Published in AgroLife Scientific Journal, Volume 3, Number 1
Written by Mekjell MELAND, Clive KAISER, John MARK CHRISTENSEN
Rain-induced fruit cracking in sweet cherries can cause heavy losses in yields and returns. Several advances in the use of different cultural practices, which reduce the incidence of fruit cracking have been made. These practices range from exclusion of water from the fruit surface during growth and maturation of the fruit, to reducing osmotic potential across the fruit skins during rainfall events, to coating the fruit with elastic hydrophobic rain exclusion biofilms. Physical exclusion of rainwater may also be achieved by covering the trees with protective plastic rain covers. Two systems have been researched in Norway; retractable plastic rain covers and multi-bay polyethylene “high tunnels”. The supporting framework of the former is built entirely of wood and overhead polyethylene curtains slide back and forth on three wires per row to open or close depending on the prevailing weather. Retractable covers must be drawn over the trees manually before rainfall events and is extremely labour intensive. The system is highly susceptible to heavy winds. High tunnels, which are accessible to tractors, are constructed of steel bows, attached to metal posts and covered with greenhouse–grade polyethylene. Tunnels may be fully ventilated on hot, humid days or completely closed for extending the growing season. The plastic covering is completely removed during winter. Cherries may be covered from bloom until harvest or only during the time when fruit are susceptible to cracking. A high density planting (1250 trees per ha) of ‘Sweetheart’/‘Colt’ in Norway grown in high tunnels yielded 9 kg per tree on average in the 4th leaf and 19 kg per tree on average in the 5th leaf. Fruit size measurements found that on average, more than half the fruit were larger than 32 mm in diameter by the 4th leaf. GA3 treatment at yellow straw colour delayed harvest by one week, and significantly improved fruit size and fruit firmness.But soil moisture management is critical inside the tunnels as excess soil moisture can induce significant percentages of fruit cracking even inside the tunnels. In the United States, Pacific Northwest, Parka (powered by SureSeal®), a novel biofilm comprised of palm oil and cellulose, patented by Oregon State University, resulted in 50% less fruit cracking on average, higher total soluble solids and increased retention force between the pedicel and fruit than untreated control fruit.