PREVENTION OF SWEET CHERRY FRUIT CRACKING USING SURESEAL, AN ORGANIC BIOFILM

Rain-induced fruit cracking in sweet cherries can be a major problem. In the Pacific Northwest United States, due to high labor costs, when fruit cracking exceeds 25% at harvest, fruit are not picked. Oregon State University Horticulture and Pharmacy Faculty have collaborated in producing and patenting a novel, elastic, organic biofilm, SureSeal, which significantly reduced sweet cherry fruit cracking by up to 250% in Milton Freewater, Oregon and Loftus, Norway. Formulations of SureSeal are hydrophobic and consist of a copolymer of complex carbohydrates, phospholipids and calcium. Collaborative research undertaken over three years throughout the Pacific Northwest and overseas found that two applications of 1% SureSeal applied at straw color, and again ten days later, reduced fruit cracking consistently when compared to untreated control fruit. In Norway, fruit cracking was reduced from 24.6 to 9.8% when trees were treated with SureSeal in combination with plastic ground covers and a preharvest fungicide (fenhexamid). Furthermore, studies throughout Oregon and Idaho found that SureSeal resulted in significantly (P<0.001) higher total soluble solids (TSS) and increased Stem Pull Force (g) (retention force between the pedicel and the fruit) than untreated control fruit. In 2008, ‘Bing’ fruit had higher TSS both before (18.5°Brix) and after (18.9°Brix) two weeks of regular atmosphere storage at 2°C than untreated control fruit (17.4 and 17.2°Brix, respectively). In Norway, 1% Biofilm increased TSS to 21.4°Brix compared to untreated control fruit (18.6°Brix). Two applications of 1% Biofilm applied at straw color and again ten days later has the potential to significantly reduce fruit cracking, accelerate maturity by significantly increasing TSS levels, and increase stem pull force. The concurrent reduction in fruit firmness observed may be a function of maturity but, in all instances, fruit firmness still exceeded the minimum standard of 250 g mm-1. a Email: clive.kaiser@oregonstate.edu. Proc. VIth Intl. Cherry Symposium Eds.: M. Ayala et al. Acta Hort. 1020, ISHS 2014 478 INTRODUCTION Rain-induced fruit cracking in cherries remains a problem internationally and can cause heavy losses in yields and returns (Pennell and Webster, 1996; Vittrup Christensen, 1996; Lang and Flore, 1999; Sekse, 2005). Cherry fruit cracking is the result of several factors including: morphological, physiological, environmental and genetic factors. Despite many years of research, a lack of understanding of several of the mechanisms involved in fruit cracking persists. Furthermore, different experiments have produced contradictory results leading to further controversy (Webster and Cline, 1994; Sekse, 1995a; Opara et al., 1997). Several advances in the use of different cultural practices, which limit the incidence of fruit splitting have, however, been made. These practices range from exclusion of water from the fruit surface during fruit growth and maturation using plastic rain covers (Meland and Skjervheim, 1998; Børve et al., 2003), to reducing osmotic potential across the fruit skins during rainfall events (Lang et al., 1997; Fernandez and Flore, 1998). Some cherry scion cultivars crack more easily than others (Ystaas and Frøynes, 1998; Cline et al., 1995a; Lane et al., 2000) and the question of why susceptible cultivars were more predisposed to cracking than resistant ones has not been explained satisfactorily. Several causes ranging from fruit morphology, through whole tree physiology, to rootstock effects have been shown to have an effect on fruit cracking (Hovland and Sekse, 2003) and it is most likely that these factors act in conjunction with one another and a holistic approach to reducing fruit cracking is favored by the authors. Indeed, where fruit morphology is concerned, investigations have found that genetic differences in skin morphology (Belmans and Keulemans, 1996), variable cuticle thickness, differences in stomatal density (Beyer and Knoche, 2002; Belmans and Keulmans, 1996), cutin content (Schreiber et al., 1996; Knoche et al., 2000) and exocarp polar pathways are all implicated. However, all cherry fruit cuticles consist of two distinctly different constituents, namely cutin and wax. Cutin is the largest constituent (90-99%), although it plays a minor role in water exclusion. The lesser occurring wax (1-10%), however, accounts for the water excluding properties of the cuticle. This explains much of the inconsistency of results often found in experiments correlating cuticle thickness with water absorption and fruit cracking (Belmans and Keulemans, 1996). Consequently, it may not be the thickness of the whole cuticle that determines its water diffusion properties but rather the percentage wax, as discussed by Sekse (1995a), Schreiber et al. (1996) and Vittrup Christensen (1996). Water transport across the cuticular membrane in sweet cherry fruit has been studied in Norway under defined conditions by monitoring fruit transpiration under high and low humidity. Hovland and Sekse (2004a) found that water loss from the fruit skin under low air humidity was linear with time, whereas fruit at high air humidity accumulated water for 4-6 h and this explains why in some cases, fruit cracking can take place following harvest. This also has major implications for harvest when prevailing conditions are cool and overcast, resulting in high relative humidity, and postharvest operations where hydro-cooling is utilized. An intact cuticle is a prerequisite for the prevention of free water intrusion into the fruit while the fruit surface is wet. Several studies of the fruit skin of sweet cherries have revealed the presence of a network of fine fractures across the cuticle even in the field prior to harvest (Glenn and Poovaiah, 1989; Sekse, 1995b, 1996; Knoche et al., 2000; Hovland and Sekse, 2003). Scanning electron micrographs found that these fractures traversed only the cuticle (Glenn and Poovaiah, 1989). Sweet cherry fruit only develop cuticular fractures during the last two weeks of their growth period and their incidence is greater at high air humidity (Hovland and Sekse, 2004a). Fractures occur in concentric rings around the stem cavity (distal end) and may be observed with a good magnifying glass. Fluorescence microscopy revealed marked differences in percentage of fruit with cuticular fractures among cheek (22%), suture (32%) and stylar end (90%) regions on visually inspected crack-free fruit (Knoche et al., 2002). Dye solution infiltrated these cuticular fractures and indicated that they represented potential pathways for water transport through the sweet cherry fruit surface. Cuticular fractures increased the