Technical Abstract: Cucumis melo accession PI 313970 possesses numerous genes for race-specific resistance to powdery mildew incited by Podosphaera xanthi, but it also exhibits non-race-specific, resistant blister leaf reactions in California, U.S.A. to several races of P. xanthii. Light microscopic examination confirmed the presence of mycelia, conidiophores and conidia, and limited numbers of necrotic cells in the water-soaked blisters. Interactions of PI 313970 with Czech isolates of cucurbit powdery mildew incited by P. xanthii and Golovinomyces cichoracearum, were followed in a comparative study with susceptible cucumber (Cucumis sativus L.) 'Stela' on leaf discs through 144 h post inoculation (hpi). Spore production, germ tube development and infection rate of both pathogens were reduced on PI 313970 which expressed delayed resistance responses including limited hypersensitive response (HR) to these two powdery mildew pathogens. At least two different resistance mechanisms are operating in PI 313970. The first mechanism delays pathogen development (conidial germination and appressoria formation). The second mechanism is probably based on specific physiological and biochemical processes leading to reduced pathogen reproduction with minimal or no HR. This second mechanism may, under different parameters (intact leaves, longer time period, pathogen isolate to which PI 313970 expresses race-specific resistance, etc.), be found to produce the resistant blister symptom with its accompanying features of water soaked tissues and higher incidence of HR-like, necrotic cells. It must be noted that development of resistant blisters on PI 313970 have yet to be demonstrated in response to infection by G. cichoracearum. Further research may, thus, demonstrate that resistant blisters, which are very unusual in plant-powdery mildew interactions, result from a third and unique mechanism of resistance to infection by P. xanthii.