Browsing by Person "Riehle, Andreas"

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Maschinelle Blütenausdünnung von Apfelbäumen
(2020) Riehle, Andreas; Wünsche, Jens Norbert
To achieve regular yields with good fruit quality, thinning is one of the most important and at the same time most demanding orchard management measures undertaken in commercial apple production (Malus domestica L.). Recently, the use of mechanical flower thinning as a weather-independent and environmentally friendly method has increased steadily. To better understand the effects of mechanical flower thinning on generative and vegetative processes in apple trees, a three-year field trial (2014 – 2016) with the cultivars Elstar (planting year 2001; M 9) and Gala (planting year 1998; M 9) was carried out at the Competence Centre for Fruit Growing (KOB) in Ravensburg, Germany. Relevant fruit quality and yield parameters, as well as physiological parameters for leaf and fruit development, flower and fruit drop, shoot growth and return bloom were determined. Thinning treatments were carried out with a Darwin-machine (Fruit-Tec; Markdorf) and took place in all three experimental years shortly before full flowering (BBCH stage 61-63). The spindle speed was varied between 200 and 400 rpm at a constant tractor speed of 8 km h-1. In addition, mechanical thinning treatments were combined with hand thinning and spray applications of prohexadion calcium (Regalis® Plus; BASF). Results of all trial years showed that mechanical thinning can reliably reduce the crop load of apple trees. In addition, the thinning effect was improved by gradually increasing the spindle speed without causing over-thinning of the trees. In both Gala and Elstar, a positive linear correlation between crop load per tree and spindle speed could be observed. In Gala, a single mechanical thinning treatment could reduce alternate bearing to such an extent that an adequate return bloom and crop load in the following year could be expected. In Elstar, a cultivar which is highly susceptible to alternate bearing, a combination of mechanical thinning followed by hand-thinning proved promising to increase return bloom in the year following treatment. Mechanical thinning treatments increased fruit size and blush colour depending on the spindle speed. An additional hand thinning treatment led to a further improvement in fruit size. A reduction in leaf mass of up to 37 % in Gala and up to 36 % in Elstar could be measured in the experiments. For leaves and flowers, the ratio between undamaged and damaged/ removed flowers and leaves increased as a function of spindle speed. However, a direct correlation between the loss of leaf mass and flower drop or growth of the young fruitlets was not observed. In addition, no direct influence of mechanical thinning on flower drop immediately postbloom or June fruit drop was observed. In both Gala and Elstar, the number of flowers or fruit per tree before the corresponding fall period was mainly responsible for the strength of the flower or June drop. The probability for undamaged flowers to fall off the tree in the untreated controls was just as high as in the mechanical thinning treatments. Based on this result, it can be assumed that the leaf and fruit damage caused by the mechanical thinning treatment has a very small effect on the following flower or fruit drop. Furthermore, these results do not support the so-called "ethylene shock theory" which implies an ethylene-based stress reaction within the tree after a mechanical thinning treatment resulting in increased flower and fruit fall. As an indirect influence of mechanical thinning, an increase in shoot growth was measured for both cultivars in all mechanical thinning treatments, this effect is most likely attributed to the reduction in crop load. An additional hand thinning treatment showed no influence on shoot growth. However, a direct link on shoot growth between the mechanical thinning treatments and the crop load could not be determined, since among other things various other factors (tree health, general tree growth or vigour, previous years yield etc.) could have also influenced the shoot growth response. The observed increase in shoot growth in the mechanical thinning treatments was reduced to the same level as in the untreated controls by the Prohexadion-Calcium treatment.