Zucchini yellow mosaic virus (ZYMV) causes considerable losses of cucurbitaceous vegetables grown nearly all over the world; indeed, the commonly planted cultivars are highly susceptible to ZYMV. In all, 3 cultivars of American and 8 of European summer squash (Cucurbita pepo), and 6 Japanese and 21 European cucumber lines (Cucumis sativus), including both slicing and pickling species, were selected for the evaluation of their resistance to the most virulent Czech strain, ZYMV-H (GenBank accession number DQ144054). Butternut squash (Cucurbita moschata) 'Menina 15', Chinese slicing cucumber 'Taichung Mou Gua-1' (TMG-1), and watermelon (Citrullus lanatus) accession PI 595203 were included in the experiment, because they were reported to be resistant to ZYMV. The tested plants were mechanically inoculated by ZYMV-H and their resistance was assessed through a comparison of the relative virus protein concentrations and visual symptoms. Butternut squash Menina 15, Chinese slicing cucumber TMG-1, Japanese slicing cucumber breeds 'G22' and 'A192-18', and watermelon PI 595203 were evaluated as immune: the virus concentration in their leaves was zero, as verified by polymerase chain reaction. American summer squash 'Cougar' and Japanese slicing cucumber breeds 'A202-18', 'R10', and 'S93-18' were clearly resistant, because the virus multiplied at a low rate in these plants. The remaining tested cultivars were tolerant or susceptible to ZYMV.
