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<div>Lima bean, the cornerstone crop of Delaware, is adversely affected by downy mildew caused by the oomycete pathogen <i>Phytophthora phaseoli</i>. When uncontrolled, downy mildew results in devastating yield loss in the mid-Atlantic region (MAR). Chemical control and host resistance are important tools for the MAR and are being improved upon to increase lima bean profitability. Genetic resources have been developed for cultivar development, including the phenotypic characterization of a 256-member diversity panel for traits which include resistance to race F of <i>P. phaseoli</i>. Chemical control is however the major management strategy, with mefanoxam (Ridomil Gold) being the most widely applied fungicide. Reports of resistance have initiated assessment of the mefanoxam tolerance of MAR isolates collected over the past four seasons. Isolates were tested for mefanoxam sensitivity by assessing their growth on frozen lima bean media containing mefanoxam concentrations of 0ppm, 50ppm and 100ppm. Single sporangial isolate subsamples of field populations were compared to their corresponding field isolate, with five of 101 isolate sets showing mefanoxam insensitivity. Morphometric differences observed between these insensitive isolates and their composite counterparts suggested mixed populations in the field, a possible explanation for genotype data that shows a lack of diversity between isolates. This data will allow farmers to be informed on better fungicide usage practices.</div>