Studies were conducted on a field of soybean monoculture at Czesławice near Nałęczów in the years 2002–2004. The object of the studies were soybean plants ‘Polan’ cv. and chitosan. A water solution of chitosan micro-gel at the concentration of 0.1% was used for studies. The studies tested the effect of this compound on the inhibition of soybean diseases caused by soil-borne pathogenic fungi. The solution of chitosan was used a few times, i.e. for seed dressing, seedling spraying, plant spraying at anthesis and in mixed combinations. During the experiment field observations were conducted in particular developmental stages when the number and healthiness of plants were established. Infected plants were submitted to mycological analysis. Results from the field observations and mycological analyses showed that chitosan used in a mixed combination (for seed dressing + seedling spraying + plant spraying at anthesis) was the most effective in protecting soybean from the infection of soil-borne pathogenic fungi. In those combinations the best density of soybean seedlings and plants at anthesis was obtained. The mycological analysis of infected parts of soybean seedlings and plants at anthesis showed that chitosan proved effective in inhibiting the infection of soybean plants from fungi from the genera of Fusarium, Pythium, Phomopsis and Botrytis cinerea, Sclerotinia sclerotiorum and Rhizoctonia solani.

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