Rain-Induced Cherry Cracking
Cracking of sweet cherries induced by rain is often the greatest single cause of fruit cullage. Even in the arid regions east of the Cascade Mountains in the Pacific Northwest, cherry growers know how devastating a rain can be if it occurs when cherries are nearing maturity. Cherry cracking has been studied for several decades but the causes of cracking are still not fully understood.
Evidence exists for two causes of cherry cracking. The first relates to excessive water being supplied to the fruit through the tree's vascular system. This builds up tensile forces acting on the skin from inside the fruit. This cracking can occur in orchards that have been over-irrigated, and can be exacerbated when rain follows shortly after irrigation. There are no well-established remedies for this type of cracking.
The more common type of cracking occurs as a result of direct osmotic water absorption through the fruit skin or cuticle. Consequently, factors affecting permeability of the skin are of major importance in determining fruit resistance to water absorption. Penetration of the cuticle occurs by diffusion or by mass flow through cuticular cracks and other surface structures. As water penetration through the cuticle increases, the cuticle can separate from the epidermal cell wall. As more water is absorbed, the inner epidermal cell wall swells and detaches from sub-epidermal cells. Cellular contents are lost from epidermal cells near the fracture. Swelling in the epidermal cell wall region results in cuticular fracturing that generally precedes fruit cracking. This is the putative mechanism for postharvest cracking which can manifest in the packinghouse or during shipping.
Fruit losses from excessive uptake of external water in the orchard can be reduced by drying cherries with airblast sprayer fans or helicopter rotor wash shortly after a rain event. Alternatively, many cherry growers mitigate cracking losses with applications of chemical salts (i.e. calcium chloride) which increase the ionic concentration in the water on the fruit surface, thereby easing the osmotic gradient across the fruit cuticle and reducing water uptake by the fruit.
Another class of sprayable protectants reduce cherry cracking by repelling water from the fruit surface. Similar to applying wax to a car, these products (RainGard and Parka) feature hydrophobic components which can help fill small cuticular cracks and reduce absorption of water. These products have the greatest efficacy when they are applied before the onset of rain and have fully dried on the fruit surface. As cherry fruit grow, the protective coating from these products breaks apart, so 2-3 repeat applications at 7-10 day intervals are advised to maintain adequate coverage throughout the period of cracking susceptibility.
It is widely accepted that cherries generally become more susceptible to rain-induced cracking as they mature, but recent research has demonstrated considerable variability in cracking susceptibility from site to site within the same year, as well as from year to year at the same site. As such, it can be difficult for growers to anticipate how vulnerable their fruit may be to rain damage at any given time. A simple benchtop test has been recently developed and validated to help cherry growers make informed decisions about when to protect their fruit vs. saving money on sprays or helicopters. More information on this test is available at http://www.treefruitresearch.com/images/stories/2013_Cracking_Susceptibility_Grower_Test_Kit.pdf.
See General Recommendations for guidelines on table use. Read all product labels carefully.
|Except where noted, rates are amount per acre (amount per 100 gallons in dilute sprays)|
|Parka||1-2 gal (0.5-1 gal)|
|RainGard||102 fl oz (102 fl oz)|
Parka: Do not exceed 1% V/V Parka in spray solution. Do not apply with surfactants, stickers, or pinolene-based materials. Consult product label for more details. Parka has been reported to cause phytotoxicity in some sensitive cherry varieties; growers are advised to consult with warehouse field staff or Extension personnel for more information.
RainGard: Follow manufacturer's label, and apply 102 ounces RainGard in 100 gal. water. Best efficacy is obtained with 0.8% (v/v) dilution and must be maintained with larger spray volumes to improve coverage. In addition, to maintain good coverage of fruits as they expand, three applications should be made. First application is made about 4 weeks before harvest with additional applications at 7-10 day intervals thereafter. RainGard is usually compatible with commonly used agricultural products to be tank mixed.