The aim of the study was a laboratory evaluation of the antifungal effect of leaf extracts from yarrow (Achillea millefolium L.), tansy (Tanacetum vulgare L.), sage (Salvia officinalis L.) and wormwood (Artemisia absinthium L.) on fungi of the genus Fusarium, major cereal pathogens. The study used 5%, 10%, and 20% concentrations of plant extracts, evaluating their effect on the linear growth of Fusarium avenaceum, F. culmorum, F. graminearum, F. sporotrichioides and the percentage of their growth inhibition compared to control. The study also included the assessment of the content of selected biologically active compounds in plant extracts and their impact on the development of the aforementioned pathogenic fungi. The total content of polyphenols and flavonoids in the extracts was assessed by spectrophotometry, and antioxidant activity was determined using the synthetic 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical. Plant extracts from sage were characterized by the highest polyphenol contents (81.95 mg/mL) and flavonoids (21.12 mg/mL) compared to other plant extracts, and also showed the highest antioxidant activity (102.44 mM Trolox). Wormwood extract contained the lowest amount of phenolic compounds (flavonoids—5.30 mg/mL, polyphenols—43.83 mg/mL). Plant extracts inhibited the mycelia growth of fungal pathogen depending upon the fungus species, type of extract and its concentration. The extracts of sage (S) and tansy (T) plants at a concentration of 20% demonstrated strong inhibitory effect against the tested fungi (the highest inhibition coefficient for S20: 83.53%; T20: 72.58%), while 10% and 5% extracts of these plants were less effective in inhibiting the growth of Fusarium (highest inhibition coefficient for S10: 71.33%; S5: 54.14%; T10: 56.67%; T5: 38.64%). Yarrow (Y) and wormwood (W) extracts showed low fungistatic effect. Their 20% concentration inhibited the development of mycelia growth of fungi at the level of 63.82% (W20) and 67.57% (Y20). The 5% and 10% concentrations of these plant extracts had the weakest effect on the tested fungi (Y5: 34.09; W5: 42.06%; Y10: 45.01%; W10: 57.44%), even stimulating the Fusarium growth compared to the control (Y5: −23.7%). Based on the study, it was found that each species of fungus reacted differently to the addition of the extract to the culture medium and its concentration, however F. avenaceum and F. culmorum were the most sensitive fungi, while the least sensitive was F. graminearum. The results of the research are the preliminary phase for further field tests to determine the fungistatic effect of plant extracts in field conditions, their phytotoxicity and biological stability, as well as the possibility of producing a biopreparation to protect plants against fusariosis.