Inheritance of Resistance to Zucchini Yellow Mosaic Virus In Tropical Pumpkin

In Cucurbita moschata Duchesne (tropical pumpkin; butternut squash) the landrace ‘Nigerian Local’ (NL) has been identified as having resistance to Zucchini yellow mosaic virus (ZYMV). Previous studies indicate that resistance from NL is completely dominant. Our experience during a backcrossing program to incorporate resistance from NL into the local cultivars Taina Dorada (TD), Soler (SOL) and Verde Luz (VL) suggests that inheritance of ZYMV resistance is more complex than a single dominant gene.  The resistant parent NL, three susceptible parents (TD, SOL, VL), the F1 between NL and each susceptible parent, and F2 populations of NL x VL and SOL x NL were planted in the greenhouse and mechanically inoculated with a local, molecularly characterized ZYMV isolate. Readings were taken thru 17 days post-inoculation when the 4^th true leaf was submitted to DAS-ELISA. In a follow-up study, additional parental and F1 plants were inoculated, evaluated for symptoms and tested with ELISA. Inoculated plants of NL were completely free of symptoms. All susceptible parents showed mild to severe symptoms. All F1 plants showed clear, albeit very mild, symptoms (usually streaks) on the 1^st and 2^nd true leaf (occasionally the 3^rd). ELISA readings were slightly elevated compared to NL (whose readings were similar to the negative control).  A follow-up study with parents and F1 plants gave similar results, indicating that the resistance from NL is incomplete. The two F2 populations showed a continuous range of symptoms, making classification into a “resistant” and “susceptible” category difficult, and classification into an “intermediate” category nearly impossible.  ELISA readings did little to aid in classification. When plants with mild symptoms on the 1^st or 2^nd leaf were classified as “resistant” and all others as “susceptible”, the two populations segregated 3 resistant: 1 susceptible. However, only 1 of 24 resistant plants of (NL x VL)F2 was completely free of symptoms, while 13 of 26 resistant plants of (SOL x NL)F2 were symptomless. This suggests that additional genes for resistance are involved and that SOL may carry an additional gene or genes for resistance that are not found in VL.  We have observed that many backcross-derived F3 families originating from seedlings identified as resistant in the BC:F2 are completely susceptible rather than either segregating for resistance or being completely resistant, as would be expected with a monogenic completely dominant resistance. Nevertheless, we have been able to identify some lines that show both horticultural promise and ZYMV resistance.