it can be a primary cause of starvation and today developing countries are more integrated into the global economy than in past decades. Stepping up investment in the agriculture sector can be one of the solutions to combat the situations. Such initiatives can achieve success when new diseases are recognized early in their emergence and before they have spread beyond a reasonable containment zone that can only be managed by quarantines or eradication efforts.

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