Development of fruit growing and fruit science in Latvia has always been closely linked to the development of the whole country. After the founding of the independent Latvia state in 1918, fruit growing developed rapidly. Although in the Soviet times the situation was not favourable for quality fruit growing, research and breeding continued with good results. After Latvia regained independence, private land property rights were restored, and interest in intensive orchard establishment and growing technologies increased rapidly, which demanded change in the research focus. At present, the Latvia State Institute of Fruit-Growing is the leading institution in this field, working in cooperation with Pūre Horticultural Research Centre, Latvian Plant Protection Research Centre, Institute of Agrobiotechology, and Faculty of Food Technology, Latvia University of Agriculture, Laboratory of Plant Mineral Nutrition, Institute of Biology, University of Latvia. Research is carried out in the following directions: breeding and cultivar evaluation; genetics and molecular biology; plant pathology and entomology; orchard management; experimental processing and storage.

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INTRODUCTIONCracking of large, firm sweet cherry fruits and the subsequent fungal decay are two great problems in many areas with wet climate such as in Latvia. These factors cause serious losses of yield. It is known that fruit cracking is caused by water absorption through the skin (Christensen, 1973;1996;Sekse, 1998;2008). To protect the fruit surface from rain and subsequent cracking, as well as rotting, and to maintain fruit quality, plastic covers are often used (Meland and Skjervheim, 1998;Balkhoven-Baart and Groot, 2005;Meland, 2005;Simon, 2006). However, some of the systems have not only a positive influence on fruit quality, but also some negative aspects (Lang et al., 2011).Under cover, pest damage caused by cherry aphids (Myzus cerasi) can increase above damage thresholds (Lang et al., 2011) causing reduced growth and yield. However, the slightly shaded conditions under the constructions can promote tree development, growth and yield, and the air temperature can be more stable and suitable. In young orchards, trees under cover can faster fill in the available space with fruit-bearing laterals (Lang et al., 2011).The production of fruit crops under cover (also tunnels) requires a high potential market value to justify the added expense of constructions and orchard management costs. The market returns for fresh sweet cherry fruit could be effective, especially in a market insufficiently supplied with local production, as in Latvia (Skrivele et al., 2008). In this study, we determined the effect of a cover system set up in an orchard at the stage of production. It was thought that the investments of the cover system could be returned in comparably shorter time. The objective of this investigation was to assess the reaction of the cultivars grown under the VOEN cover system. MATERIAL AND METHODSThe study on the effects of the VOEN cover (www.voen.eu) was conducted in 2009-2012 on two cultivars (early ripening 'Iputj' (syn. 'Iput') and medium ripening 'Krupnoplodnaya') in an orchard, where one part of the trees was covered. The trial was performed with three replications. Cultivars were placed alternately with three trees per plot.The study was set in a trial orchard planted in 1998 (Ruisa and Rubauskis, 2004), in which the effect of rootstock and soil ammendment was studied previously. In the present study, the effect of rootstock and soil ammendment not considered and the average data for trees on rootstocks Gisela 4, Gisela 5 and Weiroot 154 were used. The VOEN cover system was installed in 2008. Tree canopy was fitted to the cover system by reducing tree height, narrowing and renewing tree canopy in the second part of the growth season. The same procedure was performed in In 1998, a trial was established with cultivars 'Iputj' and 'Krupnoplodnaya' at the Latvia State Institute of Fruit-Growing. A VOEN covering system was installed on part of the orchard in summer 2008. Considerable lowering of the canopy and renewed pruning were carried out throughout the trial. During 2009-2012, sweet cherry growth, y...

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