Performance of Tropical Pumpkin Lines Derived from Backcrossing for Zucchini yellow mosaic virus Resistance

Very few ZYMV resistant cultivars of Cucurbita moschata are available to growers in the tropics. ´Soler´ [SOL], ´Taina Dorada´ [TD] and ´Verde Luz´ [VL] are three Puerto Rican cultivars of tropical pumpkin susceptible to Zucchini yellow mosaic virus (ZYMV). SOL is known to be widely adapted in the tropics. These cultivars were backcrossed to the resistant landrace ´Nigerian Local´ (NL) in order to develop new lines with ZYMV resistance. Resistance to ZYMV in NL has been reported to be a single dominant gene. During backcrossing, selections were made on the basis of symptom severity (0 = no symptoms) and ELISA readings (<0.300 negative for ZYMV) following artificial inoculation. After one or two generations, backcross-derived F1s were selfed. BC-derived F2s were inoculated and evaluated for severity, ELISA and horticultural traits, and the best F2 plants were selfed. For each backcross-derived population, 8 F3 lines were tested in a field trial in Isabela, Puerto Rico along with NL and the three susceptible recurrent parents. Seedlings were inoculated before being transplanted to the field. NL remained completely free of symptoms and tested negative for ELISA through all stages of this research, including in the field test. Average severity scores and ELISA readings of lines derived from backcrossing were significantly less than scores and readings of the corresponding susceptible parent (VL, TD or SOL), and usually not significantly greater than that of NL (the exception being lines derived from TD which had scores intermediate between susceptible TD and resistant NL). Although backcross lines were not significantly more susceptible than NL, the values for both disease severity and ELISA were always higher in the backcross lines, suggesting that the complete resistance seen in NL was not fully incorporated into these lines. Severity and ELISA readings were much more variable among plants in plots of the backcross-derived lines compared to plots of NL, and segregation for some degree of susceptibility continued to be observed. Despite NL having very poor horticultural qualities and only 1 or 2 backcrosses being made, all three populations produced some lines that combined good horticultural traits of their recurrent parent along with a better level of ZYMV resistance. However, the level of ZYMV resistance in derived lines was not quite as strong as that of NL. This suggests ZYMV resistance is controlled by more than one gene in these genetic materials.