AbstractPyricularia grisea has been identified as a foliar pathogen on buffelgrass (Cenchrus ciliaris) in North America and was studied as a potential source of phytotoxins for buffelgrass control. Two monosubstituted hex‐4‐ene‐2,3‐diols, named pyriculins A and B, were isolated from its culture filtrate organic extract together with (10S,11S)‐(−)‐epipyriculol, trans‐3,4‐dihydro‐3,4,8‐trihydroxy‐1(2H)‐napthalenone, and (4S)‐(+)‐isosclerone. Pyriculins A and B were characterized by spectroscopic (essentially nuclear magnetic resonance [NMR], High‐resolution electrospray ionization mass spectrometry [HRESIMS]) and chemical methods such as (4E)‐1‐(4‐hydroxy‐1,3‐dihydroisobenzofuran‐1‐yl)hex‐4‐ene‐2,3‐diols. The relative and absolute configuration of these compounds was determined by a combination of spectroscopic (NMR, electronic circular dichroism [ECD]) and computational tools. When bioassayed in a buffelgrass coleoptile and radicle elongation test, (10S,11S)‐(−)‐epipyriculol proved to be the most toxic compound. Seed germination was much reduced and slowed with respect to the control and radicles failed to elongate. All five compounds delayed germination, but only (10S,11S)‐(−)‐epipyriculol was able to prevent radicle development of buffelgrass seedlings. It had no effect on coleoptile elongation, while the other four compounds caused significantly increased coleoptile development relative to the control.