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<div>Our lab has observed that the medium on which <em>P. infestans</em> is grown can strongly influence the results of experiments with sporangia produced on those media. The first observation was that sporangia produced from pure cultures (culture sporangia) on agar (Rye B or Pea) medium were more sensitive to solar radiation than were sporangia produced on leaflet lesions (leaflet sporangia). The second observation was that arachidonic acid induced resistance in potatoes to culture sporangia, but not to leaflet sporangia. To quantify the effect of “source” of sporangia on the virulence of those sporangia, we conducted inoculation studies in the field and in moist chambers. Leaflet sporangia produced three- to twenty-times as much disease per sporangium as did culture sporangia. In order to develop hypotheses concerning mechanism, gene expression in both types of sporangia was assessed. The ratio of expression of an RxLR effector to expression of INF1 differed more than 50-fold in the two types of sporangia. The ratio declined gradually as an isolate was obtained from leaflets and then passaged on agar medium. Thus, we conclude that for <em>P. infestans,</em> leaflet sporangia are physiologically and pathogenically different from culture sporangia, and leaflet sporangia should be used for the most accurate assessment of host-pathogen interactions under natural/agricultural conditions.</div>