Phytophthora infestans distinguishes artificial growth medium from a living host by consistent differential expression of thousands of genes that lead to large differences in phenotype. For example, sporangia produced in pure culture (plate sporangia) are less aggressive and produce more necrosis than do sporangia from an infected tomato leaflet (leaflet sporangia). We analyzed gene expression in different genotypes of P. infestans to test if this pathogen distinguished among different genotypes of tomatoes. We found no significant differentially expressed genes (DEGs) between sporangia produced on susceptible tomato cultivar Jetstar versus susceptible tomato cultivar Rutgers. This was true for each of two different genotypes of P. infestans (US23 and US24). In contrast there were 352 significant DEGs when the gene expression of US23 growing on the tomato cultivars was compared to that of US24 on the tomato cultivars. US23 is a good pathogen of both potatoes and tomatoes and grows biotrophically on tomatoes. In contrast US24 is primarily a pathogen of potatoes and produces more necrosis on tomatoes than does US23. The gene expression differences are consistent with these phenotypes. Eight crinklers were significantly up-regulated in US24 compared to US23. Crinklers are so named because of an initial association with a crinkling and necrosis phenotype. Seven of these eight crinklers were also up-regulated in one comparison of plate sporangia to leaflet sporangia. A next goal is to determine if P. infestans distinguishes between potatoes and tomatoes.