The effect of two potyviruses on development and yield in tropical pumpkin and the inheritance of resistance to papaya ringspot virus
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Viruses of the Potyviridae can infect all cucurbit crops. General symptoms are well-described, but there are no known studies documenting yield and fruit quality loss in tropical pumpkin (Cucurbita moschata Duchesne). Papaya ringspot virus (PRSV) is among the most common of the potyviruses found in Puerto Rico. Two sources of resistance are well known in C. moschata: ‘Nigerian Local’ (NL) and ‘Menina’ (MEN). The inheritance of resistance from NL has been previously studied, but no inheritance studies have been reported for MEN, nor is it known if resistance to PRSV in NL is allelic to that in MEN. Therefore, the objectives of this research were to study (1) the impact of potyvirus on tropical pumpkin at the field level and (2) the inheritance of PRSV resistance in mechanically inoculated F2 populations derived from both sources of resistance, and test for allelism in a NL x MEN F2 population. Control plants and plants inoculated with PRSV, ZYMV or PRSV+ZYMV of six different genotypes were transplanted to the field. Number of fruit and fruit weight per plant, and average fruit size were reduced up to 50% in virus-inoculated plants. In the inheritance study susceptible genotypes were ‘Verde Luz’ (VL), ‘Taina Dorada’ (TD) and ‘TP411’ (TP). The third to fifth leaf of inoculated seedlings were rated on a 0 to 4 scale for disease severity and scores were combined to convert to a 0 to 12 scale. F2 populations using NL as the source of resistance had a nearly normal distribution with an average disease severity of 5.23 in NL x TD and 6.25 in VL x NL. In contrast, the F2 populations with MEN were strongly skewed towards resistance with an average severity of 3.38 in MEN x TD, 2.27 in VL x MEN and 2.80 in TP x MEN. The NL x MEN F2 population was highly skewed, with an average combined severity of 0.840. It segregated 224:14 (R:S) when a combined severity of <4 was considered resistant. Segregations in resistant x susceptible F2 populations were variable, depending on how severity scores were combined into the resistant versus susceptible classes. However, most segregations suggested that at least two genes are involved in the inheritance of resistance to PRSV for both NL and MEN. The data clearly indicate that at least some of the genes for resistance in NL and MEN are different. Considering the level of resistance conferred by both NL and MEN, both sources, either alone or combined, will be useful in a breeding program.