Selection and breeding for insect and disease resistance

Conclusions

In Breeding Pest-resistant Trees, the only dedicated book on the subject published in 1966, the authors state that "very few intensive efforts to breed insect resistant trees have been started, and even these [are] in their infancy". This statement is still valid today although the use of biotechnology has brought about significant changes in the research area.

2002 saw the first commercial deployment of genetically-modified forest trees in China, but because the trials are relatively new there is limited follow-up and monitoring of their status and impact so far. Genetic modification is a quick, 'short-term fix' that uses total resistance and single gene constructions that could be bypassed by insects. It could be necessary to insert various Bacillus thuringiensis (Bt) and insect resistance genes, and possibly to continue patching genetically-modified trees as insects break resistance. Genetic modification-based resistance thus entails an agricultural model, and is justified financially and ecologically in only a few cases. This is a very controversial issue and public opinion should be considered very important.

Based on the information gathered in this review, there are two important conclusions that should prompt further discussions in government and private agencies that fund or undertake research into disease and pest resistance breeding.

First, there has been an enormous amount of research into many of the biological details of many host–disease/pest systems, whether with molecular markers associated with the resistance or more basic population or individual tree surveys for the expression of resistance. A great deal of this research has given us substantial insight into many types of tree responses and mechanisms of resistance to numerous biotic challenges. However, although of great biological interest, much of it has had a difficult time in finding its way into applied breeding programmes and deployment of improved resistant material. There are likely several reasons for this, but the most common seems to simply be that the species/populations do not have well-developed tree improvement programmes to support the delivery of any resistant selections. Another may be that resistance is present, but it may not be silviculturally useful for the scale of the biotic challenge, but this is difficult to definitively pin-down from literature.

Second, the relatively few programmes that have advanced far enough to be actually deploying improved resistance in reforestation efforts, have been in developed countries that have had substantial resources to maintain the needed efforts of selection, testing and seed production. For example, the disease resistance programmes in the southeast United States for fusiform resistance in loblolly pine, needle-cast resistance in radiata pine in New Zealand, blister rust resistance in white pine in western North America, and resistance to spruce terminal weevil in spruces in western North America largely represent the biggest successes. In conclusion, this represents a substantial challenge to forest tree breeders, forest managers and funding agencies to try and find ways to focus efforts on high priority disease or pest resistance in regions, countries and areas with relatively fewer resources available. This may be particularly true if factors such as climate change continue to increase the prevalence of insect outbreaks, and as more exotic pests and diseases become established in new countries.