Risk assessment with current deployment strategies for fusiform rust-resistant loblolly and slash pines

In the southeastern USA, fusiform rust resistant loblolly and slash pines may be deployed as 1) ulked seed orchard mixes, 2) half-sibling (sib) family mixtures, 3) single half-sib families, 4) full-sib family mixtures, single full-sib families from 5) "bulking up" or producing large numbers of controlled cross seeds, or as 6) clones of individual genotypes. These deployment types are respectively less genetically variable and less well buffered against environmental stress, but provide respectively greater genetic gains from higher selection intensity. Currently, bulked seed orchard mixes are deployed by all state organizations and many smaller companies, but about half the 1.1 billion loblolly and slash pines deployed annually are planted in half-sib family blocks. The most aggressive landowners plant virtually all of their land with a small number of half-sib families. Full-sib families and/or clones are currently planted on a small fraction of the total area regenerated, but research and development seeks to make the deployment of full-sib families and clones economical to increase the genetic gains from applied tree improvement programs. Resistance to fusiform rust currently being deployed is likely due to resistance based on both major genes and genes of small, cumulative effects. However, major genes for resistance to fusiform rust have been discovered using molecular genetic techniques, and deployment strategies are currently being developed. "Boom and bust" cycles of pathogens on other crops when major genes were deployed against them create concerns that these same problems might arise when deploying major genes for resistance against fusiform rust. We assessed the risk that fusiform rust might overcome one to few major genes for resistance if they are deployed widely and strategies to mitigate the risk that this will occur. We concluded that the deployment strategies currently in widest use (bulked seed orchard seedlings and half-sib family blocks) robustly resist fusiform rust infection. Plantations are probably sufficiently genetically buffered to present little risk of cataclysmic failure, as current resistance is likely to be based on both major and minor genes. Furthermore, these same deployment strategies are likely to provide robust protection against risk factors other than fusiform rust. We concluded that deploying pine cultivars with known genes for major resistance to fusiform rust in regions where their associated virulence genes are absent or in low frequencies is a practical near term strategy and that deploying a mosaic of different resistance genes may mitigate the presumed greater risk of deploying full-sib family blocks or clones.