Selection and breeding for insect and disease resistance
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.
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 still represent the biggest successes. However, a few new programs, such as root disease resistance in Port-Orford cedar, can now be considered operational as well.
Lastly, the 4th International Workshop on the Genetics of Host-Parasite Interactions: Diseases and Pests in Forestry, held in 2011, was one of the most important conference in tree breeding for disease and pests resistance since its inception. It was one of the few conferences we are aware of that brought tree breeders, pathologists, entomologists, ecologists and evolutionary biologists together to discuss pest and disease resistance breeding; all five disciplines are now considered important in this field for future success in developing and deploying more resistant germplasm in forested ecosystems (in planted or semi-natural forests).
The conference had research and development presentations broken down into seven topic areas:
- Mechanisms of Resistance
- Breeding for Resistance in a Changing Environment - Durable Resistance: Hopes, Pitfalls, and Management Strategies
- Ecology and Evolutionary Biology of Resistance and Tolerance, Natural Systems
- Molecular and Genomics Tools in Resistance Programs
- Evaluating Resistance
- Fusiform Rust Resistance
- Resistance Breeding Programs
While it is not possible to summarize all aspects of the conference, or the papers presented there, several key messages or major new developments that surfaced at the conference were:
- Developing genetic resistance to all current pests of forest trees is obviously impossible, but certain classes of the most prevalent and damaging types of insect or diseases may be an approach for focussing a wider body of research towards. There was little doubt that overall the Phytophthora’s are representing some of the biggest concerns across the world.
- With new pests and disease problems becoming more prevalent, and surfacing at a pace faster than we perhaps have seen previously, we may “...need to combine multiple-defence traits to fight against the wide range of potential pests”. So, we are probably in a situation of trying to find where ‘cross resistance’ is present, to our best guess of what pests or pathogens we could be facing, and then combining or ‘stacking’ various resistance mechanisms.
- While there are some large successes in ‘commercial focussed’ tree breeding programmes, the approaches and products of resistance selection and screening we use in tree breeding, could be used and applied in restoration strategies for semi-wild or even natural ecosystems that have been devastated by exotic pests and diseases. Several examples of work that is underway, that may lead to such ‘enrichment’ plantings to re-establish species devastated or being devastated by exotic pests, were/are; Sudden Oak Death syndrome for tanoaks), elm in America, elm in Europe, Pacific madrone, Hawaiian koa, and emerald ash borer, beech bark disease and butternut canker in America. All of these examples are similar in the general objectives to the work that has been underway now for decades now in American chestnut. Perhaps this represent the majority of the work in the future, if exotic pest introductions continue around the globe.
In conclusion, while noteworthy progress and successes can still be expected, the 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, still remains significant. As noted many times here, 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.