Good Fruit Grower

July 2013

Issue link: https://read.dmtmag.com/i/138107

Contents of this Issue

Navigation

Page 13 of 39

GOOD TO KNOW A research report from Gregory Lang, Michigan State Pruning for large cherries T en years ago, when growers were struggling with their first experiences of growing sweet cherries on precocious, vigor-controlling rootstocks like the Gisela series, it was common to hear comments like "Small trees grow small cherries." Since that time, it has been clearly demonstrated, by scientists and growers alike, that rootstock has little, if any, direct effect on fruit size. In our research trials, we have grown dozens of varieties on dwarfing Gisela 5, even some on very dwarfing Gisela 3, and harvested fruit that averaged 11, 12, 13 grams per fruit, with top sizes of 8-row and even larger (up to 35 or 36 millimeters in diameter, see Figure 1). These include Benton, Cowiche, Early Robin, Rainier, Regina, Selah, Skeena, and Tieton (sadly, not Bing!). The key issues for producing large fruit are management factors: 1) supplying adequate water and nutrients proportional to the size of the root systems (that is, more frequently for smaller root systems), and 2) managing crop load and leaf area, which can largely be done with pruning. Pruning for large cherries will be the focus of this article. Understanding the basic fruiting unit for a sweet cherry tree is helpful for thinking about the impacts of pruning on sweet cherry growth and fruiting. Cropping University occurs on fruiting spurs that form on wood that is at least two years old, and on nonspur flower buds that form at the base of last year's shoot growth (see Figure 2). The most fundamental units of fruiting in cherry includes both of these two distinct fruiting populations, plus the new current season shoot growth. This reveals that there are three distinct leaf populations on the fundamental unit of fruiting: —the new shoots' leaves, which are very large and form only one per shoot node; —the nonfruiting spur leaves on the portion of the shoot that grew last year, which are usually moderate in size and form 6 to 8 leaves per shoot node; —the fruiting spur leaves on the portion of the shoot that grew two years earlier, which are usually moderate in size and form 6 to 8 leaves per shoot node. The idea that fruit require a minimum number of leaves (for photosynthesis) to achieve adequate growth leads to the concept of leaf-to-fruit ratios. However, it is difficult to count exact numbers of fruit or leaves, leaf sizes can vary dramatically, and as we've seen in the fundamental fruiting unit, leaf number per node, leaf size, and leaf distance from the fruit make it difficult to determine what would be an appropriate target leaf-to-fruit number to achieve in orchard management. Suffice it to say that the leaves closest to fruit tend to provide the most carbohydrates for fruit development, and the larger the leaves and more exposed to sunlight, the better. EARLY B Gets the W ird orm! Call today f 2014 & 2015or deliveries! Pear Trees on ® 9 7 97 OHxF 6 , 8 , & Rootstocks OHxF® 69, 87, & 97* – Match your specific variety and orchard to one of these rootstocks for maximum results. *OHxF® 69 Rootstock Shown Patent Information Available Call today for more detailed information (800) 675-6075 14 July 2013 GOOD FRUIT GROWER Figure 1 Very large fruit of Benton and Rainier sweet cherry grown on Gisela 5 rootstock. An 8-row-sized fruit is about 33 millimeters; these fruit are larger than 36 millimeters. A valuable illustration for thinking about pruning is to take the concept of leaf-to-fruit ratios and imagine how a cherry branch with a standard leaf size (new shoot leaves being twice as large as spur leaves) and a standard number of fruit grows year by year and how those leaf and fruit populations are affected by pruning. In Year 1, a new shoot grows: there are 10 new shoot leaves (equivalent to 20 spur leaves since they are twice as large), no spur leaves, and no fruit (see Table 1). In Year 2, the shoot extends, adding 10 more new shoot leaves, while each of the nodes on the portion of the shoot that grew the previous season now has 6 nonfruiting spur leaves (60 spur leaves total), resulting in an effective leaf area of 80 equivalent spur leaves. There are also 10 fruit that form at the base of the previous season shoot growth, resulting in a leaf area-to-fruit (LA:F) ratio of 8.0 and very good fruit size. In Year 3, the shoot extends to add another 10 new shoot leaves, the nodes on the portion of the shoot that grew the previous year now have 60 nonfruiting spur leaves, and the original shoot growth now has 60 fruiting spur leaves, for an effective leaf area of 140 equivalent spur leaves. However, the fruit population in Year 3 is comprised of 10 basal fruit on the previous season's new growth plus 75 spur fruit, for a total of 85 fruits. Thus, as soon as Year 3 on precocious rootstocks, the LA:F ratio falls to 1.7, and so the carbohydrate supply to those fruit is not as great as in Year 2; therefore fruit size may be limited. In Year 4, the LA:F ratio falls again to 1.2, and projections into future years show that the ratio will remain close to 1.1 without any intervention by pruning. Pruning for size Now, to examine the impact of pruning for fruit size, we first return to the fundamental fruiting unit branch (see Figure 2). Note that on the section of the shoot with fruiting spurs, the spur density is greater towards the terminal portion compared to the basal portion of the spur section. Not only are the spurs closer together, but there are more flower buds per spur in those more terminal spurs. Therefore, the crop load is much greater near the terminal portion of that section. This is the portion that would be ideal to thin out to reduce the crop load and improve the LA:F ratio. Typically, flower or fruit thinning is accomplished with chemical sprays, mechanical means like string thinners, or by hand. The problem in sweet cherries of removing excessive flowers or fruits by pruning is that cutting into the dense clusters seen on two-year and older sections of wood also means removing a lot of valuable nonfruiting leaf area (the nonfruiting spurs and new shoots). So, to effectively balance sweet cherry crop loads by pruning, one needs to anticipate where the future dense clusters will occur and make preemptive pruning cuts to reduce the node density before it becomes fruitful. That allows the nonfruiting leaf area to develop normally while the fruiting section matures. www.goodfruit.com

Articles in this issue

Links on this page

Archives of this issue

view archives of Good Fruit Grower - July 2013