An aqueous extract of the sisal agave, A. sisalana, was prepared from dried stem tissue by boiling in water. The sisal extract (SE) displayed a broad inhibition of the seed-borne mycoflora when applied to farmer-saved seeds of Sorghum bicolor. Species of Fusarium (belonging to the Gibberella fujikuroi complex) were inhibited >75 % on seeds. Accordingly, seed-borne crown rot, associated with Fusarium thapsinum was significantly (p < 10⁻⁷) and dose-dependently reduced in plants from SE treated seeds. The SE treatment furthermore stimulated seedling vigour and increased dry weight of 4-week-old plants (p < 10⁻⁴) with 35 %. The growth promoting activity of SE was found to be similar to what has been reported for the extract of another agave, Yucca schidigera. Both extracts were found to reduce seed-borne Fusarium and crown rot more efficiently than fungicides based on tebuconazole, enilconazole and fludioxonil (p < 0.0042). The stem of the sisal agave is a waste product in fiber production. From our findings A. sisalana stem appears to represent an abundant source of antifungal activity, simple to extract, easy to store and with a promising potential in biological seed treatment.