The 3rd International Cherry Research Symposium

By Gregory A. Lang, Prunus Research Program Leader, Washington State University, Prosser

The Third International Cherry Symposium, sponsored by the International Society for Horticultural Science (ISHS), was held last year in Norway and Denmark. This quadrennial conference was attended by some 125 international scientists representing expertise in sweet and sour cherry production, breeding, and physiology, as well as about 25 cherry growers, packers, and nurserymen from around the world. Cherry scientists from the Pacific Northwest included Anita Azarenko, Tim Facteau, Lynn Long, and Daryl Richardson from Oregon State University; Frank Kappel and Norm Looney from British Columbia; and myself, funded by Washington State University (WSU) and the Washington Tree Fruit Research Commission. We also were joined by a strong contingent of cherry growers from Oregon.

The symposium provided a unique and efficient opportunity to learn about current and future cherry varieties, rootstocks, and production technologies that are being researched elsewhere in the world. This article will briefly summarize key areas of sweet cherry research from the symposium that may be of interest to sweet cherry growers in the Pacific Northwest.

INTERNATIONAL ROOTSTOCK RESEARCH

Several years ago at the annual Cherry Institute meeting in Yakima, Washington, during discussion of the WTFRC cherry research assessment, one grower's opinion was voiced that no research should be done on rootstocks. A glance at current international cherry rootstock research (see Table 1) indicates how quickly the Pacific Northwest could fall behind the rest of the world if we remained oblivious to new plant materials, content with our overly vigorous, delayed-fruiting rootstocks. There are increasing numbers and types of precocious, size-controlling rootstocks in test across Europe, hinting at significant reductions in labor costs, earlier returns on investment, reduced environmental impacts, and more intensive management strategies.

A brief review of the rootstock trials discussed at the symposium indicated that rootstocks became a very hot research topic during the late 1980s and early 1990s. Sixteen years of research was reported from Poland (see Table 1) on the use of sour cherry (Prunus cerasus) and a wild cherry species (P. fruticosa) as interstems on Mazzard or Mahaleb rootstocks. Many of these interstems promoted significantly smaller trees and higher yields. An extensive trial of Gisela rootstocks planted in 1986 in Germany demonstrated good graft compatibility of various Gisela rootstocks, including Gisela 5 (GI.148/2) and Gisela 6 (GI.148/1), with 21 different European cherry varieties. Fruit size was influenced more strongly by seasonal crop load than by individual rootstock.

One year after the North American NC-140 Cherry Rootstock Trials were established in 1987 and 1988, a European rootstock trial was planted across Belgium, Denmark, France, the Netherlands, and Norway (see Table 1). It should be noted that many, if not most, rootstock trials in Europe are conducted without irrigation; rootstock effects on vigor often are more pronounced than under irrigated Pacific Northwest conditions. For example, Colt is considered to be a moderately weak rootstock under standard European conditions, while it is a vigorous rootstock in the Pacific Northwest. Nevertheless, on a relative basis, European results generally were similar for most of the rootstocks common to the NC-140 trials, such as excessively weak growth and fruit size on Inmil (GM.9 [Gran Manier (Gembloux)]) and Gisela 1 (GI.172/9), and excessive vigor on Mazzard. Two of the Weiroot series, W.10 and W.13, were included in Europe that were not in the 1987 NC-140 trials. These were found to be similar to Colt in vigor but more productive, as was St. Lucie 64 (SL.64), a Mahaleb clone tested only in France. Tree death was greatest on Gisela 1 and 10, which we have demonstrated at WSU to likely be due to hypersensitivity to common pollen-borne viruses.

A German rootstock trial was established in 1989 at several sites, although only one remains fully intact, with 22 rootstocks (see Table 1).

T A B L E 1
International sweet cherry rootstock trials reported at the ISHS Cherry Symposium.
Trial sites	Year planted	No. of stocks	Rootstocks being evaluated include:
Poland	1981	9	Mazzard and Mahaleb with Prunus cerasus or P. fruticosa interstems
Germany	1986	10+	Gisela series (Prunus hybrids)
U.S./Canada	1987-88	18	GM series, Gisela series, Colt, MxM series, Mazzard, Mahaleb
Europe	1988-89	13	GM series, Gisela series, Weiroot series, Colt, Charger, Mazzard, others
Germany	1989	22	Gisela series, Weiroot series, Colt, Mazzard, others
Spain	1989	5	Colt, SL.64, Reboldo, P. cerasus, Mazzard
Denmark	1990	10	Prunus cerasus selections
Hungary	1990	11	Colt, MxM.14, MxM.97, various Mahalebs, various interstocks
Poland	1991	5	Colt, MxM.14, P-HL A and C, Mazzard
Italy	1993-94	20	Edabriz, GM series, CAB series, P. cerasus selections, Padum, MxM.14, others
U.S./Canada	1998	17	Edabriz, Gisela series, Weiroot series, P-50, Mazzard, Mahaleb
Europe	1999	26	Edabriz, Gisela series, Weiroot series, Pi-Ku series, P-HL A and B, Colt, MxM series

The Weiroot series W10, W.13, W.14, and W.158 were all somewhat similar to Colt and Mazzard in vigor, but had somewhat higher productivity. More dwarfing, precocious, and productive were Gisela 5, Gisela 4 (GI.473/10), and GI.209/1. A Spanish rootstock trial was also established in 1989 with five rootstocks (see Table 1). The two derived from sour cherry (P. cerasus)--Stockton Morello and Masto de Montañana--were the most dwarfing and most productive, but also had the smallest fruit.

A Danish rootstock program was begun in the 1980s to select sour cherry seedlings for use as rootstocks to improve sweet cherry precocity and reduce tree size. In 1990, a rootstock evaluation trial was initiated with clones of ten promising selections: DAN.1, 4, 5, 6, 9, 10, 12, 13, 17, and 19 (see Table 1). The highest yields were on DAN.6 and 12, both of which were 58% the size of Colt (others ranged from 22% to 51%) and all had better yield efficiencies (yield per orchard-canopy area) than Colt during the first six years of cropping. A Hungarian rootstock trial was also established in 1990 (see Table 1) with Mahaleb, Colt, MxM.14 (Mahaleb x Mazzard), MxM.97, Korponay, GM.9, and Prob 1 rootstocks, plus Mahaleb with the following interstocks: Erdi bötermö, Pándy meggy, or Prob 1. Of the interstocks, only Prob 1 (which has P. fruticosa as a parent) reduced vigor significantly; yield efficiencies thus far are highest for MxM.14 and SL.64 rootstocks and for Prob 1 interstock.

In the early 1990s, a small rootstock trial began in Poland, notable for its inclusion of the Czech rootstocks, P-HL A (Holovousy) and P-HL C (see Table 1). Thus far, the P-HL stocks are more dwarfing and productive than MxM.14, which has been more productive than Colt and Mazzard. Fruit size has been smaller on P-HL C and MxM.14 than the others, and P-HL C trees have exhibited some unhealthy appearances.

A very large rootstock trial was established in 1993-94 in Italy (see Table 1), a major sweet cherry producing country. Although still young, thus far the most dwarfing stocks include Edabriz, the GM series, and CAB.8F; semi-dwarfing are CNR.94 and Vladimir. The most vigorous are Colt, Mazzard F12/1, MxM.14, and SL.64; semi-vigorous stocks include Alpruma, CAB.6P, CAB.11E, several sour cherries, and Padum. Those exhibiting the most precocious bloom include MxM.14, Padum, SL.64, CAB.6P, and Edabriz.

A new NC-140 cherry rootstock trial being planted during spring 1998 across the United States and Canada will include new rootstocks from Gisela, the Weiroot series from Germany, Edabriz from France, and P.50 from Japan (see Table 1). A similar trial will be planted across Europe in 1999, with additional rootstocks such as the Pi-Ku series from Germany and the P-HL stocks from the Czech Republic. These stocks are currently in various stages of North American quarantine and virus screening by the Sweet Cherry Genotype Research Consortium.

T A B L E 2
International sweet cherry training system trials reported at the ISHS Cherry Symposium.
Trial sites	Year planted	Orchard systems under study
Belgium	1985	spindle, hedge
Spain	1989	palmate, oblique leader (Marchand), open vase
Hungary	1990	Zahn spindle, slender spindle, modified Brunner spindle
Norway	1991	vertical axis, free spindle, open vase, Y-trellis
Netherlands	1993	slender spindle, super spindle, Le Page hedge, vertical cordon,cordon/Guttinger V-hedge
Denmark	1994	tied-down spindle, bent spindle, Spanish bush
France	1994	solaxe
Germany	1994	spindle (spil), slender spindle, oblique spindle,modified Tatura trellis
United States (Washington)	1995	multiple leader vase, multiple leader bush, central leader spindle, central leader axe, oblique leader trellis, central leader trellis, palmate leader trellis, Y-trellis

Rootstock research conclusions

The Pacific Northwest sweet cherry industry and support scientists have been at the forefront of international sweet cherry rootstock trials during the past decade, as worldwide interest in dwarfing, precocious rootstocks has heated up. European efforts in this research area are becoming much stronger and more concerted due to decreasing nationalistic boundaries among the European Community (EC) members and the fact that nearly all of the interesting new rootstocks were initially bred or selected in European rootstock research programs.

INTERNATIONAL ORCHARD SYSTEMS RESEARCH

As vigor-controlling, precocious rootstocks move closer to commercial viability and concerns about labor availability and cost increase throughout most technologically advanced countries, high density orchard management strategies have also become a major research focal point. Such research trials generally involve interactions between canopy architecture, tree spacing, rootstock vigor, and variety.

The major training system trials on which reports were made during the ISHS symposium are listed in Table 2. As with apples, more intensive utilization of orchard space is particularly important in those countries with limited orchard land, like the Netherlands, where experimental cherry tree densities range from 865 to 4,500 trees per acre, using the dwarfing rootstocks Edabriz, Inmil (GM.9), or Damil (GM.61/1). As should be expected, fruit size generally has been related more to cropping levels than to clear influences of training system. Likewise, early production levels are directly related to tree density, pretty much regardless of training system. Most trials are, as yet, too young to predict tree performance or orchard maintenance economics "at maturity" on such high densities.

Among the interesting notes in these reports are incidents of greater tree death in Belgium for training systems that relied on limb bending, which increased susceptibility to bacterial canker. In Spain, the best trellis design varied by fruiting variety, since some varieties are naturally more upright and others are more spreading. In Germany, those systems that required greater pruning during canopy development had lower early yields, as would be expected, and thus larger fruit size. While canopy architectures varied from trial to trial, every trial included one or more variations of spindle-type training systems. Consequently, there was much discussion over colloquial names for training systems and techniques; as Professor Godini from Bari, Italy, pointed out, the "Spanish Bush" system has been used for many decades in parts of southern Italy, and he wondered why it was not known as the "Italian Bush" system.

Several scientists stressed that while some techniques developed in high density apples are certainly applicable to sweet cherry, there are significant differences in tree response to certain pruning and training techniques. Cherries are an inherently riskier crop than apples (more diseases, broader susceptibility to climatic factors), also should be considered when developing high density training strategies. Consequently, a major objective of many European high density trials (e.g., Germany, the Netherlands, Norway, United Kingdom) is to create an orchard of smaller-statured trees that can be covered easily by plastic and/or netting for protection against rain-cracking as well as bird damage to fruit (see Figures 1A and 1B).

FIGURE 1a. Sweet cherry orchard in Telemark, Norway, with bird netting.

FIGURE 1b. Note plastic rain covers under netting.

Orchard systems research conclusions

Europeans are ahead of the United States in research on high density orchard strategies for sweet cherries by probably about three to four years. Although there are many small orchard systems trials ongoing among North American cherry research and extension scientists that were not reported at the symposium, this area of research has simply been a matter of concern among European orchardists longer than it has for American orchardists. However, most high density cherry research is still less than ten years old and is influenced so strongly by variety, production site, and especially rootstock, that no definitive or sweeping conclusions can yet be made anywhere.

INTERNATIONAL GENETIC IMPROVEMENT

There were overviews of numerous international sweet cherry breeding and variety evaluation programs, which are summarized in Table 3. Notable trends are selection for increased fruit size, ripening date diversity, and self-fertility in the major cherry production areas (North America, Italy, France), cracking resistance in many areas (Europe, Australia), disease resistance (France, Germany), and cold hardiness in extreme northern latitudes (Estonia, Latvia). There was one report on the widely-produced Spanish variety Ambrunes that is picked without stems.

A wide range of international varieties were on display from test orchards in both Norway (early to mid-season varieties were ripe) and Denmark (mid- to late-season varieties were ripe). In general, varieties from Summerland (e.g., Van, Lapins, Sweetheart) have been tested the most universally around the world and are often held as the standards for comparison. In the Norwegian trials, some of the WSU/Prosser selections looked very promising.

Chelan was among the best of the early ripening varieties, being at least equal in quality and earlier than either Margit from Hungary or Giorgia from Italy (pictured on page 30). Adriana from Italy was similar to Chelan in ripening time and even lower in resistance to rain-cracking, but fruit size was smaller.

Many of the breeders who walked the Norwegian variety trials together after the symposium group tour considered PC.7146-8 from WSU/Prosser to be the highest quality cherry seen among the early-to-mid ripening season varieties; this selection is self-fertile, low in cracking, and high in flavor, but has received little interest in the Pacific Northwest since it is otherwise similar to Bing, including ripening time (pictured on page 31). The varieties Regina from Germany and Sonata from Summerland looked outstanding but were not yet ripe in either the Norwegian or Danish trials. Other interesting, high quality cultivars seen in the orchard were Skeena from Summerland, Karina, Oktavia, and Viola from Germany, and Kordia (known as Attika in the U.S.) and Techlovan from the Czech Republic. A significant proportion of test varieties, including those from France and others from WSU/ Prosser, were not yet fruiting.

Although there were several papers on classical genetic studies (such as inheritance of specific traits), there was a somewhat surprising dearth of "high tech" research reported in cherry breeding and genetics. The few areas that were addressed included the use of molecular markers for genetic mapping and potential screening of seedling populations for specific traits (United States, United Kingdom) and variety identification using DNA fingerprinting techniques (United States, Germany). The area of genetic engineering, transformation, and regeneration of genetically-altered plants (Belgium/Austria/Portugal) continues to be somewhat problematic in sweet cherry, with routine success still years away.

FIGURE 2a. Chelan and Margit cherries.

FIGURE 2b. Giorgia and Chelan cherries.

Genetic improvement conclusions

It was clear that the breeding programs at Summerland and Prosser have had perhaps the most significant success to date in releasing the type of high quality varieties important to Pacific Northwest production. However, several European breeding programs, most notably in those higher production countries of France, Italy, Germany, and Hungary, are beginning to release interesting varieties with fruit size, firmness, and self-fertility that warrant evaluation under Pacific Northwest conditions. Additionally, selection of a few long-standing local varieties in Europe also hold some potential interest for the increasingly competitive international sweet cherry marketing scene.

INTERNATIONAL RAIN-CRACKING RESEARCH

With much of western and central Europe's 1997 cherry crop devastated by record rainfall, cherry cracking was a topic of widespread interest. In some locations, even the most "cracking resistant" varieties suffered very significant damage. During a pre-symposium trip to visit the Czech Republic's breeding program, entire villages were being evacuated due to flooding from continuous rains, and it was difficult to find any cherry selections that had less than 95% culls due to cracking or brown rot.

FIGURE 3. Nearly ripe PC.7146-8 cherries from WSU/Prosser, at Norway.

A significant number of small orchards in Norway, plus at least some in England, Belgium, Germany, and Denmark, have recently installed retractable or stationary rain covers. As noted above, these are sometimes combined with bird netting, which is considered an absolute necessity in some locations. In France, orchard application of fruit waxes ten days before harvest reduced cracking. In Oregon, antitranspirants were noted to be ineffective or to aggravate fruit cracking.

There was only moderate interest in the ongoing, widespread North American automated calcium-sprinkler strategy to reduce rain-cracking, since most orchards in Europe do not already have irrigation. A variant of this strategy was being conducted in Denmark, with results not yet tabulated at the time of the symposium, and in France, with some positive but as-yet variable results due to a lack of automation. In Turkey, speed sprayer application of calcium hydroxide at ten-day intervals during ripening was reported to reduce rain-cracking but also fruit size.

One of the fundamental cracking studies reported a correlation between variety cracking susceptibility, postharvest disease incidence, and "cuticular fracturing," the formation of microscopic cracks in the skin of different cultivars after exposure to rainwater. Variety, rootstock, and turgor pressure of both the fruit and tree influenced the amount of cuticular fracturing. This relationship explains the observation that noncracked fruit from protected trials (either under covers or under calcium sprinklers, with fewer micro-cracks) tend to exhibit better postharvest performance (longer shelf life, lower incidence of fruit rots) than would noncracked fruit that had been sorted out from cracked fruit in unprotected orchards. The latter fruit, while not cracked in the classical and economic sense, in fact would have a higher incidence of microscopic cracks that affect postharvest quality and longevity.

OTHER AREAS OF INTERNATIONAL RESEARCH

There was very little postharvest research reported, probably an indication that the Pacific Northwest, which had only one postharvest scientist participating, leads the world in this research area. A Norwegian "state-of-the-art" packing line (it could be used for both cherries and fish!) observed did not appear to approach the level of technological sophistication or capacity seen in current Washington and Oregon packing lines.

Also surprising was the lack of research focusing on integrated fruit production (IFP) practices, since Europe is widely considered to be on the forefront of such research. The only IFP papers reported were from Oregon State University for sweet cherries and Michigan State University for sour cherries.

There were general overviews of the increasing cherry production in Australia, Turkey, and China. In Australia, production (primarily for fresh market) of up to 8,000 metric tons is expected to double over the next five years. Orchardists grow from 5 to 20 varieties to spread their harvest over a six- to eight-week period. As everywhere, it seems, there is increasing Aussie interest in export markets and expanding into nontraditional production areas.

In Turkey, sweet cherry production reached 186,000 metric tons in 1995. The primary high quality sweet cherry variety is 0900 Ziraat (also known as Aksehir Napoleon), which is exported over a six-to eight-week period to European and Arabian Gulf countries. Production in the Taurus mountain region is favored due to minimal rainfall during harvest. Current research is focused on improving harvest and postharvest techniques to increase the quantity of fruit suitable for export.

Sweet cherry production in China is currently minimal, but has been targeted as a development project by the United Nations for Shaanxi Province, which has already seen large increases in apple and peach production. Major environmental hurdles exist, such as high pH calcareous soils, disease pressures, and frequent rainfall, but Chinese scientists have begun collaborations with cherry breeders from Hungary and Ukraine to improve both their varieties and their rootstocks, and are actively seeking production expertise from international consultants.

2001: THE 4TH INTERNATIONAL CHERRY SYMPOSIUM

There were formal invitations from three countries--Australia, Italy, and Slovenia--to host the 4th ISHS Cherry Symposium in 2001. However, at the business meeting of the International Cherry Working Group, a majority of the scientists urged the United States to consider hosting the next symposium in either Michigan or the Pacific Northwest. In subsequent discussions between the working group's new executive committee (Lars Sekse from Norway, chair; Ron Perry from Michigan State, past chair; and myself, chair-elect), it was decided the Pacific Northwest would be the best host site for 2001. The scientific meeting itself will be held in Washington or Oregon, with pre- and/ or post-conference tours encompassing the entire cherry-producing region from Oregon to British Columbia. I am excited about the learning opportunities for our Northwest cherry industry, with local access to such a concentrated international slate of cherry researchers and their ideas in just over three years.

T A B L E 3
International sweet cherry breeding and selection program reports at the ISHS Cherry Symposium.
Programs	Year begun	Breeding results or variety evaluation activity
Australia	mid 1980s	Breeding: selection and advanced evaluations in progress
Belgium	mid 1970s	Primarily evaluation of varieties from other programs
Canada	1930s	Breeding: Van, Lapins, Sweetheart, Summit, Santina, Sonata, Cristalina, Sandra Rose, Celeste, Samba, Skeena, Symphony
Denmark	??	Primarily evaluation of varieties from other programs
Estonia	early 1950s	Breeding: Priima, Elo, Eva, Juku, Kriistina, Meelika, Nord, Tontu
France	early 1980s	Breeding: Ferprime, Fercer Arcina, Fernier, Ferbolus Verdel, Fernola
Germany	1950s, early 1970s	Jork breeding: Regina, Karina, Oktavia, Viola, Alma, Annabella, Valeska, Pillnitz breeding: Nadino, Nalina, Namosa, Nanni, identification of disease-resistant germplasm
Hungary	early 1950s	Breeding: Margit, Linda, Katalin, many new selections in progress
Italy	early 1980s	Bologna breeding: Early Star, Blaze Star, Lala Star, others. Bari: Primarily evaluation of varieties from other programs
Latvia	??	Some breeding: primarily identification of cold-hardy germplasm
Norway	late 1950s	Primarily evaluation of varieties from other programs
Romania	1940s	Breeding: Iva, Severin, Roze, Somesan, Daria, Clasic, Tentant, Simbol
Switzerland	??	Limited breeding: primarily evaluation of other program varieties
Turkey	??	Primarily evaluation of local and standard varieties
United States
New York	1910s	Breeding: Hartland, Hudson, Kristin, Royalton, Somerset, Ulster
Washington	late 1940s	Breeding: Rainier, Chelan, Tieton, Index, Glacier, Olympus, Cashmere, Simcoe

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