The Obisu type ‘Delica’ buttercup squash (Cucurbita maximaDuch.) was inoculated at early, mid, and late stages of growth with zucchini yellow mosaic virus (ZYMV) and watermelon mosaic virus 2 (WMV2). The results of these field trials demonstrated that early‐ and mid‐season ZYMV infections and early‐season WMV 2 infections caused serious yield losses and adverse quality effects in comparison with yields from uninoculated plots. Total yield losses from ZYMV were 48% for early‐season and 26% for mid‐season infections. Yield losses from WMV 2 were statistically significant only after early infection (38%). Numbers of marketable fruit were similarly affected as was marketable fruit weight and incidence of fruit blistering. For example, early season single or combined infections of ZYMV or WMV 2 reduced the mean marketable yield of ‘Delica’ from 84 to 23% of the total yield of uninoculated plots. In a second trial, the effect of a mid‐season infection on three squash types showed that the Miyako type was more sensitive to losses in number and fruit weight than Obisu or Arjihei types. In both trials, mixed infections of ZYMV and WMV 2 caused yield effects that were similar to those associated with ZYMV alone. We conclude that ZYMV and WMV 2 infection either before or during the flowering period will reduce fruit size, affect marketable quality through fruit blistering, and reduce fruit numbers. It is therefore important that measures to reduce or eliminate virus infection are taken early in the growing season. Seed from infected fruits were grown and assayed for virus transmission. Incidences of up to 3.5% for ZYMV and 5% for WMV 2 were recorded, demonstrating that both ZYMV and WMV 2 can be transmitted by the seed of infectedC. maximafruit. We believe that the reintroduction and spread of ZYMV, and possibly WMV 2, into buttercup squash growing areas will continue if seed health is not addressed, in addition to practicing normal virus disease control strategies.