the more subtle effects of low infestation levels is probably a vast underestimate. Globally, a wide range of crops are produced, with some grown in specific areas (Table 2.1). Others have a broader geographical plasticity, which can result in a greater range of pests, varying according to region, continent and climate. Moreover, some crops are produced in regions of varying levels of economy, leading to different levels of nematode management, often as a consequence of awareness as well as the availability of options for their management. The degree of damage a nematode causes can also be dependent upon host and age. In addition, prevailing soil, environmental and climatic conditions all influence the threshold population density, above which measurable damage occurs. For example, Tylenchorhynchus martini causes damage on sugarcane at populations between 600 and 6,400/plant, whilst on onions just 5 individuals per seedling of Pratylenchus penetrans will result in serious damage (www.enclyclopedialive.com).
Nematode attack can also predispose plants to attack by other pathogens either through mechanical damage but also on a genetic basis. For example, Sidhu and Webster (1974) determined the genetic basis of the Meloidogyne incognita—Fu-sarium oxysporum lycopersici disease complex on tomatoes, from sequential inoculations of F2 progeny and further demonstrated the role of nematodes in disease interactions through the breakdown of resistance to F. oxysporum lycopersici in the presence of M. incognita.
In addition to the immediate concerns surrounding global food security issues, there is growing concern for pest and disease management under the predicted climate changes and the threat of the emergence of new pests, including nematodes. The Intergovernmental Panel on Climate Change (IPCC) assessments (2007) have concluded that, even if concentrations of all greenhouse gases had been kept constant at the levels present in 2000, a further overall warming of ~0.1°C per decade would be expected, due to the slow response of the oceans. About twice as much warming would be expected if emissions are within the range of scenarios used in IPCC assessments. Resulting changes would include an increase in frequency of heat extremes, heat waves and heavy precipitation; changes in wind, precipitation and temperature patterns; precipitation increases at high latitudes and decreases in most sub-tropical land regions. This would impact on species range shifts; water scarcity and drought risk in some regions of the dry tropics and sub-topics; and coastal damage from floods combined with sea-level rise. For example, Radopholus similis occurs only below ~1,400 m altitude in the East African Highlands where it is a principal pest of banana and plantain, a regional key starch staple for over 20 million people. A small raise in temperature would result in R. similis, which is cold-sensitive, infecting millions more bananas grown at higher altitudes. In an alternative example the rice root knot nematode, Meloidogyne graminicola, can be maintained under damaging levels through good water management. However, with reduced availability of water following climatic changes and/or competition for urban use, reduced quality of water management, or the introduction of water saving mechanisms such as direct wet seeding is favouring the development of high populations of M. graminicola, drastically raising its economic significance as a damaging pest (de Waele and Elsen 2007).
Nematodes are excellent bio-indicators for environmental change as once they are present in a habitat and in proximity of hosts conducive to their development, they may rapidly multiply. Indigenous species that have remained in balance may emerge to pest status on agricultural crops with small changes to their habitat, either through changes in cropping practice (crop, cultivars, rotation cycle, etc.) or climate. A good example of this is illustrated by the rapid and alarming emergence of Meloidogyne minor in Europe (Karssen 2004). Plant damage symptoms were first observed in The Netherlands on sports turf in 1997 and on potato in 2000 (Karssen 2004). Since then this nematode has been recovered from a range of pasture, amenity turf and potato crops in mainland Europe and the British Isles (PRA 2007). Its varying morphology on different hosts creates confusion with the quarantine root knot species M. chitwoodi and M. fallax, with both morphological and molecular characterisation essential for accurate diagnosis (C. Fleming, personal communication). Consequently, we can be sure that nematodes will continue to emerge as new or more aggressive pests of crops as farming practices adapt to fashion, as climate change occurs and as cropping systems intensify in response to an increasing global demand for food. In a world of limited means for nematode management, focus on plant parasitic nematodes as a significant affliction of crop production is highly pertinent.
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