Resistant cultivars: An option for environment-friendly management of plant parasitic nematodes
Authors: Zakaullah Khan, Bharat H. Gawade and A. Kandan
ICAR-National Bureau of Plant Genetic Resources, Pusa Campus, New Delhi 110012

Plant parasitic nematodes constitute one of the major obstacles for the production of adequate supply of food in several countries. In general, they are polyphagous and occur in polyspecific communities. These are found in wide range of habitats and are highly adaptive to both food and environment. They are among the most damaging and uncontrollable pests of cultivated crops causing severe economic losses in agriculture. Worldwide losses as a result of plant-parasitic nematode infection have been estimated to be more than $100 billion per year. Majority of crop losses caused by plant parasitic nematodes are inflicted by relatively a few genera i.e. root-knot nematodes (RKN), Meloidogyne spp. and cyst nematodes (Heterodera spp. and Globodera spp.). The high impact of these specific nematodes on world agriculture is a result of their wide distribution and broad host range. As cyst and RKN are the most damaging groups of nematodes, most of the efforts to study parasitism and screening of germplasm to find resistance sources and other control strategies have been focused on them. RKN and cyst nematode are both obligate sedentary endoparasites that spend most of their active lives within the plant roots. Sedentary endoparasitic nematodes enter the roots as motile second-stage juveniles (J2) that induce very specialized and complex relationships with their hosts.

RKN J2 hatched out from eggs in soil, locates and penetrates a root to establish a feeding site, usually within the pericycle and vascular tissues. A gall is formed due to hypertrophy and hyperplasia of the root cells. Later, the J2 becomes sausage shaped and undergoes three further molts to become an adult. The female feeds and starts laying eggs in a gelatinous matrix secreted by the rectal glands, which is clearly visible outside the roots as an egg mass. The gelatinous matrix has lytic properties that has role in housing and protecting eggs from the external environment. When susceptible plants are infected with RKN, typical root galls formed that are two to several times large in diameter as that of healthy root. The losses are heavy and may result in complete destruction of the crop. Similarly cyst forming J2 induce the formation of feeding sites, called syncytia, in which few cells merge by dissolving their cell walls. Once their feeding sites have been established, J2 become sedentary and begin to swell into enlarged adult females through subsequent molts to J3 and J4. Contrary to RKN, cyst-forming adult females, protrude from the roots with the majority of their body, lay eggs inside the body and at the end of their life-cycle, have their external cuticle brown colored and hardened. These hardened, brown and dead females are called cysts, which protect the eggs from adverse conditions are also serve means for spreading the infestation.

Management of plant-parasitic nematodes has always been a difficult task but its management has been achieved by adopting various methods either singly or in combination. These methods are oriented toward the host and/or pathogen. Host management has non-genetic and genetic components. The non-genetic components include cultural, physical and chemical techniques. The genetic component involves the identification of sources of resistance by employing reliable screening method(s) and utilization of selected sources of resistance in the breeding programs for development of nematode resistant cultivars (Narayanasamy, 2002). Chemicals are used to control nematodes but due to their high cost and hazardous effects, nematicides are not always attractive to farmers. The era of nematicides is ending and we must develop alternative nematode management systems. Use of cultivars resistant to nematodes is one of the alternatives which are environmentally benign, secure and economically feasible means of controlling nematodes. Search for resistant varieties/lines of crops or for genotypes that may serve as a source of resistance against target nematode species has been an important activity of the nematologists all over the world (Park et al., 2004, 2007; Moon et al., 2010). The use of host resistance has been the goal of nematologists, biotechnologists and plant breeders to transfer genes to susceptible crops and genotypes with other desired traits in order to obtain acceptable nematode resistant crop cultivars. With an advent of genetic engineering in plant biotechnology, it has become possible to isolate the genes responsible for nematode resistance and transfer them into the genomes of economically important plant germplasms which are otherwise susceptible to nematodes. The use of series of R genes like Mi, Ma, etc. providing resistance to root-knot nematodes has been a success story.

Resistance has been proved to be an effective management tool that improves crop yields, lowers nematode population densities, and favors the developments of effective rotation. Perusal of latest reports indicates that, resistance, when available, can generally be considered as the best option for nematode management mainly because of its cost effectiveness and safety to environment. The current availability and/or use of resistant cultivars and root stocks for nematode management reflects the success of research efforts in identifying and evaluating resistance sources, incorporating them into commercially acceptable crop selections, and implementing them into management programs (Molinari, 2011). Resistant cultivars can also be used as a component of integrated nematode management along with other control strategies such as organic soil amendments, biocontrol, and soil solarization for controlling nematodes. However, despite its great potential as a management tactic, resistance appears to have been underutilized, probably because of its problems like unawareness among farmers and supply constraints which need to be properly addressed. Most of these impediments can be overcome or minimized with intensive research on screening of germplasm for the resistance to nematodes, effective grower education programs, and by a closer collaboration between nematologists and plant breeders.

Literature cited:

Molinari, S. (2011). Natural genetic and induced plant resistance, as a control strategy to plant-parasitic nematodes alternative to pesticides. Plant Cell Reporter 30:311"323

Moon, H. S., Khan, Z., Kim, S. G., Son, S. H. and Kim, Y.H. (2010). Biological and Structural Mechanisms of Disease Development and Resistance in Chili Pepper Infected with the Root-knot Nematode. The Plant Pathology Journal 26:149-153

Narayanasamy, P. (2002). Microbial Plant Pathogens and Crop Disease Management. Oxford and IBH Publishing Co. Pvt. Ltd, New Delhi, India.

Park, S. D., Khan, Z. and Kim, Y. H. (2007). Evaluation of medicinal herbs for resistance to root-knot nematode, Meloidogyne incognita in Korea. Nematropica 37:73-77.

Park, S. D., Kim, J. C. and Khan, Z. (2004). Host status of medicinal plants for Meloidogyne hapla. Nematropica 34:39-43

About Author / Additional Info:
Senior Scientist (Nematology), Division of Plant Quarantine, Indian Council of Agricultural Research (ICAR)-National Bureau of Plant Genetic Resources (NBPGR), Pusa Campus, New Delhi-110012.