The Horticultural Improvement of the American Persimmon

(originally published in the Nutshell, Nov. 2006; updated Apr. 2007)

John W. Raymond

INTRODUCTION

Figure 1.   Persimmon Tree 'Keener'

In stark contrast, the Japanese persimmon (D. kaki), originally native to China, has been under cultivation and genetic improvement for at least several hundred years.  In the Oomachi, Kagami-cho, Okayama Prefecture of Japan, there is a grafted tree of the 'Saijo' cultivar purported to be over 600 years old.²  It has been written that there have been over 1,000 varieties of D. kaki in cultivation and that it exhibits the most variation in fruit shape of all fruits now in cultivation.³  In the warmer climates of this country, it is has been the primary choice for those interested in growing persimmons owing to its larger size, fewer seeds, and more extensive choice of cultivated varieties.  Several varieties of D. kaki could be found growing and producing as far north as Tennessee over a hundred years ago.⁴  For the cooler, northern climates, though, growing persimmons has meant cultivating the more wild but winter hardy D. virginiana.

It appears that the propagation of selected wild specimens of American persimmon started to occur in the latter half of the 19th century.  Notable enthusiasts of this era included amateur horticulturalists such as Logan Martin (Borden, IN),^5-6 James A. Little (Cartersburg, IN),^7-8 Samuel Miller (Bluffton, MO),^9-10 and F. O. Harrington (York Center, IA).⁸  These and other early adopters accomplished much in the way of preserving superior, clonally propagated varieties and maintaining a smoldering interest in the cultivation of native tree fruits.  Over the years the torch was passed to other growers who continued to add new varieties discovered in the wild as well as selections made from open-pollinated seedlings of known cultivars.  Many of these growers published their findings and observations for the benefit of others in their local fruit and nut growing associations.

University sponsored interest in the improvement of the American persimmon began in the late 19th century with the work of Prof. James Troop of Purdue University.  His enthusiasm for the prospects of the American persimmon is clearly evident in his writings on the subject.^{7, 11-13}  A research substation was established in Danville (Hendricks county), IN under the direction of O. M. Hadley for the evaluation and improvement of best known cultivars of the time.  The list of cultivars under study included Daniel Boone, Early Bearing, Early Golden , Golden Gem, Hicks, Kansas, Shoto, and Smeech.  Unfortunately little is known of their findings, and the author was unable to find any published information describing the outcomes of their efforts.  It is unknown whether any living remnants of their work remain.  Of the cultivars described in these early writings, only Early Golden remains readily available.   Concurrent with the work being conducted in Indiana, the American persimmon was also being evaluated by the Horticultural Department of the Kansas Experiment Station.¹⁴  Recently, an American persimmon evaluation program has been initiated at the Missouri Agricultural Experiment Station, Southwest Center.   This test plot has under study many of the modern cultivars as well as several Claypool selections.

The fortunes of the American persimmon began to change about fifty or so years ago when a handful of enterprising enthusiasts embarked in earnest to collect, test, and trade many clonally propagated varieties selected from the wild.^15-23  During this time, the late Professor J. C. McDaniel continued his life-long calling to extol the virtues of the lowly American persimmon which began around 1915 at his childhood home in Alabama.²⁴  He was responsible for selecting and helping others select improved, open pollinated seedlings from the best of the wild selections.  He also initiated the first known forays into controlled hand-pollination for horticultural improvement; however, his interests at that time focused primarily on unsuccessful attempts to hybridize Japanese and American persimmons.^{25, 26}   Prof. George Slate of Geneva, NY also worked, to some extent, with the native persimmon.  His work focused primarily on identifying northern-hardy varieties capable of surviving harsh winters as well as ripening their fruit in a shortened growing season.¹⁸

In the 1970s, a persimmon enthusiast named James Claypool with guidance from his friend Prof. McDaniel embarked on an effort to improve the status of the unassuming American persimmon.  Over the course of almost three decades, Mr. Claypool created thousands of controlled crosses between the best known cultivars of the time, keeping detailed records.^27-32  This Herculean effort represents the most extensive research and breeding effort ever instituted for the improvement of the American persimmon.

After some time, he contacted members of the Indiana Nut Growers Association (INGA) about preserving the best progeny from his breeding experiments.  Thus, several members of the INGA responded to his request by accepting oversight of his orchard in St. Elmo, IL.  They have been tabulating and organizing his hand-written breeding notes into a more computerized format as well as establishing standardized evaluations of the best specimens and distributing scion from this priceless bit of horticultural history.  They have recently posted a detailed description of Claypool's breeding efforts as well as the breeding data.  It is the objective of this article to examine this data to discover whether any interesting observations may be drawn from this rich information source.

CULTIVAR DESCRIPTIONS
Persimmons are typically dioecious - meaning that reproduction requires a male tree (staminate) and a female tree (pistillate).  Therefore, every cross will ostensibly require a male and a female parent.  This gets a little more complex because there are some specimens that are occasionally capable of producing male, female, or even perfect flowers on the same plant which means that it is possible for certain varieties to serve as the male or female in crosses.  Claypool found that when pollen from a female cultivar (i.e., Early Golden, Garretson, and Killen) is used to pollinate another female cultivar that all of the progeny will be female.³²  This has led to some degree of speculation regarding the mode of sexual expression in D. virginiana.

Figure 2.   Male tree with female and perfect flowers

And if this wasn't enough to muddy the waters, D. virginiana also occurs as two distinct subspecies with 60 and 90 chromosomes, respectively.^{33, 34}  As a loosely applicable rule, the 60 chromosome race tends to have slender, smooth leaves; whereas, the 90 chromosome race typically has pubescent leaves.  The distribution of the tetraploid (60 chromosome) race is centered in the southern Appalachian mountains and adjacent areas, and the hexaploid (90 chromosome) race occupies the range north and west of the tetraploid range.³⁵  Both ranges of both races overlap, and the two types can be found growing in close proximity.  This has led to some difficulty in establishing seedless cultivars as the supposed "seedlessness" of several varieties has been found to be due to fertilization incompatibility between the two races when a particular variety is grown in proximity to pollinators of the other race.^{23, 36}  Here, the focus is on 90 chromosome race since for various reasons, almost all of the cultivated varieties are of the northern type.

Figure 3.   Blue persimmon discovered in Iowa

The data used here consists of Claypool's controlled crossing data as well as pedigree information gathered from the literature.^{28, 32, 35, 37-39}  Figure 1 depicts the pre-existing persimmon pedigree including most of the varieties used in the Claypool experiments.  There is speculation that Juhl and Yates are actually the same cultivar, but they will be denoted as separate cultivars here as the author has been unable to reconcile this assertion with written record.^{18, 40}  Early Golden, Garretson, and Killen are the principal ancestors of the Claypool breeding line, chosen for both their high fruit quality as well as their tendency to produce the occasional male flower.

Figure 4.   Pre-Claypool Persimmon Pedigree

A brief description of the fruiting cultivars that were used in Claypool's best crosses follows. Some of them are still available through mail order nurseries.

Beavers: A second place winner at Mitchell Persimmon Festival (Mitchell, IN) in 1954. Noted to have good flavor.  Its primary claim to fame is due to its excellent pulp which has been used in prize winning persimmon puddings.  The tree has an upright growth with deep green, glossy leaves which are resistant to leaf spot leading to speculation that it may have value as a landscape tree.  It is also a heavy bearer.  The fruit is small to medium.^{21, 41}

Craggs: Discovered by the superintendent of Prof. McDaniel in Harrisburg, IL on a coal mine property around 1950.  The fruit ripens medium to late and is yellow fleshed.  Fruit size is medium to large but soft fleshed with red blush on the skin.  Reputed to have good flavor.^{15, 38, 42}

Early Golden: Probably the oldest cultivar still in cultivation.  Described in the literature as early as 1894 along with contemporary varieties such as Golden Gem, Hicks, Josephine, Marion, and Ruby.  It was discovered in Alton, IL around 1880 by the noted horticulturalist E. A. Riehl.  Sometimes sports male limbs.  Fruit is medium size with firm flesh and clear skin.  Seeds vary from 2 to 8.^{4, 10, 12, 15, 39}

Garretson: Selected in Adams county, PA circa 1920.  Is a relatively early ripener and has been noted to ripen as far north as Geneva, NY and Grant, MI.  Fruit is a rich orange color, firm, and of good flavor.  Presumed seedling of Early Golden with leaves slightly smoother and seeds larger than its parent.^{15, 18, 23}

Golden Supreme: A very large fruited variety originating from Illinois, ripening in October/November.¹⁵

Juhl: Fruit is very early and very large for an American persimmon.  It ripens over a short time period, and the fruit tends to contain black specks of precipitated tannins.  Records mention that Juhl pulps out at approximately 10 fruits per pound.^{18, 43}

Killen: Introduced from Felton, DE by Joseph Killen.  Ripens a few days later than its parent Early Golden with firm fruit slightly larger than that of Garretson but smaller than Early Golden.  Mentioned as possibly being a selfling of Early Golden since Mr. Killen was attempting to pollinate Early Golden with D. Kaki.  This seems somewhat unlikely given the results of the experiments with selfing and inbreeding within the Early Golden line conducted by both McDaniel and Claypool.  It sometimes sports male flowered limbs.  Claypool noted that when flowers were bagged, few fruits set, but they were seedless and about the same size as the seeded fruits with good flavor.^{15, 29, 38, 44}

Lena (Mitchellena): Winner of Mitchell Persimmon Festival in 1955.  Fruit is squat and soft with tender skin and an attractive red color. Good in cooked products.  Flavor holds up well in frozen pulp.  Ripens early and over a long period.^{20, 43}

Figure 5.   Marion persimmon

Figure 6.   Miller persimmon

Morris Burton: Discovered on the Burton farm in Indiana when Mr. Burton noticed several animal trails leading to a single persimmon tree.  It is a 1957 winner of the Mitchell Persimmon Festival contest.  The fruit is small with tender skin.  It is often quoted as having the best flavor of all persimmons.  The pulp is very sweet.  It is a beautiful red color and nearly free of black specks.  The foliage is reputed to turn bright yellow in autumn.  The tree is slow to come into bearing, and the small to medium fruits pulp out at approximately 25 fruits per pound.^{23, 43}

Szukis: Male cultivar often bearing fruit - no need for a pollinator.  Believed to be a seedling of Early Golden.  It is known to perform well in the northern limits of persimmon culture.^{45, 46}

Table Grove: Selected by Homer Wilson from Table Grove in northern Illinois.  The fruit ripens early and over a short harvest period.  The fruit is shaped like an acorn, and the calyx remains attached when the fruit falls.  The pulp contains some black specks and loses flavor when frozen. Pulps out at approximately 20 to 25 fruits per pound.^{21, 43}

Wabash: Originating in Pinkstaff (Lawrence county), IL.  The early fruit is red-orange, aromatic, sweet, and firm.  However, it is small and seedy.  The leaves turn a bright, ornamental red in fall - unusual for an American persimmon.^{15, 38}

Yates: Introduced by Edward Yates of Cincinnati, OH.  Tree was discovered in a pasture in southern Indiana.  It won first place in the Mitchell Persimmon Festival in 1983.  Fruit is large, early and flavorful.  Readily available through the mail order nursery trade.⁴⁰

CLAYPOOL CROSSES
The raw data used in the analysis is currently available through the internet.  Most of the values in the original tabulation are attributes associated with the fruit quality of each tree under observation.  The raw data lists several attributes associated with each tree.  They are Shape, Size, Skin, Sugar, Taste, Fiber, Color, Seeds, Production, Market Potential, Black Spot, After Taste, and No Black.  For the purposes of this analysis, Production, Market Potential, and No Black were omitted since these fields tend to be sparsely populated and/or are highly correlated with some of the other listed fields.  The Shape field was also excluded from the analysis to concentrate efforts on more quantitative features.  Size of the fruit is measured in centimeters.  The eight fruit quality fields Skin, Sugar, Taste, Fiber, Color, Seeds, Black Spot, and After Taste are rated by human testers on a scale of 1 to 10 with 10 being the most desirable Score.  In the raw data, each tree was valued by its cumulative Score, summing its respective fruit quality fields as well as the fruit Size field.

To account for differences in the variances of values within each field, the Score was redefined.  Each of the nine Score fields were normalized to a standard deviation of 1.  The purpose of this is to prevent any of the attributes from dominating the total Score for each cross.  For instance, the standard deviation of the Black Spot attribute is more than six times as large as the standard deviation for the Size attribute.  This was accomplished by subtracting the within-field mean from each field value and then dividing by the within-field standard deviation.

To avoid seedlings with a modified Score less than zero, a value of 0.5 was then added back to each field so that a value of 0.5 denotes an average Score.  The total Score was then defined to be the mean value over all nine Score fields for those fields with measured value.  If a tree had four or less values listed for the nine Score fields, it was assigned a Score of zero.  Therefore, the new Score value range is centered on 0.5 with Score > 0.5 indicating above average scoring selections.  It is important to note that this analysis is based solely on the available data.  It is likely that there are additional superior specimens that have not been thoroughly scored and included in the analysis.  This is especially true for some of the earlier breeding progeny.  In addition, a selection that scores well on the ortet may not necessarily perform well when grafted onto other rootstock.⁴⁷  This criteria reduced the more than 2,000 Claypool crosses to a subset of approximately 400 considered for more detailed examination.

Table 1.  Best Scoring Claypool Crosses

When multiple evaluations were listed for a single variety because it was grafted in more than one location (i.e., Early Golden, Garretson, Killen, etc.), the values were averaged over all replicates.  The records of some trees were unrecoverable.  These entries were deleted. Male trees were not assigned any Score values unless they exhibited the tendency to reliably produce fruit and were judged by the evaluation committee (i.e., Szukis, F-100, etc.).  It is interesting to note that Early Golden, Garretson, and Killen (the standards by which other cultivars are compared) score slightly below average with respect to the subset of Claypool crosses which have been evaluated.  There are approximately 160 Claypool crosses which Score better than Early Golden.  Table 1 lists the 51 best Claypool crosses in order of decreasing Score.

Figure 7 provides a convenient representation of all of the Claypool crosses with an INGA evaluation Score along with the pertinent named cultivars.  In this figure, the circles represent individual trees where the size of the circle represents the relative Score for that tree (bigger circle means better Score).  Note that some selections are not represented by a noticeable circle. This is because no INGA Score was available for these respective selections.  The red lines indicate a female parent relationship from the named cultivar to the Claypool cross, and the blue lines indicate a male parent relationship.

Figure 7.   Network representation of the best Claypool crosses (depicted using NetDraw)

The most stark observation offered by this visualization is that crosses where the pollen donor was a female cultivar (i.e., Early Golden, Garretson, or Killen) are noticeably superior on average to those crosses where the pollen donor was a male.  While this is not exactly an epiphany, the difference is notable nonetheless.  This underscores the need for the discovery of other superior fruiting female cultivars that are capable of sporting the occasional male flower as well as the development of males possessing a larger percentage of genes attributable to superior fruiting females.

WHAT NEXT?
While it is hoped that the previous discussion provided some insight as to which of the Claypool crosses appear to be the most promising, it doesn't provide much in the way of guidance as to what steps could be taken for additional breeding attempts.  It has been published that the Japanese persimmon is highly susceptible to inbreeding depression causing a reduction in both fruit size and vigor.^{48, 49} This appears to also apply to the American persimmon.  In his attempts to explore the mode of sexual expression in D. virginiana, Claypool performed many inbred crosses between cultivars such as Garretson x Early Golden and Szukis x F-100.  As a rough gauge of the effect of inbreeding on the American persimmon, all of the Claypool crosses were entered into an MS Access database, and the coefficient of inbreeding for each tree was calculated using the freely available, PediTree plant pedigree software.⁵⁰  The coefficient of inbreeding is a computed measure (0 to 1) of the expected level of accumulated homozygosity induced by intra-family heredity which significantly increases the probability of the accumulation of deleterious alleles.  By inspection of the computed results, it appears that a range of 0 to 0.1 appears to be a relatively safe target range for future breeding efforts.

From this data, a virtual breeding network was constructed.  First, all of the male Claypool crosses were identified and grouped according to their pedigree so that all males with the same parents were in the same group. Then a single male was randomly selected to represent each group.  These males were then crossed to all of the high scoring selections represented in Figure 2.  The cultivars Early Golden, Garretson, and Killen were also included in the list of male parents since they have been known to produce superior offspring when used as pollen donors.  This resulted in approximately 3,500 potential crosses.  The coefficient of inbreeding for each potential cross was then computed using the Peditree software.

While many of the potential crosses resulted in coefficients of inbreeding greater than the 'rule-of-thumb' value of 0.1, many others proved to be of potential interest.  Of particular interest are those selections that can be safely back-crossed with one of the females known to produce viable pollen.  Most of these prospective crosses resulted in coefficients of inbreeding greater than the 0.1 threshold; however, some of these crosses meet the threshold criteria.  A handful of these of these are listed in Table 2.  They are ranked by the Score value of the prospective female parent. Note that the female parent can be replaced by any selection with an equivalent parental combination and still result in the same value for the coefficient of inbreeding.

Table 2. Potential Early Golden family back-crosses.

The list of prospective crosses involving Claypool females and Claypool males with coefficients of inbreeding meeting the threshold was also determined.  Some of these appear to also be of interest in future breeding efforts especially considering that a greater proportion of their genetic background is due to superior fruiting ancestors than the males represented in Figure 2.

Although there is still much opportunity to capitalize on the benefit of using pollen from cultivars such as Early Golden and some of her progeny, the long term future of the improvement of the American persimmon will depend on the breeder's ability to directly address the fitness of the male parent.  Clearly, increasing the proportion of genes expected to be due to superior female ancestors is an indispensable tactic.  However, there may also be other techniques that could be employed to help achieve the desired results.  Japanese researches have found that treating the male flowers of D. kaki with benzylaminopurine (BA) can induce male flowers to set fruit.⁴⁹  This may offer the opportunity to screen male breeding candidates for the desired breeding targets such as ripening time and fruit weight if it has a similar effect in D. virginiana. BA is an inexpensive and readily available cytokinin.

Two methods are described to initiate the sexual conversion process.  1) Spray the BA as a 1,000 ppm solution on the desired branch, applied six times prior to the initiation of the flower primordia (June 2 through July 6 in Japan).  Treated branches had significant levels of pistillate flowers the following spring.  2) Applying BA solutions at the same concentration were also noted to convert staminate flowers to hermaphroditic the same year as the application when applied during April (in Japan).  If this effect can be replicated in D. virginiana, an interesting experiment regarding persimmon sexual expression would be to use pollen from a female cultivar such as Early Golden, Garretson, or Killen to pollinate treated flowers on a male tree.

One problem confronting 'would be' breeders of D. virginiana is the loss of genetic diversity.  Increasingly, the genetic background of the most prominent cultivars is becoming narrower.  Unfortunately, many of the caretakers of historical collections have passed away, orphaning many valuable discoveries.  In addition, some of the older varieties discovered in the wild that managed to traverse multiple generations have been superceded by more recent acquisitions selected from seedlings of established cultivars - in effect further decreasing genetic diversity.  Many of the historical varieties were propagated for over a hundred years.  While many of the newer varieties are superior to these older varieties, the fact that the historical cultivars were propagated for so long acknowledges the fact that they were notably superior to the typical wild tree.  At some point, further improvement will require more genetically diverse selections, and the work of the fruit explorers of the past will have to be recreated if improvement is to continue.  Some of the notable varieties of the past that appear to have been lost to cultivation include Golden Gem (discovered by Logan Martin),^{5, 12} Hicks (discovered by Hicks Trueblood),¹² Josephine (discovered by Samuel Miller)⁵³.  These cultivars have a storied past and were propagated for around a century before being lost.

It has been lamented that the future of most non-commercial fruits such as the American persimmon is relegated to the ranks of the amateur grower, but this could a disappointment or a blessing depending on your perspective.  How often does one have the opportunity to make a lasting impact in an activity that one enjoys void of intense academic and commercial competition?  For those interested in trying their hand at creating a living legacy of their own, a thorough tutorial on hand-pollination of persimmon has been published,⁵¹ and two good places to learn more are the persimmon interests groups in both the North American Fruit Explorers (NAFEX), and the Indiana Nut Growers Association (INGA).

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