Disease resistance studies and breeding in Papua New Guinea
Disease resistance studies and breeding in Papua New Guinea (PNG) were previously done mainly on two major diseases, Vascular Streak Dieback (VSD) caused by the fungus #Oncobasidium theobromae# and black pod disease caused by #Phytophthora# spp. (Ppr). The development of Trinitario x Amazonian polycross hybrids was the major breeding strategy. Hence, the breeding work was concentrated on the identification of parental clones with the best possible levels of resistance to the two diseases, the assessment of their progenies and on genetic studies. Results of the genetic studies showed that for both VSD and Ppr the resistance is polygenic (horizontal) and largely inherited as additive genes. Two polycross hybrids were released, SG1 in 1982 with major emphasis on resistance to VSD, and SG2 in 1988 with additional improved levels of Ppr resistance. CCRI has recently changed its breeding strategy from the development of polycross hybrids to hybrid-derived polyclonal varieties. All clones will be carefully examined in multilocation trials in different environments around the country before their official release. Presently, priority for resistance breeding is being given to Ppr. The development of reliable screening methodologies to assess a large number of progenies is considered as the most important step for improving the level of disease resistance. Attempts are being made to widen the genetic base of resistance by introduction of cultivars from overseas and collections of locally adapted Trinitario germplasm.