Weather conditions in California are generally favorable for the production of high quality cherries; however, historical weather records show that spring rains occur during maturation and the harvest period in seven out of 10 years in the Fresno/Bakersfield areas, five out of 10 years in the Stockton/Lodi areas, seven out of 10 years in the Yuba City area and two out of 10 years in the San Jose area.
Rain induced cracking injuries may vary from minor breaks, which are only skin deep to ruptures extending the length of the fruit and deep into the flesh. Three categories of cracking are generally noted. These include: 1) the small star or crescent-shaped or concentric rings at the apical end, 2) the circular or semi-circular cracks at the stem end, and 3) the long irregular cracks along the sides. These first two areas are commonly cracked since water accumulates at the tip of the fruit, which is the area of highest sugar content and in the cup surrounding the stem.
The severity of cracking depends upon a number of factors, which include the variety, the stage of maturity, the length of the rainy period, and the air temperature. The varieties most susceptible to cracking include Bing, Brooks, Chinook, Lambert, Royal Ann, Early Burlat, Macmar, Knight's Early Black, Sue, and Early Republican. Varieties considered of moderate susceptibility include Rainier, Van, Corum, Stella, Vega, Lamida, and Spalding. Low susceptibility varieties include Black Tartarian, Jubilee, Sam, Vista, Seneca, Viva, Schmidt, Vernon, Vic, Emperor Francis, Windsor, and Hudson.
As the sugar content increases within the fruit, the greater the cracking potential within each of these categories. As the fruit becomes more mature its potential for cracking increases and reaches its maximum cracking at tree ripeness. Cracking is not a problem in the sour cherry industry.
Causes of cracking
For many years it was thought that the rainwater was taken up through the root system and moved into the fruit causing the cracking; however, more recent studies have proven that rain cracking is caused by absorption of water through the skin of the fruit. The firmer varieties are most susceptible to cracking and fruit which is turgid prior to a rain will crack more severely than fruit which is somewhat water stressed prior to a rain event.
The potential for skin cracking increases with warmer air temperatures. Oregon research noted that little splitting occurred at 40 degrees while the splitting potential increased by 50 percent at 77 degrees over that noted at 59 degrees.
The osmotic potential of the fruit dictates how much and how quickly water is absorbed through the skin of the fruit. As the sugar contents of the fruit increases so does the movement of water through the skin and into the flesh of the fruit.
The architecture of the wall or skin of the cherry is rather complex. It is made up of three parts that include the cuticle, epidermis, and hypodermis. The cuticle consists of a waxy outer layer, which is imbedded into a spongy carbohydrate matrix that provides some body to the waxy layer. This is glued to the outer layer of cells of the cherry by a pectin substance. This outer layer is called the epidermis, which is only one cell thick. In this surface there are pores, which develop in the fruit wall, which become nonfunctioning lenticels, but do allow gas exchange and moisture loss through transpiration. Below the epidermis is the hypodermis, which consists of smaller rounded cells from three to seven cells deep and below this is the flesh of the fruit. As the sugar contents increases inside the fruit so does its ability to absorb water through the skin; thus, causing the skin to crack.
Harvesting fruit as soon as possible following a rain is the best option to reduce cracking. Lightly cropped trees or trees in weaker ground should be harvested first. The longer the fruit remains on the tree and the higher the humidity the greater the cracking potential. The larger fruit will crack before the smaller fruit.
Remove as much water as possible from the surface of the fruit and leaves as soon as you can enter the field. The use of speed sprayers, low flying helicopters, wind machines, orchard heaters, or limb shaking can be helpful depending upon how much water is removed.
There have been many methods attempted to reduce rain cracking including nutrient and mineral sprays, the use of plant growth regulators, plant growth retardants, and anti-transpirants, limb shaking, and blowing water off trees with speed sprayers. All of these methods have produced variable results.
The application of chemicals prior to the rain such as gibberellic acid can help toughen the skin somewhat, but delays maturity a few days.
A few growers who use gibberellic acid also use the metalized reflective film to enhance the color, which is delayed by the gibberellic acid treatment. Materials that provide a water penetration barrier such as the anti-transpirants may provide some protection provided good coverage of the fruit surface is provided.
Boron is responsible for increasing the elasticity of cell membranes and preventing the breakdown of vegetative tissues. Experimental applications of soil-applied boron have reduced cracking from 25 percent to 50 percent. Spraying Solubor on the soil surface two weeks before harvest may reduce cracking.
Calcium containing materials such as Bordeaux mix were first noted to reduce cracking in the 1930s. Several research trials from around the world have demonstrated that calcium in various forms (calcium chloride, calcium caseinate, calcium hydrate, calcium sulfate, and others) can reduce cracking in cherry by reducing but not stopping water absorption. Field-tested calcium applications with speed sprayers and overhead sprinkler applications of calcium chloride have successfully reduced cracking provided they are applied during the rain event and are repeated as each rain event occurs with good spray coverage. Multiple applications are generally necessary if the rain is persistent.
The calcium solution must be on the surface of the fruit when there is rainwater present on the fruit surface to prevent water uptake. Field tests have demonstrated that there is very little difference in reduced fruit cracking between applying calcium chloride with a speed sprayer or by applying it through an automated over tree micro-sprinkler system.
The automatic over tree rain activated systems are very expensive; however, they do offer other potential benefits. These additional benefits might include temperature activated frost protection or evaporative cooling during hot periods or during the time of fruit bud differentiation to prevent spurs or doubled fruit, to apply foliar nutrient sprays, or increase chilling hours during the winter months.
Rain covers are another option, but they are very expensive and delay maturity from three to five days. These covers must be well ventilated on the sides to prevent condensation and fruit rot from developing.