Inheritance of Papaya Ringspot Virus Resistance In Temperate and Tropical Genotypes of Pumpkin

Cucurbita moschata Duchesne (butternut squash; tropical pumpkin) is an important vegetable crop in tropical and subtropical regions worldwide. The disease caused by Papaya ringspot virus (PRSV) has a negative impact on the yield of this and other cucurbit crops. In Puerto Rico, PRSV is one of the two most common viruses affecting tropical pumpkin. One of the goals of our breeding program at the University of Puerto Rico is to incorporate resistance to PRSV into local cultivars. C. moschata cv. Nigerian Local (NL) has been used as a source of resistance to various cucurbit viruses, including PRSV. Inheritance of PRSV resistance was previously reported to be a single recessive gene based on a study in the population NL x ‘Waltham’ (a temperate butternut type of C. moschata). Nevertheless, our experience when attempting to backcross NL resistance into tropical genotypes of C. moschata suggests that the inheritance might be more complex. The objective of this study was to confirm the inheritance of resistance PRSV in additional populations. The F₁, F₂ and BC₁ generations derived from the cross of NL x various susceptible C. moschata genotypes (‘Waltham’ and tropical genotypes ‘Soler’, ‘Taina Dorada’, ‘Verde Luz’) were used to test the inheritance of PRSV resistance. The cotyledons of 9 to 12 day old plants were mechanically inoculated with a local, molecularly characterized PRSV isolate. Symptoms were evaluated from 10 to 40 days post-inoculation. The virus was monitored by ELISA. Inoculated plants of NL remained completely free of symptoms.  When only F₂ plants completely free of symptoms were classified as resistant, we did not observe a 3:1 (susceptible:resistant) ratio as would be expected with a single recessive gene for resistance. When plants with very mild symptoms were included in the “resistant” category, the segregation fit a 3:1 ratio in each of the F₂ populations. Many of the resistant F₂ individuals showed some degree of very mild symptoms, rather than being symptom-free. When these F₂ individuals classified as resistant were planted in the field, many of these plants developed mild to severe PRSV symptoms. Very few plants remained symptom-free. This study demonstrates the importance of progeny testing of resistant individuals when selecting for PRSV resistance.