Authors: Zakaullah Khan, Bharat H. Gawade and A. Kandan
Division of Plant Quarantine, ICAR-National Bureau of Plant Genetic Resources (NBPGR), Pusa Campus, New Delhi â€" 110012
Among biotic stresses, root-knot nematodes, Meloidogyne spp. is an important factor that limits the vegetable cultivation, more so in protected cultivation. Root-knot nematodes are sedentary endoparasites of roots, attacking a wide range of crops resulting in significant economic losses worldwide, (Sikora and Fernandez 2005). The infection starts with root penetration by second stage juveniles (J2) hatched in soil from eggs encapsulated in egg masses laid by females on the infected roots. The association of plant parasitic nematodes with the fungus is reported to produce greater loss than caused by either pathogen alone. Infection of roots by root-knot nematodes predisposes plants to infection by soil-borne root-infecting fungi and bacteria resulting in the development of root-rot and wilt diseases. Disease complexes in soil caused by root-knot nematodes, Meloidogyne spp. and soil-borne fungal pathogens such as Fusarium spp. often damage plants more severely and render the disease control more difficult than single pathogens alone.
During last few decades, plant disease control has been based largely on the use of chemicals. Control of root-knot nematodes has been accomplished primarily through nematicides, crop rotation and resistant cultivars wherever available. Environmental and health hazards associated with chemical pesticides and the recent loss of methyl bromide as a multipurpose soil fumigant have spurred research into alternative methods for disease/nematode control. Because of environment and food quality concern in recent years, there have been worldwide swings to the use of eco-friendly methods which are bio-efficacious, economical, biodegradable, environment friendly and could be ideal candidates for use as a reliable tool to control plant diseases. The use of beneficial microorganisms to manage plant diseases offers an attractive alternative to the use of synthetic chemicals. Several studies have shown that the interaction between plant and some endophytic bacteria was associated with beneficial effects such as plant growth-promotion and biological control potential against plant pathogens. Genus Paenibacillus is gram positive, aerobic, rod shaped, endospore forming bacteria, belongs to Plant growth promoting rhizobacteria (PGPR). Paenibacillus spp. are known to suppress several plant pathogens including root-knot nematodes (Khan et al., 2008, 2012; Kim et al., 2009). Studies have demonstrated that some rhizobacteria can induce systemic resistance in the plants towards soil-borne fungi and plant-parasitic nematodes (Khan et al., 2012).
Application of PGPR strains onto the seeds or bare root dip treatment of vegetable crops to enhance the seedling growth and to control the soil-borne pathogens is one of the most convenient and ideal methods to introduce microorganisms to plant materials instead of supplying greater amount of biomaterials to large area as soil drench or spray. Numerous instances have shown that bacterial treatments of seeds and root dip treatment achieved plant growth promotion and disease suppression. The bacterial spores of P. polymyxa and P. lentimorbus inhibit egg hatching of root-knot nematodes, causing lyses and disruption of egg shell and destroy fungal hyphae of Fusarium oxysporum f. sp. lycopersici (Son et al., 2009).
Tomato seedling roots soaked in bacterial suspension of two strains, P. polymyxa GBR-1 and P. lentimorbus GBR-15158 @ 108CFU/ml for a period of 30 min reduced the interactive impact of both F. oxysporum and M. arenaria drastically, as no wilting symptoms and root-gall formation were observed in treated tomato plants (Figs. 1&2), and increased plant growth significantly as compared to untreated plants (Fig. 1).
Fig.1. Beneficial effect of Paenibacillus strains on the disease complex caused by Fusarium oxysporum (FO) and Meloidogyne arenaria (MA) in tomato plants. A- infected with MA and FO only; B- MA+FO+GBR1; C- MA+FO+GBR158; D- Control: Uninoculated, untreated healthy plants.
Fig. 2. Effect of P. lentimorbus GBR 158 on root gall formation by M. arenaria in tomato plants. A. Roots of untreated plant showing heavy root galls, B. Roots treated with GBR 158 showing no root galls.
P. polymyxa strains may be important from biocontrol point of view, given that such effect interrupt the life cycle of the pathogens. Bacterial antibiotics and other compounds present in cultural metabolites might be responsible for inhibition of egg hatch and mortality of J2 of nematodes. Several reports have suggested that P. polymyxa produces many antagonistic substances and suppress root-knot nematodes on tomato, cucumber and clover as a result of application as soil drenches or root dip and seed coating treatment with bacterial suspension. It is also reported that seed coating treatment with P. polymyxa induced systemic resistance against root-knot nematode in tomato roots (Khan et al., 2012). Thus it may be concluded that the tested strains of P. polymyxa and P. lentimorbus had no adverse effect on plants, rather suppress root-knot nematode and wilt fungus and promote plant growth. Thus, the protective and nutritional properties of these Paenibacillus strains make them as environment-friendly useful tool to reduce adverse impact of root-knot nematode and Fusarium wilt on plant growth, especially in protected cultivation where, management of disease complex caused by root-knot nematode and wilt fungus becoming a major limiting factor.
References:
1. Khan Z, Kim S G, Jeon H Y, Khan H U, Son S H and Kim Y H. 2008. A plant growth promoting rhizobacterium, Paenibacillus polymyxa strain GBR-1, suppresses root-knot nematode. Bioresource Technology 99: 3016â€"23.
2. Khan Z, Son SH, Akhtar J, Gautam NK and Kim YH (2012). Plant growth-promoting rhizobacterium, Paenibacillus polymyxa induced systemic resistance in tomato (Lycopersicon esculentum) against root-knot nematode (Meloidogyne incognita). Indian Journal of Agricultural Science 82: 613-617.
3. Kim S G, Khan Z, Jeon Y H and Kim Y H. 2009. Inhibitory effect of Paenibacillus polymyxa GBR-462 on Phytophthora capsici causing Phytophthora blight in chili pepper. Journal of Phytopathology 157: 329â€"37.
4. Sikora R A and Fernandez E. 2005. (in) Plant Parasitic Nematodes in Subtropical and Tropical Agriculture, pp 319â€"92. Luc M R, Sikora A and Bridge J (Eds). CABI Publisher, Wallingford, UK.
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.