Reinventing the Fruit Research Wheel

I have been involved in fruit research for 48 years—long enough to observe trends and cycles in the fruit industry and fruit research. I have long admired Professor Maurice Adin Blake, who Professor Ernie Christ (former New Jersey tree fruit specialist) called "The Father of the New Jersey Peach Industry." Before taking over the peach breeding program at Rutgers University, M.A. Blake researched and published many aspects of peach tree physiology and orchard management. I am in the process of summarizing Blake's work for a paper in the Journal of the American Pomological Society.

While searching the old literature, I realized that if I had spent more time reading old experiment station bulletins early in my career, I could have avoided repeating experiments that we performed by pomologists in the early 1900s. In my defense, 40 years ago, it was much more difficult to locate old bulletins and required spending many hours in the library looking at the indices of journals and experiment station annual reports. Some, but not all, of these reports are available electronically, or at least the titles can be found with an electronic search. This greatly improves the efficiency of searching for information on specific topics.

For many years I have criticized graduate students and young faculty members for spending too little time in the library reading older literature. As a result, they are unaware that their research is similar to research published before 1990 because electronic versions of the three journals published by the American Society for Horticultural Science are available only after 1990, and most experiment station bulletins have yet to be scanned. While reading some publications written by Blake before 1920, I realized that I am also guilty of not spending enough time in the library. As a result, I unknowingly repeated research by Blake and others. Below are a few examples of my reinventing the wheel.

Summer Pruning

I studied summer pruning of apple for my Ph.D. research, so I thought I was familiar with the older literature on the subject. However, I recently learned that based on his research, in 1912, Blake recommended removing upright shoots and water sprouts on young peach trees during the summer to improve tree form. In 1983, I reported that summer removal of water sprouts and upright shoots improved light in the interior of young, vigorously growing peach trees. When summer pruning was performed annually to reduce shade, I could produce fruit all the way to the trunk and keep the fruiting zone of mature trees within 8' of the ground. Until now, I thought I was the first to advocate summer pruning in young peach trees, but Blake scooped me. I extended this work by covering trees with different levels of shade cloth at other times of the summer, and I found that peaches need about 30% full sun during mid-June and July to form flower buds. Heavy shade after early August does not influence flower bud formation.

Since summer pruned trees grow later in the summer, Blake hypothesized that summer pruned trees might be more susceptible to winter injury than dormant pruned trees. I performed an experiment where I compared dormant pruned trees to trees that were pruned or topped in mid-August. I counted leaves per shoot each week until total defoliation in the fall. When I measured cold hardiness with artificial freezing, flower buds and wood on one-year-old shoots from summer pruned and summer topped trees were more susceptible to cold temperatures in the early winter. As Blake predicted, I found that summer pruning delayed leaf abscission and cold acclimation of flower buds compared to dormant pruned trees.  

Heading Trees at Planting

At planting, peach trees are generally headed at about 24" to 30" above ground. Sometimes growers receive large trees with trunk diameters close to one inch in diameter. Such trees often have no branches or visible buds at the desired height for scaffold branches. Ernie Christ said that these trees—he called them "broom handles"—should be headed high at planting; then, during the summer, they can be headed above the highest desirable limb. This method often resulted in trees with poor tree structure and scaffold branches higher than desirable. In the mid-1980s, I established a planting to compare central leader and open center trees. Unfortunately, I received large trees from the nursery.

I decided to modify my experiment and headed some trees at 28" and some at 6" above ground. I hoped that the low-headed trees would produce epicormic shoots that I could develop into a tree with good structure. Epicormic shoots arise from buds beneath the bark and are released from dormancy following trauma, such as a heading cut. Ten percent of the low-headed trees died, but the rest produced vigorous upright shoots with many wide-crotch limbs from which I could select ideal scaffold limbs.

Last week I came across a bulletin written by Blake in 1916, "Effect of Pruning Peach Trees at Different Heights Previous to Planting in the Orchard," and I realized that I was scooped again. He obtained one-year-old trees that ranged in size from 3/8" to 7/8" in diameter, and he headed them at 6, 12, 18, 24, or 30" above ground and measured the total linear twig growth after the first growing season. He repeated this experiment with freshly dug trees, trees from cold storage, and trees that had dried out in storage. His results were more variable than I would expect, probably because he had only five single-tree replications per treatment. The optimum heading height varied depending on the size and condition of the tree. For freshly dug trees, large trees (3/4") had 23% more growth than small trees (3/8"). Trees grew best when headed at 36" regardless of initial tree size. For stored trees, moderate-size trees (5/8") generally grew best. Large trees (3/4") grew best when headed at 36", but small trees (3/8") grew best when headed at 6". For dried-out trees, 38% of the smallest trees (3/8") died, whereas less than 22% of larger trees died. For dried-out trees, averaged over all heading heights, moderate size trees (5/8") grew best. Small trees headed at 36" grew best, but most tree sizes headed at 18" or 24" grew the best.   

Another of my favorite pomologists is Ronald Hatton, who evaluated dwarfing apple rootstocks from European nurseries and is responsible for classifying the East Malling rootstocks. He also served as a director at East Malling for 30 years and significantly expanded its size and range of activities. By the 1960s, it was probably the leading research facility for fruit crops. When I was in Virginia, some apple growers felt that trees on M.26 produced larger apples than trees on M.9 rootstock. I had not seen this in my rootstock trials, and in most of the earlier rootstock trials, rootstock did not consistently affect fruit size.

Since I coordinated several NC-140 regional apple rootstock trials, I had all the data from these trials, so I decided to take a closer look. When I used a statistical technique called analysis of covariance to adjust average fruit weight (g/fruit) for crop density (fruit/cm2 trunk cross-sectional area) for 'Gala,' I found that trees on M.9 had larger fruit than trees on M.26. The weakness in this approach was that rootstocks could greatly influence the number of fruit on a tree, and crop density was often not similar for different rootstocks. NC-140 then designed an experiment where we adjusted the range of crop densities on 'Golden Delicious' trees, so we could better evaluate the effect of rootstock on the relationship between crop density and average fruit weight.

The results showed that for most years and locations, trees on M.9 produced larger fruit than trees on G.16, and trees on M.26 had an intermediate fruit size. In 2002 I thought ours was the first study to definitively show that trees on M.9 produced larger fruit than trees on other rootstocks. However, in 2017, as I was reviewing the rootstock literature, I found a 1935 report by Hatton in the Journal of Pomology and Horticultural Science where he stated that trees on M.9 produce larger fruit than trees on other Malling rootstocks – scooped again.  

Peach Position on a Shoot

In the 1980s, there were conflicting opinions about thinning peaches. Most people tried to space peaches uniformly along a shoot. Dr. Gary Couvillon in Georgia reported that peaches at the terminal end of the shoot were larger than those at the base of the shoot. Luca Corelli Grappadelli was evaluating fruit position as part of his master's research at Clemson University with Dr. D.C. Coston and found that fruits developing near the base of the shoot were larger than those at the terminal end. Based on several experiments, I concluded that fruit position on the shoot did not affect fruit size, but fruit developing on shoots with leafy axillary shoots produced the largest fruit, and those shoots tended to be at least 12" in length. A few years later, Luca told me that he accidentally came across an article published in an Italian journal in the 1920s where peach size was related to the number of axillary shoots developing on the fruiting shoot. Scooped again!

Summer Pruning Apples

Summer pruning apples is again a topic of conversation. In the late 1800s, some pomologists recommended summer pruning to suppress tree vigor and induce fruiting of young trees. Since there were few data to support the practice, from 1910 to 1925, several researchers across the country compared various types of summer pruning to similar pruning performed during dormancy. Although results varied for many reasons, summer pruning was not recommended because there were few benefits compared to dormant pruning.

As I began my doctoral research in 1977, summer pruning was again discussed as a method of suppressing tree vigor and reducing shade that would lead to improved flower bud formation, fruit set, and fruit size and quality. At that time, summer pruning involved heading all one-year-old shoots to about three leaves in mid-August. Results from my research and several others showed that summer pruning improved post-pruning light distribution in the canopy and sometimes improved fruit red color. Still, fruit size and soluble solids were often reduced.

Although summer pruning suppressed late-season trunk and root growth, it did not suppress shoot growth which is responsible for shading the tree interior. Since summer pruning had few economic benefits, the practice has not been recommended for the past 40 years. However, summer hedging (which involves the mechanical heading of shoots growing into the row middles) is being discussed as a method for suppressing tree vigor and improving flower bud formation, fruit set, and quality. Does this sound familiar?

Thiago Campbell just finished his master's research with Dr. Jim Schupp and me, and his results are similar to those reported in the 1980s. It seems that interest in summer pruning resurfaces about every 40 years. There may be several reasons for this. Maybe we assume that trees grown on different rootstocks and trained to modern systems will respond differently than trees in previous experiments. Still, more likely, we are not familiar enough with previous research results. Maybe I will be proved wrong, but when I first heard that summer hedging was being recommended for maintaining tree walls in intensive apple plantings, I was reminded of the saying (often falsely attributed to Albert Einstein): "The definition of insanity is doing the same thing over and over and expecting a different result."

The amount of information being published is increasing at an increasing rate. As new journals are being established, it is impossible to keep up with the literature. As we tend to focus on newly published papers, there is also a tendency to overlook the older literature, partly because it is more difficult to find. In graduate school, I read most of the papers on apples, some on peaches and strawberries, published in the Proceedings of the American Society for Horticultural Science from 1903 to 1978. I also spent many nights in the archive section of the library basement reading old experiment station bulletins. In some cases, those old reports included raw data that I analyzed with modern statistical techniques. Therefore, I prided myself on my knowledge of the older pomological literature.

But as I approach retirement, I have been humbled as I realize that I am unfamiliar with much relevant information published before World War II. I hope that the next generation of pomologists can find the time to familiarize themselves with the work that has been published so they can avoid repeating as many experiments as I did. Before recommending a supposedly "new" idea, I hope extension workers will consult with their predecessors. I also encourage fruit growers to discuss it with their parents and grandparents to learn if the idea really is new!