2019 ASHS Annual Conference

Viruses of the Potyviridae impact all cucurbit crops. Papaya ringspot virus (PRSV) is among the most common of the potyviruses found in Puerto Rico and has been observed to reduce yields of tropical pumpkin (Cucurbita moschata) by up to 50%. Two sources of resistance are well known in C. moschata: ‘Nigerian Local’ (NL) and ‘Menina’ (MEN). The inheritance of resistance from NL has been variously reported as due to a single recessive gene or due to two genes with dominant suppression epistasis. To our knowledge, no inheritance studies have been reported for MEN, nor is it known if resistance to PRSV in NL is allelic to that in MEN. We studied the inheritance of PRSV resistance in mechanically inoculated F2 populations derived from both sources of resistance, and tested for allelism in a NL x MEN F2 population. Susceptible genotypes were ‘Verde Luz’ (VL), ‘Taina Dorada’(TD) and ‘TP411’ (TP). The second to fourth 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 close to normal distribution with a medium disease severity of 4.5 in NL x TD and 5.0 in VL x NL. In contrast, the F2 populations with MEN were strongly skewed towards resistance with a medium severity of 2.5 in MEN x TD, 0.5 in VL x MEN and 1.5 in TP x MEN. The NL x MEN F2 population segregated 215:23 (R:S) when a severity of ≤3 was considered resistant, indicating that NL and MEN resistance genes are not allelic. 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, alone or combined, will be useful in a breeding program.