Beneficial effects of cyanobacteria in the management of plant parasitic nematodes in agricultural crops
Author: Zakaullah Khan
ICAR-National Bureau of Plant Genetic Resources, Pusa Camus, New Delhi 110012

Among the plant parasitic nematodes, root-knot nematodes (RKN), Meloidogyne spp. are the most damaging nematode pest in agriculture, causing an estimated US $100 billion loss/year worldwide. They are found wherever plants are grown and any agricultural crop may be a host to one or more root-knot nematode species. RKN cause galls in roots of several crops impeding normal uptake of water and nutrients. Infection of roots by RKN predisposes plants to infection by soil-borne root-infecting pathogens resulting in the development of root-rot and wilt diseases. Disease complex caused by RKN and soil borne fungal pathogen such as Fusarium spp. often damage plants more severely and render the disease control more difficult than single pathogen alone. Use of chemical nematicides is one of the primary means of controlling plant-parasitic nematodes. However, their potential negative impact on the environment and human health has led to a total ban or restricted use of most nematicides. In addition, uses of chemical nematicides are prohibited in organic farming. Therefore, several options are currently being assessed around the world to identify and develop ecologically sustainable management options for controlling nematode damage to plants. Nowadays biological control of plant parasitic nematodes has become one of the most significant and intensive research areas. Several types of organism, including fungi, bacteria, viruses, nematodes, insects, mites and some invertebrates, have been found to parasitize or prey upon nematodes. Recently one of the biological control practices attempted is the study of cyanobacteria that parasitize plant parasitic nematodes.

Cyanobacteria (blue-green algae) are primitive groups of organisms, which grow as single cells, in colonies or in filaments. The endospores of the species of Microcoleus and Oscillatoria killed nematodes and their culture filtrates inhibited egg hatching of root-knot nematode and killed hatched juveniles, and have also been reported to provide protection against RKN on tomato and okra crops (Khan et al. 1997, 1998, 2005, 2007; Khan and Park, 1999). Incorporation of freeze-dried cyanobacterial powder into the soil suppressed the population of RKN on tomato and increased plant growth. Such effect was expressed both by reducing the amount of root galling and the amount of secondary nematode inoculum in infected plants. The growth of tomato plants was significantly improved with the use of a liquid concentration of the brown alga, Ecklonia maxima (Osbeck) Papenfuss, and RKN infection was reduced substantially. Endospores of a cyanobacterium, Microcoleus vaginatus (Vaucher) Gomont have been reported to parasitize and killed plant-parasitic nematode and reduced root galling caused by M. incognita on tomato (Dhanam and Kumar, 1994). Soil mixed with 0.8% powder of a cyanobacteria (Oscillatoria chlorina Gomont) at 5 days before tomato planting is the most appropriate for RKN management, whereas the lowest effect was observed when applied at planting time (Khan et al., 2007). When soil treated with freeze-dried marine algal powder, Spatoglossum Schroederi (Mertens) Kutzing, Botryocladia occidentalis (Borgesen) Kylin, and Caulerpa prolifera (Forsskal) Lamouroux at concentration of 0.5-1.0% by weight gave significant reduction in tomato root galling induced by RKN. Application of 20ml endospores suspension of Microcoleus vaginatus @ 2.4x106 endospores/ml was found optimum to reduce root galling on tomato caused by RKN (Khan and Park, 1999). Thus selection of optimal dose and application time of cyanobacteria for management of RKN are of primary importance to use this approach.

The mode of action of cyanobacteria against nematode is not precisely known. However, nematode damage reduction may be attributed to a direct effect of cynobacterial neurotoxins. Anatoxin-a (neurotoxins) is made by many genera such as Anabaena, Oscillatoria, Aphanizomenon, Cylendrospermum and Microcystis or partially to effect resulting from nitrogen fixation in soil. Several cyanobacteria are capable of nitrogen fixation in soil. The Cyanobacteria such as Microcystis, Anabaena, Nostoc and Oscillatoria produce a great variety of secondary metabolites, such as Nitrogen-containing compounds, polyketides, lipopeptides, cyclic peptides and many others. Anatoxin-a(s) is the only naturally occurring organophosphate and has been isolated from cyanobacteria, Anabaena flos aquae (Lyngb.) Breb. and A. lemmermannii Richt. Thus the protective and nutritional properties of some cyanobacteria make it an attractive eco-friendly management of RKN or alternative to chemical nematicides, especially in an intensive organic farming system, where plant nutrition and disease control, including nematodes, are the main limiting factors.

Literature cited

Dhanam, M., Kumar, A.C. and Sowajanya (1994). Microcoleus vaginatus (oscillatoriaceae), a blue"green alga (cyanobacterium) parasitizing plant and soil nematodes. Indian Journal of Nematology 24: 125"132.

Khan, Z. and Park, S.D. (1999). EVect of inoculum level and time of application of Microcolues vaginatus on control of Meloidogyne incognita on tomato. Asia Pacific Journal of Entomology 2: 93"96.

Khan, Z., Jairajpuri, M.S. and Khan, M.W. (1997). Effect of culture filtrate of a blue"green alga, Microcoleus vaginatus on mortality and hatching of root-knot nematode, Meloidogyne incognita. International Journal of Nematology 7: 100"102.

Khan, Z., Jairajpuri, M.S. Khan, M.W. and Fauzia (1998). Seed-soaking treatment in culture filtrate of a blue"green alga, Microcoleus vaginatus, for the management of Meloidogyne incognita on okra. International Journal of Nematology 8: 40"42.

Khan, Z., Kim, Y.H, Kim, S.G. and Kim, H.W. (2007). Observations on the suppression of root-knot nematode (Meloidogyne arenaria) on tomato by incorporation of cyanobacterial powder (Oscillatoria chlorina) into potting field soil. Bioresource Technology 98: 69"73

Khan, Z., Park, S.D., Shin, Y.S., Bae, S.G., Yeon, I.K. and Seo, Y.J. (2005). Management of Meloidogyne incognita on tomato by root-dip treatment in culture filtrate of the blue"green alga, Microcoleus vaginatus. Bioresource Technology 96: 1338"1341.

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.