The aim of this Master's thesis was to assess the action of the antagonistic fungus Trichoderma asperellum on seedling development and yield of lettuce (Lactuca sativa L.) grown outdoors. The fungus formulation was a 0.35 % aqueous spore suspension, prepared from Trifender® Pro. We raised seedlings of two batavia green lettuce varieties 'Tourbillon' and 'Joliac'. The preliminary experiment with seedlings took place in the greenhouse, where we immersed the seedlings in the prepared suspension for 35 seconds. After 18 days, we evaluated the seedlings, and planted the remaining outdoors. The seedlings treated after planting were watered with the help of a backpack sprinkler so that each seedling received 18 mL of suspension. In the main experiment, we had six treatments repeated in four blocks. Treatments include combinations of two lettuce varieties 'Joliac' (J) and 'Tourbillon' (T), and seedling treatment by soaking in a preparation suspension (t1), seedling treatment on field (t2), and control (k). The effects of the fungus on the seedlings were different, as the average mass of the treated seedling of the 'Joliac' variety was 36 % higher than in the untreated seedling of the same variety, and the average weight of the aerial part was 48 % higher than untreated seedling. There were no significant differences in the cultivar 'Tourbillon'. In technologically mature lettuce of the 'Tourbillon' variety, the rosettes and the stem were the shortest at t1. At t2, cultivar 'Tourbillon' had 35 % more damaged leaves removed than cultivar 'Joliac'. The latter had 32 % more removed leaves in t1 compared to t2. There were no clear effects of treatment with the preparation on lettuce yield, except for the potential later bolting in the variety 'Tourbillon' in t1. According to our results, the use of Trifender® Pro did not give a better yield. The efficiency of fungi from the genus Trichoderma sp. depends on many biotic and abiotic factors.
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