Program seeking durable Pyrus for small sites

A cooperative project with the Landscape Plant Development Center (LPDC) is underway at Washington State University Puyallup targeting the development of durable new varieties of small-statured Pyrus (pear) trees. Durable plant tolerate difficult environmental and biological conditions like disease, drought, poor soil, extreme temperatures and pests. At the same time these new plants must have improved form, aesthetic qualities and landscape function.

Trees with small stature are important because as lot sizes shrink and home sizes grow, the space for tree planting becomes more limited in the modern American landscape. Another important consideration for anyone breeding and selecting new plants is the invasive potential of an introduction. The goal is to produce durable plants for sustainable landscapes, not plants that could become weedy and threaten native vegetation.

Highly variable genus

Tree breeding requires a long-term commitment with careful observation and testing before any variety is released. Pyrus was chosen because there is considerable genetic variation for climate and soil adaptation, landscape traits and disease and pest resistance among species in the genus.

Five-hundred hybrid pear tree seedlings, some with as many as four species in their parentage, were planted to the field in 1993 and 1994 and now have enough age and size to give a good indication of their performance in the landscape.

Part of the pear plot is in this photo, taken from an aerial lift bucket in spring 2007. The photo gives a good indication of the variability in timing and display of the blossoms. Some of the trees were past bloom when the photo was taken; some were in full bloom; and some had yet to bloom. You can see trees that put on their flower show first and then grow leaves, while others expand their leaves before opening their flower buds. This characteristic along with the size and number of flowers determines the spring floral display of an individual tree.

To identify trees with superior landscape characteristics, we have been recording data on growth, flower, fruit and foliage characteristics. Tree heights and trunk calipers at 6 inches above ground are measured. Fruit size and the number of fruit on a tree are rated, and fall color has been evaluated yearly since 1996.

As you can see in this photo, the pear hybrids show a great deal of foliage color variation in fall. Fall color evaluations involved periodic visual evaluation of three characteristics:

* Percent defoliation of the tree.

* Leaf color.

* Numerical rating of the quality of the whole-tree fall color display.

Invasive potential, pest resistance studied

The potential of a tree to escape cultivation and become an invasive species is another factor taken into consideration. For example, trees that produced root suckers were destroyed.

Selection for insect and disease resistance is important in developing new trees and will reduce the use of pesticides in our landscapes.

Fire blight can be a devastating disease of pears, and any tree released for landscape use must be resistant to this disease. Susceptibility to fire blight has been tested by cooperators at North Carolina State University and Oregon State University and is being tested at WSU Puyallup by inoculation with the pathogen.

Pseudomonas blossom blast and dieback disease has been prevalent in the Puyallup planting, and trees in the field are rated for susceptibility to this disease.

Promising selections

In deciding which trees have potential for release as a new variety, we combine all of the data collected with evaluations of individual trees in the field and expectations based on their genetic background. In 2001, 17 individual trees were selected for advanced testing and were budded onto rootstock by a cooperating nursery.

In spring 2003, these trees were shipped to cooperators around the United States, where performance evaluations are underway. Parentage of these selections indicates they should possess good drought and heat tolerance. A survey of the cooperators will help us determine how well their selections performed.

The LPDC’s first pear selection, Silver Ball, was introduced in 2006. A later selection has Pyrus ussuriensis, the most cold-hardy pear species, in its parentage. This tree has good form, flower and fall color characteristics but requires testing in additional geographic areas before it can be considered for release. Another promising pear selection has the most brilliant fall color we’ve seen.

We are continuing to collect data and make selections with the goal of introducing superior, durable trees for modern landscapes.

Center’s goal: superior plants for sustainable landscapes

Developing improved cultivars of North American native plants is another important part of the Landscape Plant Development Center’s efforts to provide superior plants for sustainable landscapes.

The center’s first shrub introduction, ‘Center Glow’ ninebark, is an improved cultivar of the native Physocarpus opulifolius. The leaves of ‘Center Glow’ emerge rosy red with a bright-golden base and darken with maturity to deep burgundy.

A variegated form of Diervilla sessilifolia, bush honeysuckle, has been selected as the center’s next introduction. Native to the eastern United States, Diervilla is an attractive, small shrub with interesting foliage and yellow flowers . Application has been filed for a plant patent and it has been given the trademarked name Cool Splash. It will be available in 2009.

Ceanothus americanus, commonly called New Jersey tea or mountain sweet, is another native shrub with considerable merit for landscape use. Thought to be the most cold-hardy of the Ceanothus, it is drought tolerant and adaptable to full-sun or shady conditions. The flowers, produced in early summer, are borne in terminal clusters and give rise to another common name for C. americanus, wild snowball. The center is making and evaluating selections from seedling populations for possible introduction of superior cultivars of this species.

In addition to these plants, the LPDC is working to improve the landscape qualities and tolerances to environmental stresses of a number of other tree, shrub and herbaceous perennial species.

For more: Landscape Plant Development Center, (952) 443-1505; www.landscapecenter.org.

- Rita L. Hummel

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Rita L. Hummel is associate professor in the Horticulture and Landscape Architecture Department at Washington State University Puyallup, (253) 445-4524; hummelrl@wsu.edu.

April 2008