Production of the delicious and nutritious tropical fruit papaya (Carica papaya) is limited throughout its growing regions by the devastating disease of Papaya Ringspot Virus type P (PRSV-P). Since 1998, breeding strategies have focussed on transferring resistance alleles from two sources of resistance from crop wild relatives. Vasconcellea pubescens is the strongest source of resistance but produces infertile hybrids when crossed to papaya. Loci with resistance alleles from the immune V. pubescens source were identified in an interspecific population derived from crossing V. pubescens and V. parviflora. V. parviflora is susceptible to PRSV-P but can be crossed with both papaya and V. pubescens and is therefore being used as a bridging species for transferring useful alleles between the two species. A fourth species, V. quercifolia also carries alleles for PRSV-P resistance, although more quantitative in action with different levels of resistance to PRSV-P. This species also produces some fertile intergeneric hybrids when crossed with papaya.