Resistance to fusiform rust disease is measured by what southern pine tree breeders call R50. If an unimproved loblolly pine seedlot would be expected to have 50% rust on a site, then its R50=50. A very resistant family might have an R50 value of 20% (e.g. it should have 20% rust when the unimproved seedlot would have 50% rust). A susceptible family would have a R50 value above 50%. We have converted these rust scores to relative R grades of A, B, C, and D. The most rust resistant would have R grades of A, and the most susceptible would have R grades of D. An R grade of C means that it is average for rust resistance.