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Fusiform Rust

Fusiform Rust
USDA Forest Service,
Fusiform Rust

Fusiform rust is a widespread and damaging disease of loblolly pine (Pinus taeda) and slash pine (P. elliottii) in the southeast. Caused by the fungus Cronartiium quercuum f. sp. fusiforme (Cqf), the disease leads to rust galls or cankers on the main stem and/or branches of trees. Rust galls effect wood quality and yield by causing deformed and broken stems in young trees. Research has identified families of these pines with improved genetic resistance to the disease, allowing production and planting of resistant seedlings in areas at risk.

An Economic Evaluation of Fusiform Rust Protection Research compared the cost of fusiform rust research to the simulated benefits of rust resistant seedlings in plantations that have been or are projected to be established southwide between I970 and 2020. Results showed likely benefit-cost ratios of about 4:1 to 6:1 for fusiform rust research. Currently, anticipated improvements in resistance will not eliminate all physical and financial damages from the disease; simulation results indicate substantial financial benefits yet remain for additional research and development.

Researchers at the Southern Institute of Forest Genetics at the Southern Research Station continue to work to improve the diversity of resistance sources in southern pines using cutting edge genomic tools. Progress that has been made includes the mapping of several pine resistance genes and the release of a reference genomic map. Having a pine genomic sequence positions researchers well to more precisely map resistance gene loci in highly resistant material, and further, to identify the resistance genes themselves.

The fact that the fusiform rust fungus has coevolved with its pine host allows the study of interesting genetic interactions. It can be inferred that all the mapped resistance genes have corresponding avirulence genes in the pathogen (Cronartium quercuum f.sp. fusiforme, Cqf). The genome of the fungus has been mapped and now sequenced. Cqf genome sequence has enabled the identification of an interval containing 12 candidate genes for Avr1, the gene conditioning avirulence to Fr1 gene (Fusiform rust resistance gene 1). The identification of Avr1 paves the way to determine the genomic locations of more Cqf avirulence genes. This knowledge would not only inform the biology of host-pathogen interaction in general, but also allow monitoring of virulence levels across the range of southern pines and allow the development of more advanced applications in breeding pines for rust resistance and managing the disease in the forest. To learn more, contact Dana Nelson, Project Leader and Research Geneticist for the Southern Institute of Forest Genetics.

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