An update on efforts to grow sterile cultivars of this important shade tree for Oregon growers

The nursery industry in the Pacific Northwest markets a great deal of plant material to the upper Midwest and New England. Overall, Oregon ships 75% or more of its nursery plant material out of state. Shade trees are among the most important plants marketed to these areas, and nationwide, Oregon accounts for nearly a quarter of the total deciduous shade tree sales.

Historically, American elm (Ulmus americana) and ash (Fraxinus spp.) have been principal trees in the urban canopy, making them trees of great economic importance to the nursery industry. However, the emergence of Dutch elm disease and, more recently, emerald ash borer have reduced the viability of American elm and ash.

Norway maple (Acer platanoides) has been an important staple shade tree that could be reliably used as an ash alternative. Norway maple grows well under a wide range of conditions, including sand to clay soils and acid to calcareous soils. It is hardy to USDA Zone 4 and tolerates heat, drought, and compacted soils common in urban settings. Furthermore, Norway maple is tolerant of air pollution — ozone and sulfur dioxide in particular — making it ideal for use as a street tree.

However, Norway maple has naturalized in forests of New England. As a result, its planting has been essentially eliminated in these states, including bans in Connecticut and Delaware.

We have been breeding for sterile forms of Norway maple by developing triploids (Contreras, 2017). These would benefit the nursery industry by allowing these cultivars to be marketed in areas where its planting has been reduced due to the tree’s weediness.

Breeding with two goals in mind

At Oregon State University, the Corvallis, Oregon site where we conduct our breeding work on maples has what appears to be a substantial infestation of verticillium wilt. We did not know about this when the plants were first planted.

After one to two years in ground, we observed trees beginning to decline; some died. Importantly, we observed some trees with less or even no symptoms of wilt. We have since focused our seed collection on these superior tetraploids as potential sources of resistance to verticillium. Additionally, since confirming the causal agent as verticillium, we have discarded seedlings that appear to be infected.

Breeding work continues on both goals — sterile triploids, and resistance to verticillium wilt. We are making positive progress on both fronts.

Currently, Norway maple cultivars are primarily propagated by budding onto seedling rootstocks. However, this method of propagation is largely incompatible with our breeding goals. To facilitate reintroduction of Norway maple into regions where it has been banned, it will need to be propagated on its own roots. This is because the fertile seedling rootstock would represent an opportunity to escape cultivation due to outgrowth of the rootstock and/or failure of the scion that would lead to stump sprouts and ultimately a fertile flowering tree.

Furthermore, we are breeding for resistance to a soilborne pathogen (verticillium wilt) and it does little good to introduce a resistant cultivar (scion) only to be budded onto a susceptible seedling rootstock.

Propagation from stem cuttings is feasible and widely practiced in other maples, such as red maple, to avoid graft incompatibility. In Norway maple, cutting propagation is not widely used due to its difficulty and low success rate.

In 2007, Wolfgang Spethmann, professor at the University of Hannover in Germany, reported success using 70-90cm cuttings treated with 5,000 ppm indole-3-butyric acid (IBA) under high-pressure fog. This method may be useful to propagate relatively small numbers, but using this size cutting does not lend itself to efficient production.

We have ongoing studies to optimize cutting propagation using more traditional sized cuttings and in preliminary studies from 2018, we had up to 60% success. During that year, we collected cuttings in August while the shoot meristems were active.

Cuttings were treated with a wide range of IBA forms (talc, quick dip, foliar, and long basal soak) and we also separated into terminal vs. sub-terminal cuttings. From that study, the best results were found in terminal cuttings treated with 16,000 ppm IBA-talc and subterminal cuttings treated with 1,000 ppm IBA foliar spray. Both of these treatments resulted in 60% rooting — but this preliminary study was conducted on small numbers of cuttings (Chart. 1). Overall, subterminal cuttings had better success than terminal cuttings across IBA application methods and rates and produced a more robust root system.

In 2019, we focused on using two node subterminal cuttings treated with 16,000 talc and compared wounding vs. unwounded cuttings — again collected during August when meristems were active. Wounding improved rooting and resulted in 85% overall rooting compared to 63% in unwounded (Chart 2). Visually, wounded cuttings had superior root systems (Figure 2). These results are encouraging and provide a basis for the next phase of research.

The 2018 and 2019 studies were performed on seedling rootstocks. It is well-established that juvenile plants have greater capacity for adventitious root formation and mature cultivars will likely prove more challenging due to reduced regeneration potential associated with adult plants. With that in mind, we will continue trying to optimize cutting propagation of mature cultivars by manipulating stock plant handling, timing, and hormone applications.

In 2020, we will collect cuttings from several superior forms of triploids that have potential as future releases (Figure 1) and compare to several industry standards.

Take-aways from 2018, 2019 studies

Sub-terminals responded best overall (especially on cuttings of a thicker caliper). 16,000 ppm IBA-talc produced the best quality roots with foliar IBA being a close second.

Wounding of sub-terminal cuttings produced a nice improvement in both the total number rooted and the number of those with robust root systems.

Primary question for the future

Will these methods translate to industry standard cultivars and any forthcoming sterile cultivars? We will repeat some of the more promising methods using several industry standard cultivars and several of the triploid selections in development at OSU.

Will rooted cuttings finish the production schedule on time? Rooting is important but timing must work out for commercial production. To address this question, we will work with Dr. Lloyd Nackley, nursery production researcher, to conduct formal studies in partnership with commercial nurseries to compare production scheduling between budded plants and rooted cuttings.

To view our ongoing breeding efforts in Norway maple or for a tour of the breeding program, please contact either of us. We’d love to have you.

Ryan Contreras is an associate professor of ornamental plant breeding at Oregon State University. He can be reached at [email protected]. Tyler Hoskins is a faculty research assistant at OSU. He can be reached at [email protected].

Literature cited

Contreras, R.N. 2017. Developing triploid maples. HortScience 52(9S):S276. (Abstract).

Spethmann, W. 2007. Increase of rooting success and further shoot growth by long cuttings of woody plants. Propagation of Ornamental Plants 7:160-168.