Viral wilt of cucumber plants grafted on Squash rootstocks.

Wilt diseases were observed in cucumber plants grafted on squash rootstocks in Kagawa, Japan, in 1986. In the wilted plants, mosaic symptoms were appeared on the upper leaves whereas the stems and roots failed to exhibit symptoms. Among the 32 leaf samples collected in the field, 15 samples were infected with zucchini yellow mosaic virus (ZYMV), 9 with cucumber mosaic virus (CMV) and ZYMV, 7 with CMV, ZYMV and watermelon mosaic virus-2 (WMV-2), and 1 with CMV and WMV-2. The non-grafted cucumber plants inoculated with these samples did not show any wilt symptoms. When the grafted cucumber plants were inoculated with these samples, those containing ZYMV alone or CMV and WMV-2 caused only slight wilt at a low frequency whereas those containing CMV and ZYMV or CMV, ZYMV and WMV-2 caused severe wilt at a high frequency. To identify the causal virus of wilt, grafted and non-grafted cucumber plants were inoculated with a single isolate of CMV, ZYMV or WMV-2, and a combination of CMV and ZYMV; CMV and WMV-2; ZYMV and WMV-2; CMV, ZYMV and WMV-2, respectively. In the non-grafted cucumber plants, wilt symptoms were not observed after the inoculation in all cases. In the grafted cucumber plants, the inoculation with CMV or WMV-2, and with a combination of ZYMV and WMV-2 did not cause wilt symptoms, whereas the inoculation with ZYMV and a combination of CMV and WMV-2 caused slight wilt at a low frequency. However, the inoculation with a combination of CMV and ZYMV, or CMV, ZYMV and WMV-2 caused severe wilt on the grafted cucumber plants at a high frequency. Based on these results, it was concluded that mixed infection with CMV and ZYMV, or CMV, ZYMV and WMV-2, caused severe wilt on the grafted cucumber plants whereas single infection with ZYMV or mixed infection with CMV and WMV-2 caused only slight wilt.