January 2021 – 2nd Edition
Selecting Rust Resistant Seedlings for a Successful Plantation
Article by Patrick Cumbie, PhD
Selecting fusiform rust resistant loblolly and slash pine seedlings is critical for many landowners. If your stand is in a moderate to high hazard area for fusiform rust disease, it is very important to have a family with good rust resistance. Rust resistance, combined with growth and stem form are evaluated for selecting commercial families. Your ArborGen Reforestation Advisor can help ensure that you receive seedlings with the appropriate level of rust resistance for your specific location.
Rust resistance is typically reported using a score called an R50. This score is scaled against an unimproved checklot which is set at 50%. If a family has an R50 of 20%, it is expected to have 20% incidence if the unimproved family were at 50% infection on the same site. In progeny tests, we can see families range from 0 to 100% infection. Most trials have an average rust incidence level ranging from 20% to 50% which gives us a very good population to evaluate and select superior families. The R50 score is very helpful to rank families and understand the rust incidence level compared to unimproved seedlings as well as other families a landowner may purchase.
The R50 score is not a guarantee of performance, but it is based on actual data from a set of experiments in the field. It is very useful for landowners to select resistant seedlings, but it is not a guarantee of what rust levels will be in your plantation. For example, a highly resistant MCP family may have an R50 of 5%. The table below shows what could happen on a site depending on the rust hazard compared to less resistant OP families and unimproved seedlings.
If a plantation is in a lower hazard area where the hazard may be below 25%, the landowner may not need the highest rust-resistance available to achieve success. If the site is in a high hazard area where unimproved seedlings may have greater than a 50% hazard, then superior rust resistance is needed. Fusiform rust hazard maps can tell us, in general, if you are located in a higher hazard area, but they do not provide a prediction for your individual plantation. Local weather patterns and the presence of oak species (the alternate host of the fungus) can have a significant impact on the fusiform rust incidence in your plantation.
If you are located in a higher hazard area, selecting the most rust-resistant seedlings is your best defense. MCP families and varietals offer superior rust resistance along with growth and stem form performance. Varietals, as well as MCP families, are screened for rust resistance in the field as well as in greenhouse trials. Varietals, which are produced as rooted cuttings, may also offer additional resistance compared to seedlings. Two highly resistant parents can be crossed to capture rust resistance that is superior to OP families. OP families offer a range of resistance as well, so paying attention to the R50 score and your rust hazard is essential to put your best foot forward!
To ensure you get seedlings with the proper level of rust resistance for your specific site,
Randolph, K. C., E. B. Cowling and D. A. Starkey, 2015. Long-Term Changes in Fusiform Rust Incidence in the Southeastern United States. Journal of Forestry 113: 381-392. (link)
Figure 6. Estimated fusiform rust hazard for loblolly pine based on rust incedence assessments on FIA plots from the 2010s (A) and on the average of assessments from the 1990s and 2010s (B).
Figure 1. Hazard maps from Randolph et al (2015).
Severe fusiform rust incidence in loblolly pine.
A fusiform rust stem gall on a progeny test seedlings. In the spring, orange-colored aeciospores cover the gall and will infect oak leaves to continue the life cycle of the fungus.
Dr. Patrick Cumbie is manager of Pine development for ArborGen Inc. Patrick has been involved in forest tree breeding research and development for more than 16 years. His career has largely focused on coordinating and implementing accelerated breeding programs of loblolly Pine. Before joining ArborGen Inc. in 2010, Patrick worked in both industry and university-based research programs. He has received his BS in Forest Management, MS in Forestry and PhD in Forestry at North Carolina State University.
Planting for Better Survival
Shallow planting is a major cause of seedling mortality. Bareroot seedlings should be planted at least 2 inches above the root collar, i.e., 2 inches deeper than they were grown in the nursery. For container Loblolly and/or Slash Pine, our recommendation is to plant the root ball at or slightly below ground level to get the roots down to moist soil and for stability. The recommended planting depth for container Longleaf in open agriculture fields, scalped fields and wet sites is where the root ball is slightly above ground level to make sure the bud does not get covered. Cut over hand planted areas should have the root ball at ground level or slightly above.
Here Dr. Paul Jeffreys shows you what a well-planted seedling looks like – the right way.
Maximize your seedling survival by avoiding these common planting mistakes.
Root collar and roots exposed, drying out roots.
Pine bud within 2" of ground line.
"J" or "U" Roots
Roots form "J" or "U" shape resulting in poor or weak root development.
A firm pull on one needle cluster moves plant.
Excessive Tap Root Pruning
Pine seedling roots should not be pruned shorter than 5". Hardwood seedling roots should not be pruned shorter than 6".
The tap root should not be planted at more than 30˚ from perpendicular.
"L" or Drag Root
Seedling pulled along in trench by planter.
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Paul Jeffreys, Ph.D.
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