(Prunus spp.)

Japanese plums (sometimes called Asian plums) originated in China but were brought to this country via Japan in the 1800s. This non-native species (Prunus salicina) is not hardy in the northern temperate zone of the U.S. and Canada, except where meso-climate conditions moderate temperature extremes, most notably in the Great Lakes fruit-growing belt. As a rule of thumb, Japanese plum require about the same environment as peaches. Aside from climate limitations, Japanese plums are desirable for their size, quality and diversity.

American plum species (and there are a number of them), most notably P. americana, P. nigra and P. maritima, are native species well adapted to northern growing conditions. They produce edible fruit that lacks the commercial appeal of Japanese plums, but in addition to cold hardiness, these native species have evolved to have better disease resistance. Fortunately for northern growers, the American species and the Japanese plum are genetically compatible for hybridization (they have the same number of chromosomes). Many Japanese-American hybrids have been introduced over the last 150 years. The best of them have the fruit quality (size, flavor, sweetness) of the Japanese parent and the hardiness and disease resistance of the American parent.

These superior hybrids often have regional adaptability and may respond differently even on a site-specific basis. The purpose of the plum variety trial at Carandale Farm is to see how representative selections emphasizing differing traits respond to growing conditions at the site. Some observations, such as tree architecture (which has management implications) and fruit type (which has marketing implications) will have general value, but other observations such as tree vigor, disease susceptibility and fruitfulness may be more site-specific. An attempt has been made to minimize bias that could lead to incorrect conclusions. A randomized, twice-replicated planting grid was laid out to reduce standard sampling error regarding soils, shade and proximity to a suitable pollinator species. Unlike European plums that are usually self-fertile, all hybrid plums require cross-pollination.

A planting grid for 60 trees with 22 feet between rows and 8 feet in row spacing was established. For a commercial planting, the in-row spacing would be adjusted according to the managed tree size of the cultivar or cultivars involved. Four each of 15 plum selections requiring and contributing to cross-pollination were initially planted in 2004. This did not include any European selections, which are considered self-fertile. The initial planting consisted of four each of three selections known to be good pollinizers; five selections from Dr. Brian Smith’s breeding program at UW-River Falls; and eight commercial cultivars. The chip-bud grafts on three of the unnamed cultivars did not take, and these have been replaced over the years by other plum varieties requiring cross-pollination.

Note: Cherry plums and American plums are discussed separately. The following observations are for the Japanese-American hybrids.

The research area is not managed as a commercial orchard. Minimal training and pruning has allowed the trees to grow in a more natural manner. This accentuates differences in the architecture and disease susceptibility to provide a sense of management needs and costs for each cultivar. Bacterial canker is prevalent after nine years, and differences in cultivar susceptibility are emerging.

‘Alderman’

This hybrid is a cross released by the University of Minnesota in 1985. Carandale ranks it as the best all-around hybrid cross in the cultivar trials.

Adaptability: All four trees adapted well, continue to exhibit good health and have shown no dieback or other signs of disease.
Tree form: In a relatively unmanaged state, they are the largest tree (with the possible exception of ‘Toka’, which is comparable in size). As the photo shows, branch angles are acute but tend to be strong with no breakage or tearing under full fruit load.

Disease: Canker infections are minimal, and most are on the trunk, near ground level, suggesting southwest injury that could be prevented with white latex paint to reflect winter sun.
Fruiting: Consistent, uniform, does not need thinning. Had a good crop in 2012 under adverse conditions. Fruits adequately in partial shade. Displays late mid-season ripening.
Fruit characteristics: Large, exceeded in size only by ‘Black Ice’. Color is apple red, smooth surface (see photos) clingstone; flesh yellow, sweet, firm, but juicy. Somewhat reminiscent of a nectarine.
Other features: Attractive to consumers, easy to harvest and handle. Does not over fruit , so size and sugar conversion remain consistent; minimal skin splitting from excess rainfall.
Bonus: Beautiful fall color. (see photo)

‘Black Ice’

Recently released hybrid cultivar from Dr. Brian Smith’s plum breeding program at UW-River Falls, this hybrid was tested in the variety trial as ‘RF98-95-21-1’, released as ‘Lydecker cv. Black Ice’ (TM). This complex cross between a cherry plum and a very large Japanese dessert plum (P. salicina), has resulted in an early, very large, high quality dessert plum that is in a class of its own among Japanese-American hybrid plums. Experience indicates that there will be some challenges growing it commercially, but growers (and backyard orchardists) will be well rewarded for the extra effort.

Adaptability: One of the four newly chip-budded trees provided by Dr. Smith did not accept the graft, and the American plum rootstock was retained as a pollinizer species. The other three trees adapted to the site well and started to bloom and produced some fruit three years later.
Tree form: The tree has a naturally compact growth habit inherited from its sand cherry parent. Left in its natural state, branches are not capable of supporting the large fruit and heavy fruit load contributed by its Japanese parent. Broken and torn branches from the excess weight of the fruit provide access for the opportunistic bacterium that causes canker. Bacterial canker further weakens the tree, contributing to a cycle that leads to premature tree death. This problem can be avoided by the use of good management practices that include: proper timing of selective pruning, sanitation of pruning tools, fruit thinning and limb support. These management activities increase fruit quality and tree longevity for all plum varieties, but especially for this cultivar. The compact growth habit of ‘Black Ice’ makes these activities easier, and the fruit quality makes it rewarding.
Disease: Bacterial canker can be a devastating disease if left unmanaged and a source of inoculum exists nearby. Brown rot susceptibility is difficult to judge, but can be significantly reduced by pruning for good air flow and thinning to avoid fruit-to -fruit contact. Breakage at the bud union has also contributed to tree loss, probably due to structural weakening by canker that has gained access from southwest injury damage. An application of white latex paint to the trunk would reduce this.
Fruiting: Starts early, can be somewhat biennial if left unmanaged, but thinning of heavy fruit set can reduce this tendency. Reliably fruitful. Did not set fruit in 2012 due to weather extremes. Being the first plum to ripen in the test plot (late July, early August) adds to its commercial value. Some fruit splitting with excessive rainfall.
Fruit characteristics: Very large, nearly as large as a tennis ball (see photo). A blue-black dessert plum with heavy bloom (the waxy white exude shown in the photo). Excellent quality, very sweet and juicy with a reddish-purple flesh, semi-freestone.
Other features: Handles well for local fresh markets. Attractive and appealing to customers and tastes as good as it looks, generating repeat sales. Red flesh indicates high anthocyanin content, although not necessarily greater antioxidant capacity (Vizzotto et al., 2007).

‘Gracious’

This plum was supposed to have been a ‘Puget Gold’ apricot. Received from Fedco Trees in 2005, it bore a few fruit in 2006 (see photo) and was tentatively identified as a gracious plum, though confirmation is pending.

Adaptability: The single tree transplanted and adapted to the site very well and even bore a few fruit one year later. (see photo)
Tree form: The tree has grown slowly. It appears to have a natural spreading growth habit, even with minimal pruning. This could be partially opportunistic because it is somewhat isolated and has ample room to spread without having to compete for sunlight. As the photos show, scaffold branch angles are about 45 degrees, but are strongly attached to the trunk.

Disease: It appears to be very healthy with no sign of canker yet, even though the trunk is openly exposed for southwest injury.
Fruiting: Since its first hint of precociousness, the tree has been a very shy producer. 2012 was the first year that it had a heavy bloom (see photo), but it set no fruit, probably because its exposed position made it vulnerable to a severe freeze, but lack of pollination cannot be ruled out.
Location concern: Since this tree was expected to be an apricot, it was not planted in the main plum block so does not have exposure to multiple pollinizer types for cross-pollination. The closest plum is about 30 feet away, with others within 40 feet. If the lack of cross pollinizers is the reason for unfruitfulness, why did it set fruit at one year of age, but not significantly since? Observations in 2013 may be critical in determining the future of this mystery tree.

‘Pipestone’

This hybrid is a cross released by the University of Minnesota in 1942. Carandale included it in the trial due to its description as a winter hardy and vigorous tree that produces an abundance of large high quality fruit. These claims may be valid at some locations, but they have not been observed at Carandale Farm — another reason why on-site testing is important before making a major investment in a commercial planting. Based on the following observations, this cultivar would be assigned the lowest ranking.

Adaptability: The trees initially transplanted well and appeared to adapt to the site. Fruiting initiated on schedule with other cultivars, but trees started to show decline shortly thereafter.

Tree form: Scaffold branch angles tended to be very acute, usually less than 30 degrees in relation to the trunk. In retrospect, tree spreaders should have been used. Tree is upright and medium height.
Disease: The trees started to decline after the fourth year. One tree died in 2008. Cause is unknown, but it may have been girdling from bacterial canker. Exact symptoms were not recorded, and there was a lack of knowledge about the disease at that time. Two more trees were lost in the next couple of years. Trees in partial shade were the first ones to die. The one remaining tree continues to bear fruit, but a major scaffold branch broke off in 2011, so it is now quite misshapen, as the photo shows.
Fruiting: When the trees were healthy, they fruited consistently from year to year. Cropping was good, but not excessive, so fruit thinning was not required and biannual bearing was not an issue.
Fruit characteristics: Fruit is an attractive red color and has good size, but not as large as anticipated based on descriptions. ‘Black Ice’, ‘Alderman’ and ‘Superior’ all have considerably larger fruit. Fruit quality compared to other cultivars is disappointing. ‘Pipestone’ has tough skin, bland flavor and marginal sweetness. They have to be left on the tree as long as possible to get marginally acceptable quality and often fall to the ground before this occurs. Fruit may be fine for processing, but as a dessert plum, they do not highlight the qualities that should distinguish a tree ripened, locally grown plum.
Conclusion: Tree form and disease issues can be addressed by implementing appropriate management practices, but at this test site, fruit quality is the reason for eliminating it from further consideration.

‘Ptitsin #9’

Not a Japanese-American cross, this hybrid originated in Manchuria and was introduced to North America by the Morden Manitoba Research Station. This extremely winter hardy plum is a cross of Japanese plum (P. salicina) and (P. triforakoreana). Two trees were purchased from St. Lawrence Nurseries in 2008 to fill a void in the replicated variety trial. Planting into an existing orchard, even where there was no previously planted trees, puts the newly planted trees at a disadvantage because of competition, shading and disease pathogens from older trees.

Adaptability: One tree did not survive transplanting. The surviving tree is growing at a modest rate.
Tree form: The photo of the unpruned tree was taken during dormancy in March 2012 at four years of age. The tree has a spreading form with no strong central leader (similar to the ‘Sapalta’ cherry-plum).
Disease: One scaffold branch has bark damage that could be mechanical damage or the start of bacterial canker. It could also be the start of fireblight. The branch will be removed after bloom when healing response is quickest.
Fruiting: Has not yet occurred, but is anticipated in 2013. The fruit is said to be green-yellow, one inch in diameter, firm flesh, meaty, freestone: excellent for eating or preserves, productive.

‘RF98-95-17-7’

A Japanese-American hybrid cross, this a very good selection from Dr. Brian Smith’s breeding program a the University of Wisconsin-River Falls. It is ranked as the second best all-around cultivar in the test plot because it scores high in all evaluation categories. Dr. Smith has been reluctant to release it as a commercial selection because it has characteristics similar to another good cultivar (‘Waneta’), but side-by-side comparison in the test plot shows it to be superior in every evaluation.

Adaptability: Five chip-budded trees were provided by Dr. Smith. All buds grew, and the trees adapted very well. All five trees remain healthy and have shown no sign of decline.

Tree form: The trees are medium in size with a nice open growth habit (see photo). The limbs are strong and have a good attachment angle, generally well over 45 degrees. Limbs do not break, even under heavy fruit load. Easy harvest access.
Disease: Virtually no disease issues. Bacterial canker is an opportunistic disease that needs injured or dead tissues as entry points for infection. Openness and a strong frame-network seem to limit disease access.
Fruiting: Has good fruit production every year, even in areas with partial shade. It could benefit from fruit thinning some years to increase fruit size, but it has never over fruited to the point of tree damage and/or poor sugar content. Fruit ripens late-August, early September at test site. Good fruit crop in 2012 despite adverse conditions.
Fruit characteristics: Medium-large, nearly round, very uniform, deep red-yellow flesh, sweet, good flavor, firm and somewhat meaty (reminiscent of European plum). Good all purpose plum. Clingstone. Customers ask for this one. Quality is consistent.
Comparative features: Easy to handle, good storage qualities. Looks similar to ‘Waneta’ with superior flavor, handling, harvest, disease resistance and uniformity. Tree form very different and more grower friendly. A top choice for commercial production if it were to become available.

‘Superior’

This is a hybrid cross released by the University of Minnesota in 1933. A very good dessert plum, readily available and highly promoted, yet not carefree to grow. Good management practices are essential, but the extra work will be well rewarded.

Adaptability: Trees adapted well at the test site, but the trees are structurally weak and should have been staked for support, trained by selective pruning and not allowed to fruit for several years.
Tree form: Without early management intervention, the trees are willowy and develop weak scaffold branches prone to breakage and tearing away from the main trunk. This tendency is compounded by the weight of the large fruit and the over-fruiting tendency in alternate years. Photos show the structural problems and what happens if fruit is not properly thinned (excess removed). Root stock suckering seen on the dormancy photos is very prevalent underneath all of the ‘Superior’ plum trees.

Disease: Most of the trees in the test plot that show symptoms of bacterial canker on the lower trunk were probably affected by tissue damage from southwest injury. On ‘Superior’, disease symptoms also appear on the upper scaffold branches. One tree was lost in 2010 when it broke off at the graft union. ‘Superior’ fruit also appear to be susceptible to brown rot, especially when they are allowed to over fruit.
Fruiting: ‘Superior’ trees in the test plot have a tendency to go into a biennial fruiting mode: They fruit excessively one year, then very lightly the following year. This cycle can be stopped by timely thinning in the years of excessive fruit set. To be effective, excess fruit must be removed soon after fruit set. Partial shade reduces tree vigor, but does not seem to have much effect on fruitfulness. ‘Superior’ is precocious, often wanting to bear fruit the year after transplanting. Fruit should be removed until the tree is structurally capable of supporting the fruit without damage.
Fruit characteristics: If properly managed and not allowed to fruit excessively, ‘Superior’ is a large, high quality dessert plum. As the photo shows, it has a conical shape. When ripe, it is dark red. The flesh is yellow, firm, sweet, flavorful and very juicy.
Management: Superior is a high quality plum that could be profitable for commercial growers and rewarding for homeowners, if properly managed. Tree longevity depends on good training and pruning. Fruit quality depends on managing fruit load.

‘Toka’

A unique hybrid plum introduced by N. E. Hansen at the South Dakota Experimental Station in 1911, it is considered to be a superb pollinizer and was included in the test plot for that purpose. It has a very appealing flavor and aroma that puts it in a class by itself.

Adaptability: Very adaptable at the test site. Trees are vigorous and fast growing. It is assumed to be winter hardy, but there could be hardening off problems due to vigorous growth possibly contributing to disease issues.
Tree form: It has an upright growth habit and is quite dense (see photo). It is a large tree for a hybrid plum and would benefit from diligent pruning for size control and to maintain an open center.

Disease: The trees are starting to show significant decline. Some scaffold branches are showing disease symptoms, and a few have died. While good pruning would help, there is speculation that pathogens could be gaining entry through winter-damaged wood.
Fruiting: ‘Toka’ fruits consistently, but has never over produced and should not need fruit thinning to maintain quality. Yields have varied from modest to good, but there has never been a complete crop failure, even under adverse conditions like those experienced in 2012. Fruit usually ripens in late-August at the test site.
Fruit characteristics: The medium-sized fruit (see photo) has a flattened spherical shape and purple-red color resembling an oversized sweet cherry. The fruit is firm, spicy sweet and flavorful. Some refer to it as a candy plum, but what really draws attention at farmers’ market is its perfume-like fragrance.
Bonus: Beautiful fall red color (see photo).

‘Underwood’

This is a hybrid cross introduced by the University of Minnesota in 1921. This cold-hardy plum has many good attributes. It is said to be very productive, but that has not been experienced at the test site.

Adaptability: All four trees adapted well and continue to be healthy and virtually disease free.
Tree form: The medium to large tree has very good structure and tree form. As seen in the dormancy photo, the tree has strong, wide-angled scaffold branches, balanced and well distributed. It is a spreading tree capable of supporting a heavy fruit load. The photo also shows extensive rootstock suckering all around the trees. Rootstock suckering can occur in association with any cultivar grafted on American plum rootstock, but it is especially noticeable with ‘Underwood’, both in full sun and partial shade. It would be interesting to know why this occurs and if it has any significance.

Fruiting: The photo of ‘Underwood’ in bloom was taken in March 2012. In this situation the lack of fruit set could be attributed to adverse weather. Despite good bloom annually, there has never been good fruit set on this cultivar in the test plot at either location in the replicated trial. It appears to be a pollination problem, but there is proximity to all pollinizer cultivars.
Fruit characteristics: The few fruit that do set have dessert quality. The red plum has a golden yellow flesh that is juicy, melting and tender. The pit is freestone and the skin is only moderately tart. The fruit is medium-large with an oblong shape that comes to a point (see photo). This early ripening plum (about mid-August at the test site) could have good consumer appeal as a fresh fruit for local/regional markets. Pollination issues are a concern, based on observations in the test plot.
Bonus: Trees have beautiful golden-yellow fall color (see photo) that is a striking contrast to the bright red colors of ‘Alderman’ and ‘Toka’.

‘Vermont’

A hybrid cross of unknown parentage, this hybrid was provided by Herb Todd and introduced exclusively by St. Lawrence Nurseries. It was included in the cultivar trials partially because yellow-skinned hybrids are unusual,for fresh fruit sales. It has turned out to be a very desirable, high quality, fresh market plum.

Adaptability: Planting stock was marginal in quality, but three of the four trees survived transplanting and have adapted to the site, showing moderate vigor and growth.
Tree form: The trees are somewhat misshapen. Tree form is more upright than spreading. Crotch angles are variable, but limb tearing and breakage has been minimal, partially due to light cropping.
Disease: Disease symptoms have been intermediate. Some tissue damage is showing up on the scaffold branches and what appears to be southwest injury can be seen on the trunk of two trees.

Fruiting: Fruiting has been consistent from year to year, but yields have been low. There was some fruit set even in 2012 under adverse conditions. Observed yield is better than ‘Underwood’, but probably too low to provide economic sustainability for commercial fruit production. Fruit ripens in late-August or early September.
Fruit characteristics: This has turned out to be an excellent plum that deserves more testing. The golden yellow medium large fruit gets a pink blush when fully ripe. It is very attractive and provides a marketing contrast, and the fruit tastes a good as it looks. The yellow flesh is sweet and succulent. Even the skin is sweet. The fruit is nearly round (see photos). This could be a profitable cultivar with good management if pollination can be improved.

‘Waneta’

Another N.E. Hansen hybrid cross introduced by the South Dakota Experimental Station in 1913, it has been considered one of the best all around, all purpose American plums for the last 100 years. Unless Dr. Smith’s selection (‘RF-98-95-17-7’) is released, it will probably continue to have that honor.

Adaptability: Four trees were purchased from St. Lawrence Nurseries and all adapted well. One tree broke off at the graft union in 2010. The other three trees are healthy and productive.
Tree form: The medium-sized trees have a highly branched spreading growth habit. Scaffold branches have good attachment angles and do not tear easily at the trunk union. Branches are long and willowy. Lower branches often lay on the ground under fruit load, making harvest difficult (see photos).

Disease: Minimal disease symptoms have been observed in the remaining three trees. Breakage at the graft union of the tree that was lost could have been from undetected tissue damage (rot), or it could have been a physiological weakness of the graft union itself.
Fruiting: ‘Waneta’ has consistently high annual yields even under adverse conditions, similar to ‘RF-98-95-17-7’, and in contrast to adjacent ‘Underwood’ trees. It has the habit of over fruiting to the point of significantly reduced fruit size and quality. Fruit thinning would be beneficial. Fruit ripens late August to early September, just before ‘RF-98-95-17-7’.
Fruit characteristics: When not over fruiting, the plum is medium-large, about the size and similar appearance to the ‘RF-98-95-17-7’ , but oblong rather than round (see photo). Fruit is dark red over a yellow background. Yellow flesh can be juicy and sweet, but can become bland and mealy if allowed to over fruit.

Plum trial synopsis

Preliminary results from the plum cultivar trials illustrate the broad range of characteristics and marketing potential of this mostly overlooked fruit species. Genetic variability makes plums adaptable to almost any temperate climate region. Based on site-specific observations where cultivars have minimal management intervention, recommended choices from the hybrids observed would include the following, listed by category:

Dessert plums: ‘Black Ice’ and ‘Superior’ — Both will require significant management to maximize success and profitability. Very different in maturity and type, but both have outstanding fresh market appeal as dessert plums.

Low maintenance, all purpose plums: ‘Alderman’ and ‘RF-98-95-17-7’ (if it becomes available; alternatively ‘Waneta’) — Both are high yielding, high quality, and low maintenance with consumer appeal.

Specialty plums: ‘Sapalta’ — Relatively carefree, mechanical harvest potential, nearly black flesh (probably nutritionally rich), definitely a processing plum, gives a rich appearance to jams, jellies, juice blends, wine and any other processed product when used as an ingredient. Also, ‘Toka’ is a uniquely flavored, highly aromatic, great pollinizer, but will require tree size and disease management.

Honorable mention: ‘Vermont’ — A beautiful, high quality golden plum worthy of trial despite its low observed yield and management issues.

References

Vizzotto, M, L. Cisneros-Zevallos and DH Byrne. 2007.  “Large Variation Found in the Phytochemical and Antioxidant Activity of Peach and Plum Germplasm.” Journal of the American Society for Horticultural Science 132(2):334-340. <http://journal.ashspublications.org/content/132/3/334.full>, accessed 6/27/13.

This article was posted in Prunus.