In the summer of 2019, we tested the effects of eight water extracts of invasive alien plant species on the occurrence of diseases, and on the growth and development of the butterhead lettuce, the variety 'Marina'. The experiment was carried out at the Laboratory Field of the Biotechnical Faculty in Ljubljana. We divided the cultivated land into three blocks, inside this blocks we distributed 10 treatments (18 seedlings in each treatment/plot) at random: 1 – water extract of giant knotweed leaves (Solidago gigantea), 2 – water extract of Canadian knotweed leaves (Solidago canadensis), 3 – water extract of Japanese knotweed leaves (Fallopia japonica), 4 – water extract of Bohemian knotweed leaves (Fallopia bohemica), 5 – water extract of three of heaven (Ailanthus altissima), 6 – water extract of staghorn sumac leaves (Rhus typhina), 7 – water extract of false indigo leaves (Amorpha fructicosa), 8 – water extract of stinkwort leaves (Dittrichia graveolens), 9 – positive control (use of fungicides and insecticides), and negative control (unsprayed). All aqueous extracts were used in 25 % concentration. We applied the extracts on the plants during the growing season; we sprayed the luttuce three times. We found differences in the activity of differrent invasive alien plant species on the causer of the Alternaria leaf spot and lettuce production. Good results have been obtained in suppressing Alternaria leaf spot with water extracts of some invasive alien plant species. In the positive and negative control we established the highest average mass of the above-ground part of the lettuce, while among extracts of invasive alien plant species, the extract of the Canadian knotweed proved to be the best biostimulant.