Weather conditions in 2006 were not kind to Montmorency cherries in some areas of Michigan’s Great Lakes region. The two primary fruit quality problems experienced there were wind whip and soft fruit.
High winds before harvest have caused damage to fragile cherries since commercial production began in the late 1800s. Today, Michigan growers routinely employ several orchard practices to reduce wind damage: windbreak plantings; closer tree spacing in combination with pruning techniques that create a hedge-row effect; and gibberellic acid applications that enhance interior fruiting. Advances in electronic sorting also help to minimize the impact from wind damage.
Soft fruit is a different problem, which has become more of a concern in recent years due to a combination of factors including converting to mechanical harvesting, losing the fruit-firming material Alar, and higher-yielding production techniques. Since 1990, the estimated average loss from soft fruit in Michigan is more than $6 million per year. For researchers to address soft fruit and potential ways to reduce the problem, we need to explore the causes.
First, “soft fruit” refers to fruit that sustains excessively deep bruises or splits during harvesting. Since tart cherries are mechanically harvested, the design and operation of the harvester play an important role in the final product. Cherries are shaken onto a tarp and then moved into tanks of cold water. Mechanical harvesting causes some bruising and loss of fruit firmness, but some of the lost firmness is regained as the fruit is cooled in the water. Mechanical harvesting may increase soft fruit, but the harvester alone does not explain why soft problems vary year to year and block to block.
Testing for softness in tart cherries has been a challenge for researchers. Initially, we measured fruit firmness within the orchard, and often, multiple orchards would have similar firmness ratings. Yet, after fruit was harvested, cherries from some orchards would be excessively soft while other orchards had no problem. Consequently, a laboratory procedure was developed to simulate harvest bruising to allow further exploration of the problem.
Using the current firmness measurement technology, there appear to be two types of factors associated with the soft fruit problem. One type causes measurable softening of fruit on the tree. For example, fruit softens with maturation. Another type does not induce measurable differences in softness on the tree but significantly influences the ability of the fruit to maintain integrity during mechanical harvesting.
Based on our research thus far, we do not fully understand all potential factors that induce softness, but the following appear to be potential contributors:
Fruit maturity: Softness increases later in the season as the fruit matures.
Soil moisture: Lots of moisture prior to harvest increases softening. Conversely, dry conditions reduce fruits’ softness.
Harvest temperature: High harvest temperatures increase soft fruit.
Weather following bloom: There is evidence that cold and/or cloudy conditions after bloom, during the cell division period, may exacerbate the problem.
Ethephon (Ethrel): Ethephon treatment may increase fruit softening, particularly when conditions turn cool after application and maturation of treated fruit is delayed.
Boron: High boron increases softness. Research has shown that trees treated after petal fall with foliar boron are more prone to soft fruit.
Tree height: The further the fruit drops during mechanical harvesting, the greater the damage to the fruit.
Crop load: Soft fruit is more prevalent when the crop load is high; cherries seem particularly susceptible when there are fewer than two leaves per fruit. To test this, we hand-thinned fruit after shuck split and didn’t significantly decrease soft fruit. However, we continued to observe softer fruit with a higher crop load. One explanation is that the influence of a heavy crop occurs during the cell division period, which was finished at the time fruit was thinned in the studies.
Soil: Soft fruit is most common in orchards planted in heavier soils. This relationship may occur because heavier soils favor some of the factors already mentioned, such as holding more moisture, growing taller trees, setting heavier crops, and growing larger fruit.
Nutrients, other than boron, may play a role, but we have not demonstrated a reduction in soft fruit from foliar calcium applications or by reducing nitrogen. Multiple foliar applications of copper, which tend to reduce fruit size, provided significant firming in 2005, but this trend is not regularly observed.
Growth regulators are another possible source for either increasing firmness or improving the ability of the fruit to retain adequate firmness during harvest. Research has shown gibberellic acid applied shortly before harvest helps sweet cherry firmness. This result could not be replicated in Montmorency.
So, without a silver bullet to correct soft fruit, we recommend a combination of things that might minimize the problem:
• Harvest blocks prone to soft-fruit problems as early as possible in the season.
• Apply boron when recommended based on tissue analysis; make only postharvest applications.
• Minimize or eliminate ethephon application to late-harvested blocks, particularly blocks prone to soft fruit.
• Avoid leaving ethephon-treated fruit on the trees longer than necessary.
• Prune trees to minimize tree height and to maximize light penetration. Pruning will decrease the drop distance to the harvester and improve photosynthesis in the interior of the canopy.
• Minimize fruit bruising on the harvester by properly padding hard surfaces, installing padded deflectors over conveyors, and avoiding piling fruit too deep on conveyors.
• In wet seasons, when problems are likely to occur, allow weeds to develop under the trees to compete for moisture.
• Processors should consider adding pitter capacity to process the fruit in a shorter time period.