Research Update

Jason Smith and Jeff Gillman

After another Minnesota winter, summer has finally arrived in Minnesota. During the short spring season (at least the calendar said spring) several articles were published in scientific journals that may be of interest to the nursery industry in Minnesota. Subjects explored in these articles include: Japanese beetle preference between purple-leaved and green-leaved cultivars of woody plants, new ‘Lights series” azalea introductions, growth of alder and birch species after drought and flooding, effects of tree shelters on growth and gas exchange of four tree species under field and nursery conditions and the effects of irrigation and root pruning on the development of roots balls on oak trees and subsequent digging survival.

In providing this synopsis, we ask you to keep in mind that this research may have been conducted in a climate very different from Minnesota and as always, we do not endorse the use of products that are inconsistent with their labels.

· The Japanese beetle (Popilla japonica) is a new pest that Minnesota nurserymen and gardeners will have to deal with in coming years. It is important to know what plants are most susceptible and what plants are resistant. In a study conducted at the University of Kentucky, Rowe et al. (1) compared the susceptibility of purple and green leaved cultivars of woody plants to Japanese beetle feeding damage. In this study, the researchers compared Japanese beetle susceptibility of cultivars of Acer, Berberis, Cercis, Cotinus, Fagus, Malus and Prunus in the field during three seasons. Using laboratory choice tests, the researchers also investigated whether Japanese beetles prefer purple-leaved cultivars of closely related plants. The field studies indicate that the Acer, Malus and Prunus cultivars are much more susceptible to attack than the Berberis, Cercis, Cotinus, or Fagus – regardless of the cultivar tested. The results of the choice tests showed that for most species there was no preference and some preferred green-leaved cultivars, however, purple-leaved cultivars of Norway maple were preferred over green cultivars. Overall, the results of both the field and laboratory tests indicate that Japanese beetles prefer certain species, not necessarily purple cultivars. The authors suggest that the widespread use of purple-leaved cultivars of susceptible species has probably contributed to the perception that purple-leaved cultivars are more susceptible to attack by Japanese beetles.¹

· The “Lights series” azaleas, developed by University of Minnesota researchers, have long been popular for landscape use because of their beautiful flowers, cold hardiness and adaptability. In the March 2002 issue of Journal of Environmental Horticulture, Rose et al. announce the release of the three newest cultivars: ‘Mandarin Lights’, ‘Lemon Lights’, and ‘Tri-Lights’. ‘Mandarin Lights’ was selected as a seedling hybrid between Rhododendron calendulaceum and R. ‘Orangeade’. ‘Lemon Lights’ was selected as a seedling from an open-pollinated unnamed Rhododendron species. ‘Tri-Lights’ was selected as a seedling from a controlled cross between a Rhododendron species (similar to R. atlanticum) and an unnamed hybrid of the Exbury group. Both ‘Mandarin Lights’ and ‘Lemon Lights’ are hardy to at least – 32 ^oF (based on controlled laboratory freezing tests and field evaluations). The vegetative buds of ‘Tri-Lights’ are hardy to –30 ^oF and the floral buds are hardy to –25 ^oF. All three cultivars have performed well (no serious disease or insect problems) at the University of Minnesota evaluation site. The cultivars are mainly propagated via tissue culture, but softwood cuttings can be rooted (using 0.8 % IBA, stuck in 1:1 peat:perlite, with 95-100% relative humidity). Both ‘Mandarin Lights’ and ‘Lemon Lights’ are currently available in the nursery trade and ‘Tri-lights’ will probably be available at the retail level in 2003. ²

· Landscape trees must endure a large variety of pests and diseases during their lifespan. Much research is directed at selecting for resistance to these limiting factors. However, much less attention has been given to selecting species that are resistant to unfavorable climatic factors such as drought and flooding. Graves et al. (3) provide a synopsis of their work of evaluating the photosynthesis and shoot growth of five birch species (Betula albosinensis, B. costata, B. davurica, B. uber, and B. populifolia) and four alder species (Alnus hirsuta, A. incana, A. japonica and A. maritima) after they were subjected to drought and flooding. The results indicate that B. davurica and B. uber may have greater flooding tolerance than most other birch species and B. albosinensis and B. davurica are more drought tolerant. Among the alders, A. hirsuta and A. maritima are most drought and flood tolerant of the alders tested. The data provided here suggest that these underused species may have specific adaptations to stresses such as drought and flooding that makes them particularly desirable for alternatives to less adaptable species. ³

· Tree shelters such as the “Tuley Tube” have been in use for decades in both the United States and Great Britain (where they originated). They are reported to provide protection from lawnmowers, herbicide drift and browsing and have also been reported to improve first year survival of seedlings planted in the field. In the study summarized here, the authors at Auburn University compared the effects of tree shelters on growth and gas exchange of four tree species under field and nursery conditions. The species included in this study were as follows: Quercus alba, Q. acutissima, Fraxinus pennsylvanica, and Cornus florida. The survival overall (all species combined) was 35 % greater for sheltered vs. non-sheltered trees grown in the field and 10 % greater for trees grown in the nursery. Height growth and biomass production was greater for sheltered trees in the field, but less for nursery grown trees (62 % reduction in root growth). Photosynthesis was reduced for sheltered trees in both field and nursery conditions and internal leaf carbon dioxide content was greater for sheltered trees grown under both conditions. The results provided here indicate that tree shelters are beneficial for field grown trees as they help reduce stresses from ambient conditions. In the nursery however, tree shelters do not benefit the growth of seedlings, but the authors point out that they could be useful for training ornamental trees. ⁴

^· Root pruning and irrigation methods have both been implicated as factors for successful harvesting of field grown trees. In particular, it has been suggested that root pruning enhances development of small-diameter, lateral roots that are important for survival of balled, field grown trees. Use of drip irrigation techniques, especially on sandy soils, is believed to also encourage the development of small-diameter, lateral roots – and aids in successful harvest. In a study conducted at the University of Florida by Gilman et al. (5), it was found that mode of irrigation (placement or volume) did not significantly affect root balls or digging survival of field grown live oak. It was found that root pruning, by either root-pruning fabric placed under the liner at planting or by use of a spade, does significantly improve survival after digging. In addition, root pruning was not found to decrease caliper and only slightly decreased height growth. The reason for better survival of root pruned trees is likely due to the fact that root-pruned trees had more small-diameter roots than non-pruned trees. Root pruning also increases the digging time allowing successful digging of live oak during the summer as well as the more traditional winter season. This study supports earlier suggestions that root-pruning improves survival of dug trees and increases the number and weight of smaller-diameter roots. ⁵

Literature Cited

¹ Rowe, J.W., Potter, D.A., and McNiel, R.E. 2002. Susceptibility of purple- versus

green-leaved cultivars of woody landscape plants to the Japanese beetle. HortScience 37(2): 362-366.

² Rose, N., McNamara, S., Pellet, H. and Zuzek, K. 2002. Rhododendron ‘Mandarin

Lights’, R. ‘Lemon Lights’, and R. ‘Tri-Lights’. Journal of Environmental Horticulture 20(1): 62-63.

³ Graves, W.R., Kroggel, M.A., and Widrlechner, M.P. 2002. Photosynthesis and shoot

growth of five birch and four alder taxa after drought and flooding. Journal of Environmental Horticulture 20(1): 36-40.

⁴ West, D.H., Chappelka, A.H., Tilt, K.M., Ponder, H.G. and Williams, J.D. 2002. Effect

of tree shelters on growth and gas exchange of four tree species under field and nursery conditions. Journal of Environmental Horticulture 20(2): 96-100.

⁵ Gilman, E.F., Stodola, A. and Marshall, M.D. 2002. Root pruning but not irrigation in

the nursery affects live oak root balls and digging survival. Journal of Environmental Horticulture 20(2): 122-126.