Effects of different crop loads and thinning times on fruit set, yield, and fruit quality of 'Opal' plums in high tunnels

摘要

High tunnels offer an intensive and protective production system for many fruit crops. In May 2014, two tractor-accessible Haygrove multibay tunnel systems were installed on a 10% slope at the experimental farm at Nibio Ullensvang, western Norway (60°19'8.03"N, 6°39'14.31"E). Feathered 1-year old European plum cultivar 'Opal' on rootstock 'St. JulienA' were planted with two rows per bay at a spacing of 1.5x4 m during 2012. Trees were trained to a central leader as free spindles. In 2016, one tunnelwas covered (150 um clear classic polyethylene film) from before blooming until harvest and one tunnel only covered from mid-July till harvest. Different crop loads levels were established by blossom thinning (each flower 5,10, and 15 cm apart), and fruitlet thinning (each fruitlet 5,10, and 15 cm apart) at 10-12 mm fruitlet diameter at the end of June. Treatments were applied on single whole trees in a randomized complete block design with five replications. Climatic parameters were monitored inside and outside the tunnels from mid-June to mid-September. Fruit set, yield data, and fruit quality parameters for each treatment were recorded. Increased thinning distances reduced the fruit set and was highest when thinned at fruitlets. Thinning to 5 cm apart and covered the whole season and 10 cm apart covered one month gave the highest fruit sets of 17.9 and 14.3%, respectively. The yield was positively correlated with the fruit set response, 11.7 kg tree1 (201 ha1) - 5 cm between fruitlets and short-covering versus 3.4 kg - 15 cm distance between flowers and long covering. Both blossom and fruitlet thinned trees when covered got a significant yield reduction compared to covered one month. Thinning at the fruitlet stage resulted in smaller fruits at the same crop level (41.3 g on average) compared to flower thinning for both covering periods (47.2 g). Qualitative traits of'Opal' plums (bright yellow ground colour, red over colour, and soluble solid contents) were weakly correlated with the fruit set and was high (16.7% average soluble solids content). The coverage from bloom to harvest time promoted maturity of the plums. From the preliminary results, it can be concluded that fruitlets thinning from uncovered trees and one month covering before harvesting gave the largest crop of premium fruits.