A stem inoculation assay was developed to assess transmission of Phytophthora infestans from the base of belowground potato stems to emerged aboveground shoots. Sporangia of P. infestans subsequently developed on aboveground stems after belowground stems were inoculated with isolates BF05 (US 8 genotype), WA10.1 (US 24), and 110B (US 11) by (i) placing a filter paper square saturated with a spore suspension onto nonwounded belowground stem tissue, (ii) placing a filter paper square saturated with a spore suspension onto wounded belowground stem tissue, and (iii) dipping the base of the cut belowground stem into a spore suspension. Plant stems were then incubated in a humidity chamber at 10 to 21.5°C for 7 to 28 days. Incidence of aboveground stems with sporulation was significantly less when stems were inoculated with isolate 110B than with isolates BF05 and WA10.1 but length of latent period did not differ among the three isolates. Incidence of aboveground stems with sporangia was significantly (P < 0.05) greater for ‘Russet Norkotah’ than for ‘Umatilla Russet’ when inoculated near the point of seed piece attachment. Incidence of aboveground stems with sporulation did not differ among incubation temperatures of 10, 15, and 21.5°C. Latent period significantly increased as temperature decreased. Age of stem did not appear to affect incidence of transmission. Incidence of transmission of P. infestans to aboveground stems from the base of belowground stems was significantly lowered when aboveground stems were treated with selected fungicides 1 h before inoculation.