Key Nematodes Threatening Major Agricultural Crops of Importance Worldwide

A major global challenge in the coming years will be to ensure food security and to feed the increasing human population. Nowhere will the need to sustainably increase agricultural productivity in line with increasing demand be more pertinent than in resource poor areas of the world, especially Africa, where populations are most rapidly expanding. Although a 35% population increase is projected by 2050 (World Bank 2008), an increase in food demand in the order of 75% is anticipated, due to economic development and changes in food preferences (Keating et al. 2010). Significant improvements are consequently necessary in terms of resource use efficiency. In moving crop yields towards an efficiency frontier, optimal pest and disease management will be essential, especially as the proportional production of some commodities steadily shifts. For example, over half the global potato production (>150 million t) now occurs in Asia, Africa and Latin America, as a result of steady increases in recent years (FAO 2010). With this in mind, it is essential that the full spectrum of crop production limitations are considered appropriately, including the often overlooked nematode constraints. For example in a recent review of intractable biotic constraints in Africa, not a single mention of nematodes was made (Gressel 2004), while for the potato crop in the UK alone, it is estimated that the cyst nematodes, Globodera rostochiensis and G. pallida, account for an estimated ~$70 million per annum or 9% of UK production (DEFRA 2010). Given the current withdrawal from use of inorganic pesticides (UNEP 2000), the primary source of pest and disease management over the past decades, the need to consider nematode pests is more acutely brought into focus.

Although over 4,100 species of plant-parasitic nematodes have been identified (Decraemer and Hunt 2006), new species are continually being described while

CIMMYT International Maize and Wheat Improvement Centre, P.K. 39 Emek, 06511 Ankara, Turkey e-mail: [email protected]

J. Jones et al. (eds.), Genomics and Molecular Genetics of Plant-Nematode Interactions, 21 DOI 10.1007/978-94-007-0434-3_2, © Springer Science+Business Media B.V. 2011

others, previously viewed as benign or non-damaging, are becoming pests as cropping patterns change (Nicol 2002). However, the plant parasitic nematodes of economic importance can be grouped into relatively restricted specialized groups that either cause direct damage to their host or act as virus vectors (Table 2.1). Most affect crops through feeding on or in plant roots, whilst a minority are aerial feeders. In addition to direct feeding and migration damage, nematode feeding facilitates subsequent infestation by secondary pathogens, such as fungi and bacteria (Powell 1971).

On a global scale the distribution of nematode species varies greatly. Some are cosmopolitan, such as certain Meloidogyne spp. while others are particularly restricted geographically e.g. Nacobbus spp. or are highly host specific, such as Heterodera carotae which attacks only carrots. Some crops may have very few nematode pests while others have a particularly wide range of genera and species associated with them, such as sugar cane and rice, leading to difficulties for nema-tode control strategies. Distribution maps and host range data are available and updated regularly as a useful source for determining nematode damage potential (http://www.cabi.org/dmpd).

Although plant parasitic nematodes are among the most widespread pests, and are frequently one of the most insidious and costly (Webster 1987), data on their economic impact remain less than comprehensive, especially for crops produced in resource poor areas. In the tropical and sub-tropical climates, crop production losses attributable to nematodes were estimated at 14.6% compared with 8.8% in developed countries. Perhaps more importantly, only ~0.2% of the crop value lost to nematodes is used to fund nematological research to address these losses (Sasser and Freckman 1987). One difficulty with assessing nematode impact is that damage resulting from nematode infection is often less obvious than that caused by many other pests or diseases. Losses that result from nematode attack may not necessarily be as a consequence of direct cell death, necrosis or 'diseased' tissue but may derive from other more insidious aspects, such as interference with the root system, reducing their efficiency in terms of access and uptake of nutrients and water; to the unaware, nematode-affected plants present typical drought and nutrient stress symptoms, which are easily and often misdiagnosed. On Musa spp. (bananas and plantains) nematode damage affects root efficiency on the one hand, but additionally leads to root necrosis and death, undermining plant anchorage; heavily infected bunch-bearing plants can topple due to poor root anchorage leading to total loss of the unripe fruit (Gowen et al. 2005). Moreover, nematode manifestation over time leads to a gradual decline over seasons with misdiagnosis common. Plants are rarely killed outright, although impressive exceptions of full scale crop devastation can occur; Ditylenchus angustus, for instance, which causes Ufra disease on deepwater rice in Asia (Cox and Rahman 1980). More generally, Sikora and Fern√°ndez (2005) suggest that vegetable production in tropical and sub-tropical environments cannot be considered without some form of nematode management.

In the USA a survey of 35 States on various crops indicated nematode-derived losses of up to 25% (Koenning et al. 1999). More recently Handoo (1998) estimated global crop losses due to nematode attack in the region of $80 billion, which, given

Table 2.1 World Food Production for major food commodities and main nematode pests of importance

Crop

Total production

Top 3 producers

Production (million

Main nematode pests (Luc et al. 2005; Evans et al. 1993;

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