Temperature is a significant environmental factor affecting growth, pathogenicity and development of rot and crown roots of many plant pathogenic Phytophthora species. This study is conducted to compare the pathogenicity temperature of the newly described species, Phytophthora pseudocryptogea, and its sister taxa, P. cryptogea and P. erythroseptica. Selecting tomato plants as a suitable host for all three species, the seeds were cultivated in 1 Kg pots in greenhouse. One-month-old plants were transferred to growth chambers at 20, 25 and 30 ºC. Inoculums were prepared with the isolates of P. pseudocryptogea, P. cryptogea and P. erythroseptica by vermiculite and hemp seed extract. The disease symptoms in infected cultivars were monitored for two months and wet and dry weight of roots and shoots, height of the plants, mortality and percentage of colonized roots were determined. Trials were performed as a factorial experiment with three replications for each combination of isolates and temperatures, arranged in a completely randomized design. Means of the treatments were compared using least squares means test at the 0.05 probability level. SAS software was used for all the statistical analyses. The results showed 20 ºC as an optimal temperature for pathogenicity of P. erythroseptica and P. pseudocryptogea and 25 ºC for P. cryptogea. The highest percentage of mortality and root colonization, as well as wet and dry weight reduction of roots and shoots, compared with controls, were observed at 20 °C for P. erythroseptica and P. pseudocryptogea and 25 °C for P. cryptogea. Additionally, pathogenicity of P. erythroseptica isolates were completely prevented at 30 °C.