PNW341 Reprinted January 1995 Choosing Pear Rootstocks for the Pacific Northwest R. Stebbins When you establish a new pear orchard, the proper choice of rootstock is as important as the choice of variety and site. This is true because the rootstock is involved in determining two key factors: your variety's susceptibility to several serious diseases, and your tree's perfor- mance in your climate and on your orchard site. The principal diseases of pear trees that are related to rootstocks are fire blight, pear decline, and Pseudomonas bacterial blight. If the rootstock produces suckers, as many do, and if the suckers are susceptible to fire blight, the disease may enter the root system, where it The Hood River Valley is a major pear producing region of the Pacific Northwest. Climate and is almost impossible to control. soil type vary from region to region and are only two of the many factors to consider when Pear decline is a disease that choosing pear rootstocks. causes a girdling of the tree at the graft union, which in turn causes a slow or rapid decline and death. higher temperatures if the tempera- propagated from seed, and the Pear decline is transmitted by pear ture falls from above freezing in a clonal selections propagated from psylla, a common insect pest of few hours. cuttings. You'll find many good pears. Most nurseries no longer use The better rootstocks provide rootstocks in either class. rootstocks susceptible to decline, so important advantages such as Here are the key questions to ask: this disease is rare. (See Table 1 for earlier, heavier production than 1. Is the rootstock tolerant of fire susceptibility of pear rootstocks to standard roots, which are considered blight and pear decline? specific problems.) to be Bartlett seedlings. (The major- 2. Does it sucker? Rootstocks also vary in their ity of pear orchards are on this tolerance of heavy soils and cold rootstock.) 3. What will be the ultimate tree winter temperatures, their effect on Rootstocks that impart high vigor size, based on the variety and site tree vigor, and other factors. If a are needed for red pears and Asian I choose? rootstock usually will be injured pears, which tend to be low in vigor. 4. Are the resulting trees uniform in when the temperature falls slowly to Rootstocks that restrict vigor are size? -10oF, it's not considered cold-hardy needed for vigorous varieties like enough for the mid-Columbia Cornice and Anjou. Robert L. Stebbins, Extension horticul- district and districts north of there. We can arbitrarily divide pear ture specialist emeritus, Oregon State These same stocks may be injured at rootstocks into two classes: those University. A Pacific Northwest Extension Publication • Oregon • Idaho • Washington Research on pear rootstocks 0 Table 1.—Relative susceptibility of pear rootstocks to damage from various causes. began at Oregon State University (0=not susceptible; 4=highly susceptible) more than 50 years ago and has progressed considerably in recent Cause years. Out of this program have Pear Fire Cold Root Iron come pear rootstocks adapted to a Rootstock decline blight damage aphid chlorosis wide variety of sites; they produce trees either larger or smaller than OHxF clones 0 1 0 1 1 standard, and they're tolerant of Quince 0 3 4 0 3 2 4 0 4 pear decline and a number of other Bartlett seedling 1 Winter Nellis seedling 1 4 0 4 1 problems. Old Home 0 0 0 4 1 When you graft any tree on a P. calleryana 2 0 4 0 3 rootstock that imparts especially P. betulaefolia (Reimer's) 0 0 4 0 1 desirable effects (such as growth control), plant it with the graft union above ground, to prevent scion rooting and the resultant loss of (Armillaria), and fungal (Phytoph- Trees on 40 may be as small as on desirable characteristics. thora) root rot. 51, but they're more productive. Some have a tendency to produce Trees on clones 69 and 217 are European seedling types suckers, which could become similar in size to 51 and slightly (Pyrus communis) infected with fire blight. They're smaller than trees on 282 and 97, European seedling rootstocks more susceptible to crown gall which produce trees of about (called "domestic seedlings") from (bacterial) than other stocks. standard size but are more preco- Winter Nellis or Bartlett female dous than Bartlett seedlings. parents are mostly tolerant of pear Old Home x Farmingdale Trees on clone 333 have been decline and cold-hardy, but they clones healthy and productive in several locations, but they've been less lack fire blight resistance. Old Home x Farmingdale (OHxF) productive than standard-sized trees The seed for the Winter Nellis or donal rootstocks used by nurseries at the Mid-Columbia Agricultural Bartlett rootstocks usually is are decline-resistant, moderately fire Research and Extension Center, obtained from canneries. Orchards blight-resistant, generally Hood River, Oregon. with satisfactory uniformity and cold-hardy stocks that provide a The Oregon nursery that has vigor have been developed on these range of tree sizes. The extent to specialized in these stocks plans to seedlings. Some of the orchard trees which any of these stocks restricts use clones 40, 69,217, and 282 for with this root are weak and may tree size varies somewhat with site. most varieties, and 277 or 97 for have the slow form of pear decline. They're reproduced from cuttings some slow-growing red pears and P. communis types have per- off selected seedling trees whose Asian pears. formed poorly on heavy clay soil in parents were the two blight-resistant All the major pear varieties are Medford, Oregon, in a rotation P. communis varieties. Old Home graft-compatible with Old Home x following pears, compared with and Farmingdale (OHxF). They were Farmingdale clones. These clones shallow-planted quince, P. calleryam, selected for their resistance to blight, have performed less well on the and P. betulaefolia. high productivity relative to tree unusually fine-textured clay soils in Imported French and P. caucasica size, and other characteristics. The the Medf ord district. Most have from Eastern Europe aren't suitable vigor of each of these stocks usually performed well in other pear dis- because they're susceptible to is compared to the average Bartlett tricts of the Pacific Northwest on decline. Another seedling type from seedling as a standard. lighter textured soils. Germany, Kirchensaller Mostbirne, is Trees on Old Home x Farming- The clones recently selected for satisfactory but has no advantage dale clone 51 are about 80 percent of propagation in large quantities are over domestic seedlings. standard (Bartlett seedling) size, cold-hardy, aren't prone to Trees on Nellis roots are slightly depending on soil type. They're suckering or lime-induced chlorosis, more vigorous than those on Bartlett more dwarfing on heavy day soils. and aren't unusually susceptible to seedlings, and they're similar in They haven't been as productive as fire blight or Pseudomonas. other ways. In addition to the items trees on the other OHxF dones listed in Table 1, Bartlett and Winter mentioned. In British Columbia, Nellis seedlings are moderately they've shown an unsatisfactory susceptible to bacterial canker degree of cold-hardiness. (Pseudomonas), oak root fungus Other pear species extensive hand thinning, such as this rootstock are more tolerant of Pyrus calleryana. Although it's Bartlett and Seckel, "betch" may be drought but less tolerant of resistant to fire blight, mostly the favored rootstock in some lime-induced chlorosis than most. tolerant of pear decline, and vigor- situations because trees on this stock They tolerate high water tables ous, this stock is cold-hardy only to produce larger-sized fruit. when the tree is dormant—but not about -10°F, depending on how Seedlings from trees of OSU high water tables that fluctuate quickly the temperature falls. It's selection Nos. 1,2, 3, and 5 during the growing season. used in regions that have warm P. betulaefolia are true to type. The P. betulaefolia is tolerant of bacterial winters. four OSU trees are from stock canker and Phytophthora root rot. Trees on seedlings from a pure brought directly from China and are Although not enough is known source of P. calleryana have shown thought to be pure species types, not about the cold-hardiness of about 10 percent weak trees, which hybrids. P. betulaefolia roots, they've been is similar to trees on Winter Nellis or P. betulaefolia seed from most satisfactory for Bartlett at Othello, o Bartlett seedlings. These weak trees other sources appears to contain a Washington, where -20 F tempera- may be partially susceptible to pear mixture of hybrid types. These tures during the 1968-1969 winter decline disease. Because those hybrids could introduce an undesir- didn't damage the rootstocks' seedlings that come from crosses able degree of variability in the productivity. Seedlings of "betch" with the highly decline-susceptible orchard trees and the "black end" produced from hybrid seed, most of P. serotina or with P. ussuriensis are physiological disorder of fruit. which comes from Europe, probably susceptible to pear decline, it's ("Black end" occurs with European won't be as cold-hardy as the pure important to isolate nursery seed pears on certain Oriental rootstocks, "betch" from northern China. sources from those species. and it appears to be related to a Trees on P. calleryana are of about rootstock's inability to supply Quince rootstocks standard vigor; but in contrast to enough caldum to the fruit.) Clonal quince stocks are used most vigorous stocks, they begin Confusion between true primarily in southern Oregon to bearing at an early age.