With the rising awareness of the adverse effects of chemical pesticides, people are looking for organically grown vegetables. Consumers are increasingly choosing organic foods due to the perception that they are healthier than those conventionally grown. Vegetable crops are vulnerable to a range of pathogenic organisms that reduce yield by killing the plant or damaging the product, thus making it unmarketable. Soil-borne diseases are among the major factors contributing to low yields of organic produce. Apart from chemical pesticides there are several methods that can be used to protect crops from soil-borne pathogens. These include the introduction of biocontrol agents against soil-borne plant pathogens, plants with therapeutic effects and organic soil amendments that stimulate antagonistic activities of microorganisms to soil-borne diseases. The decomposition of organic matter in soil also results in the accumulation of specific compounds that may be antifungal or nematicidal. With the growing interest in organic vegetables, it is necessary to find non chemical means of plant disease control. This review describes the impact of soil-borne diseases on organic vegetables and methods used for their control.

Biocontrol potential of Rhizobium and Bradyrbizobium against soilborne root infecting fungi was tested. In vitro tests Rhizobium meliloti inhibited growth of Macrophomina phaseolina, Rhizoctonia solani and Fusarium solani while Bradyrhizobium japonicum inhibited M. phaseolina and R. solani producing zones of inhibition. In field R. meliloti, R. leguminosarum and B. japonicum used either as seed dressing or as soil drench reduced infection of M. phaseolina, R. solani and Fusarium spp., in both leguminous (soybean, mungbean) and non‐leguminous (sunflower and okra) plants.

A total of 22 ethanol extracts of seaweed species (13 brown, 6 green and 3 red) collected from the Karachi coast were investigated for brine shrimp cytotoxicity. Of all the species, only six namely Stoechospermum marginatum, Sargassum swartzii, S. binderi, Spatoglossum asperum, Stokeyia indica (brown) and Caulerpa racemosa (green) showed significant activity. n-Hexane-soluble fractions of the ethanol extract of S. marginatum and S. swartzii were found to be responsible for the activity, whereas the methanol-soluble fractions of S. asperum and S. binderi were most active. The water extract of S. indica and C. racemosa exhibited the most prominent activity (LC50 value below 70 micrograms/mL) when compared with the ethanol extracts and their fractions. Cytotoxic activity may be due to the compounds differing in polarity.

The effect of soil amendment by brown, green and red seaweeds was studied in controlling the root rot infecting fungi of okra seedlings in the greenhouse. The soil amendment with seaweeds Stokeyia indica, Padina pavonia (brown), Solieria robusta (red), at 1% w/w reduced Macrophomina phaseolina, Rhizoctonia solani and Fusarium solani infection on okra roots. Codium iyengarii (green) at 0.5 % w/w was effective against F. solani, while at 1% w/w was found phytotoxic. S.robusta showed better control of F. solani infection when used with Pseudomonas aeruginosa than either used alone. S. robusta produced better plant height and fresh weight of shoot than P. aeruginosa. Results of the present study suggest that the use of brown seaweeds S. indica and P. pavonia alone and S. robusta alone or in combination with P. aeruginosa have great potential to control root-infecting fungi of okra with enhancement of plant growth. These seaweeds alone or in combination with P. aeruginosa may be utilized as biological control of root infecting fungi of okra.

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