Oxycarboxin and benomyl were applied in water suspension to soil in which were growing seedling oat (Avena sativa) plants.  They previously had been infected with crown rust (Puccinia coronata var. avenae).  Infected leaves were fixed and embedded in epoxy resin for electron microscopy 12 and 24 hours after soil treatment.  Mitochondria and nuclei of haustoria of the fungus, fixed 12 hours after treatment with oxycarboxin, were clearly recognizable, with membranes ranging from nearly normal to obviously deteriorating.  The interior structure of these organelles, and of the cell cytoplasm was severely damaged.  Haustoria fixed 24 hours after treatment with oxycarboxin were difficult to fix and section and had greatly enlarged empty mitochondria.  The plasmalemma and cell wall were generally intact.  Haustoria from plants fixed 12 hours after treatment with benomyl showed no gross effects of treatment.  However, 24 hours after treatment, nuclear membranes and plasmalemmae were severely damaged or missing, mitochondria were greatly enlarged, and cytoplasm was deteriorated.  The ultrastructural effects of the two fungicides were shown to correlate, respectively, with physiological studies on the different modes of action of the two fungicides.

Additional key words: fine structure.