Technical Abstract: Identification and standardization of several parameters to ensure reproducible conditions for fungal germination valuable in screening fungicides in vitro were established in this study. Washing spores prior to inoculation significantly reduced germination times, increased cultural synchrony, and improved reproducibility of microtiter assays. The natural product-based fungicide, sampangine and a sampangine analog, 4-bromosampangine, plus seven commercial fungicides (captan, kresoxim-methyl, fenhexamid, iprodione, benomyl, fenbuconazole, and cyprodinil) were screened in vitro (microtiter assay) for their ability to inhibit germination and growth of seven important plant pathogenic fungi (Botrytis cinerea, Colletotrichum fragariae, C. acutatum, C. gloeosporioides, Fusarium oxysporum, Phomopsis obscurans, and P. viticola). Germination and morphology bioassays corroborated the results of microtiter assays where sampangine was effective at arresting fungal development. We hypothesize that sampangine has the same physical mode of action, germination inhibition, as the commercial fungicides captan, kresoxim-methyl, and iprodione. 4-Bromosampangine caused morphological anomalies including excessive branching of germ tubes of C. fragariae and splaying and branching of germ tubes of B. cinerea. These observations suggest that this compound may protect the host after fungi have already established themselves in or on the host. Our hypothesized physical mode of action of 4-bromosampangine, inhibition of germ tube elongation, but not conidial germination, is comparable to that of the commercial fungicides fenhexamid, benomyl, and cyprodinil.