Pathogenic fungi cause significant yield losses and quality reductions to many crops including canola, wheat, and barley. Toxic metabolites produced by fungal pathogens, along with excessive application of synthetic fungicides, can also pose risks to human and livestock health. Hydroxy unsaturated fatty acids (HUFAs) are novel alternatives to commonly used fungicides. Here, the antifungal activities of two HUFAs, coriolic acid (CA) and ricinoleic acid (RA), were assessed in vitro and in planta for their activity against the important phytopathogens Fusarium graminearum Schwabe, Pyrenophora tritici-repentis (Died.) Drechsler, Pyrenophora teres f. teres Drechsler, Sclerotinia sclerotiorum (Lib.) de Bary, Leptosphaeria maculans Ces. & De Not., and Aspergillus niger Tiegh. on selected media, monocots or dicots. The results in vitro indicated that both CA and RA showed the strongest inhibitory activity against L. maculans and A. niger, but their activities varied with different fungi. On wheat and barley, CA but not RA reduced disease severity caused by Pyrenophora spp.; on canola, treatment with high levels of CA and RA led to oxidative damage of the plant tissues, and treatment with low concentrations of CA and RA did not reduce disease severity caused by L. maculans or S. sclerotiorum on canola. Our findings suggest that the utility of HUFAs in reducing disease severity caused by pathogenic fungi depends on the dosage and the plant and fungus targeted. High concentrations of HUFA can be phytotoxic on certain plants. In addition to their direct antifungal properties, additional mechanisms may be involved in the disease reduction of CA, suggesting the need for further evaluation of its potential use in crop protection.