Summertime means delicious fresh fruits and vegetables, and July means cherries. Since starting my graduate thesis in tart cherry breeding, I have developed an adoration for cherry production, and if you love cherries, Michigan is the right place to be. Within the United States, Michigan produces around 75% of the nation’s tart cherries and Washington, Oregon, and California collectively produce 90% of the sweet cherries.
The Cherry Family Tree
Cherries belong to the Rosaceae family which originated in Eastern Europe. Other members of this family are peaches, strawberries, plums, apricots, apples, pears, raspberries, almonds, roses, and others. Within the Rosaceae family, cherries belong to the Prunus genus. The two cherry species that we grow for fruit are sweet cherries (Prunus avium) and tart/sour cherries (Prunus cerasus). Sweet cherries are a progenitor species of tart cherries: in the ancient past, sweet cherries hybridized with another species, ground cherry, to produce what we know today as tart cherry. Cherries were brought to North America by Europeans and are now grown across the continent, particularly in areas like the Great Lakes and the Northwest United States where the effects of climate extremes are reduced by the surrounding topography.
So what’s the difference between sweet and tart cherries?
The major difference, of course, is that one is sweet and the other sour, but there are several other differences in both physical characteristics and production practices. Let’s start with the different characteristics. Commercially available sweet cherries are typically large, dark purple colored, and firm. You may recognize names like ‘Bing’, ‘Regina’, and ‘Sweetheart’. Sweet cherries can also be lighter yellow with a red blush, like the popular cultivar ‘Rainier’. There are fewer commercially grown tart cherry cultivars, with the most common being ‘Montmorency’, which is smaller and soft with red skin and light flesh. Another tart cherry cultivar you may recognize is ‘Balaton’ which is dark purple in color. ‘Balaton’ is grown on a small percentage of the total tart cherry acreage and is more commonly found in farmer’s markets and you-pick locations than in grocery stores. ‘Balaton’ has a charming origin story and was brought to the U.S. by my advising professor Dr. Amy Iezzoni, read more about its history here.
Sweet and tart cherries are both grown in monoculture over many acres of orchards and require similar growing conditions. Even though the two species are closely related they do differ in some cultural practices. One example of this is the use of fungicides to control the fungal disease Cherry Leaf Spot, which defoliates trees and is devastating to growers. Tart cherries are more susceptible to Cherry Leaf Spot than sweet cherries, and therefore
require more frequent fungicide sprays.
Because my thesis is regarding different host-tree responses to Cherry Leaf Spot and breeding tart cherries for resistance to it, I will save these details for another post. Nonetheless, there are slightly different cultural inputs for sweet and tart cherries. The biggest difference, however, is the end use of these two cherry species and how this changes the way they are harvested and the ideal traits for each.
Sweet cherries are sold to consumers in the fresh market, while tart cherries are usually sold frozen and dried or as a component of baked goods, juices, and jams. Because sweet cherries are eaten fresh, they are picked and transported around the country shortly after harvest. This is why it is difficult to find sweet cherries out of season. Consumers prefer sweet cherries with the stem still attached, therefore all sweet cherries are hand-picked in order to keep the stems, which cannot be accomplished with a mechanical harvester. For fresh eating, it is preferred that the cherries be large in size and firm. Size appeals to consumers and increases grower profits while firmness allows the fruit to be transported long distances with less damage.
Oppositely, tart cherries tend to be processed just after harvest and are eaten as an ingredient in other food products. They are typically soft and therefore difficult to transport as a fresh product. About two weeks prior to harvest, tart cherries are sprayed with a plant growth regulator, Ethephon, which loosens the stems from the fruit. This allows the cherries to be more easily harvested with mechanical shakers that grasp the trunk of the tree and shake the fruits from the branches, catching them on a tarp (*see links to videos below). Once collected on the tarp, a conveyor belt moves the fruit into a large square bin of cold water. This cold water helps to preserve the quality of the soft fruit. These bins are then taken to a platform, or cooling pad, where cold water is continuously pumped throughout the bins until they can be transported to a nearby processing plant that same day. At the processing plant, the pits will be removed and the cherries will eventually be made into products like canned pie filling, dried cherries, frozen cherries, jam, and juice. Because tart cherries are harvested using mechanical shakers, fruit can only be harvested from trees that are at least 6 years old: any younger and the trunk may break. This is a big restriction to tart cherry growers, as cherries can flower and produce fruit when they are 3 years old. To address this issue, alternative methods, such as the use of precocious dwarfing rootstocks and over-the-row harvesting systems, are being explored for both young orchards and high-density plantings.
Tart Cherry Breeding Goals
As mentioned in previous posts, plant breeders tend to target specific traits depending on the crop, environment, and needs of growers. As far as tart cherry breeding goes, Michigan State University is home to the only tart breeding program in the United States, led by Dr. Amy Iezzoni. As a student in this program, I have been lucky enough to see firsthand the progress made in improving tart cherry traits and the impact this work has on growers. So what are the traits being targeted in tart cherry breeding? You may have been able to guess a few from the above descriptions of various fruit traits and production practices.
When it comes to fruit traits we are looking for larger and firmer fruits that have both red skin and red flesh, and detach easily from the stems for harvest purposes. Because consumers associate red color with cherries, it is the color preferred by processors. The current major cultivar ‘Montmorency’ has red skin but nearly clear flesh, so red food dyes are often added to cherry products; however, fruit with naturally red flesh would eliminate this use of added dyes. For processing, it is best if the fruits are round so that they sit appropriately in the pitting equipment, and firm enough that they do not break apart when the pit is poked out. It is also essential for the pit to be round so that it is not chipped by the machinery, freestone so that it easily detaches from the flesh of the fruit, and small enough so that it doesn’t take up too much of the fruit size but large enough that it is not missed by the pitting equipment. In addition to fruit traits we are breeding for increased fruit yields, resistance to Cherry Leaf Spot, and late bloom time. Later bloom time reduces the risk of yield losses caused by flower death in the occurrence of Spring frosts.
Progress in breeding for these traits has been hastened through the use of genetic markers which provide information to be used in designing breeding crosses and selecting individuals. To learn more about how this technology is being used in breeding Rosaceous crops like cherries, visit these links for the RosBREED Project and Dr. Iezzoni’s Program and look for a future Cultivate Curiosity post on genetic markers in breeding.
This summer, enjoy fresh sweet cherries while they are in season and keep an eye out for any local fresh markets with tart cherries! The rest of the year, enjoy tart cherries in pies, jams, juices, as well as dried and frozen. To learn more about tart cherry’s admirable qualities and to get some creative cherry recipes visit choosecherries.com.