Hector Garcia O., Co-Founder and General Manager of Diagnofruit Laboratories Ltda. [email protected]
Carlos J. Tapia T. Technical Director Avium SpA and Co-Founder of SmartCherry.
One of the greatest fears of cherry producers is that their fruit will be lost due to splitting caused by rain, a problem also known technically as cracking. It is not an unfounded fear; complex seasons can cause partial or complete loss of production. Due to the above, as an industry, we have implemented different types of covers, polyethylene of various densities, raffia or others that help prevent direct contact of precipitation with the fruit, thereby reducing the risk of losses quite successfully, although we do not eliminate it, as we will analyze later through the physiopathological bases of the problem as they are theorized today.
The critical turgor hypothesis.
To date, we have come to understand that rain cracking is the result of excessive net water absorption. This excess increases the turgor of the pulp, which then deforms and finally breaks the epidermis of the fruit at the weakest point. This idea was adapted from a model hypothesized in grapes in the early 1970s, called the “critical turgor hypothesis,” and assumes that the fruit is composed of pulp in a semi-fluid state, which in turn is kept under pressure by an epidermis in constant tension. Several empirical and experimental observations support this hypothesis, including that the cherry has two tissues that respond to the model, pulp and a thick skin. In turn, there is no evidence that the pit or seed has any responsibility for the disorder; certainly the skin of the cherry is under tension and it is also known that changes in irrigation can increase cracking. While the above observations are consistent with the critical turgor model, it is important to keep in mind that a recent publication reported low turgor pressure in mature cherries and, in particular, a lack of turgor response to water absorption and transpiration, all evidence that could make the postulated critical turgor model questionable.
The zipper hypothesis
Studies carried out in the last 10 years, while taking the turgor hypothesis as a basis to explain why cherries crack, conclude a new hypothetical model that describes cracking as a localized event. The mechanism is presented as the zipper hypothesis and was coined in Germany by the research group of the Dr. Moritz Knoche (Institute of Horticultural Production Systems, University of Hannover). In a very concise way and as explained in its publication, the model indicates in a temporal sequence:
1°. Rain cracking is initiated by localized water absorption, which occurs through micro-cracks in the cuticle. The micro-cracks focus or direct the absorption of water in a particular region of the epidermis, as if following the markings on a map.
2°. The epidermis cells absorb water and then, further down, the pulp cells with a more negative osmotic potential are affected. The pulp cells are structurally weaker than those located forming the epidermis, and begin to crack.
3°. The bursting of pulp cells causes their simplest contents to leak into the apoplast (extracellular space peripheral to the plasma of plant cells through which water and other substances flow); malic acid being the main osmolyte, which would alter the plasma membranes and weaken the cell walls of adjacent cells, causing the bursting to spread, like a domino or chain effect.
4°. The continuous release and delivery of osmolytes from the symplast (intracellular compartment) to the apoplast causes nearby skin cells to plasmolyze (the vacuole content leaves the cell and the cell loses its turgor). As a result, the resistance to deformation decreases and water absorption continues. The tangential propagation of these processes causes the skin to open, just as it happens in a zipper, hence the name of the hypothesis, so that an initial cuticular micro-crack soon evolves into a macro-crack of the skin in the same way that a pantyhose tears, first a small tear appears which then ends in an open tissue like a “ladder”.
The interesting thing about this hypothesis, as the German research points out, considering that much is still in theory, is that the zipper model is consistent with the higher frequency of cracking in the pedicellar cavity and the stylar scar region (“crescent” splits) compared to the cheek or suture region. Microcracks result from a mismatch between surface area expansion and cuticular deposition, a similar situation we observe in table grapes when growth regulators are abused. These areas are subjected to prolonged periods of surface moisture after rain, water deposited in the cavity and “hanging” in the stylar area.
The same group of researchers has just published a very interesting study where they put their hypothesis to the test. In a very brief way, in three seasons, they subjected controlled precipitation fruit attached to the plant, fruits harvested and placed in Mini-cages in different positions of the tree and also fruit immersion in deionized water; This formal experimental design allowed observing the behavior of fruit with water flow from the plant (fruit attached to the tree) and without it (fruit in immersion and cages). The different treatments presented different results in terms of the appearance of cracking over time and also in relation to the amount of water absorbed to cause cracking. Thus, fruits attached to the tree split slowly and at a low frequency and also required more water to crack, in relation to the fruit in immersion; in addition, the cracks were more commonly found in the region of the pedicel cavity and were noticeably less common in the region of the stylar scar. When simulating rain falling on detached fruit hanging on trees (in cages), the cracking was even slower and required even more water to occur. The fruit in immersion absorbed more water than the fruit attached to the tree under a controlled precipitation regime and the latter absorbed more than the fruit in cages subjected to the same precipitation regime, probably due to the contribution of the phloem flow. Analyzing all this behavior at the same time, it is discussed that the large moistened surface in the fruit in immersion It is the component that determines the high sensitivity to cracking, in the simulated rain experiment, regardless of whether it was attached to the tree or in a cage, only the 18% of the surface would be in constant humidity and said humidity is concentrated in the pedicellar cavity and stylar zone, hence the result of lower frequency of the problem. Next would be the phloem flow that contributes to the development of the crack, but with a very minor impact on the final result; in conclusion, everything described would be explained through the zipper hypothesis, where localized humidity would be responsible for the cracks in the field and the water flow through the phloem from the plant would contribute, but in a very secondary way.
Crack control: is there an effective method?
Due to the nature of the problem, to date the use of covers is the most effective method of controlling cracks, however, it has some negative aspects that we cannot fail to mention, high investment cost, detrimental effects on the quality of the fruit, such as less hardness (durofel) and soluble solids at harvest, among other aspects that must be coexisted with and managed appropriately to generate a sustainable crop.
According to the above and the zipper hypothesis, removing water from the fruit surface immediately after rain would be a good method, considering rains that do not last too long. Two options for this management are available, the first is to use unloaded turbo-foggers to generate jets of wind and/or helicopters using the counter-current rotor. Probably, the ease of having access to agricultural machinery in an orchard is exaggeratedly greater than having a helicopter, so the first alternative is the closest to our reality; however, in terms of efficiency a tractor must move the machine at no more than 0.9 m s-1 For treatment to be effective and safe, manned helicopters can fly at speeds of 2.3–4.5 ms-1, significantly increasing the surface to be controlled in a race against time, before the process of opening our cellular “zippers” begins. Today there is the possibility of using unmanned helicopters, but they are still in development to improve performance.
If you recall, in the commented experiment, the fruit was immersed in deionized water, this with the purpose of generating a significant osmotic gradient between the cherry cells and the surrounding medium. The simulated rain also used this type of water, with the same purpose. Rainwater is largely pure, and it has been postulated that applications of calcium (e.g. CaCl2) prior to the event can help prevent cracking by generating water with a lower osmotic potential when it touches the fruit. Although this type of application works, the easy washing of these from the fruit by rain suggests that, in this case, calcium has an interaction beyond the osmotic potential; it could also act at the wall level and it has been reported that the treatments influence the cuticular properties.
Other types of products are available to prevent cracking, waxes and various hydrophobic polymers, for example. As we have already learned, these types of products are likely to work, but it is up to us to make them effective, for this it is necessary to carry out successive and well-controlled applications to achieve the objective; the important thing is that the products are applied to the fruit in a homogeneous manner, therefore tests must be carried out in the target areas, pedicellar cavity and stylar zone, which is where we must reduce the risk, since we know that prolonged exposure to water in these areas ends in a crack.
In conclusion, we are still learning about the mechanism by which our cherries split, and everything would indicate that the water left for a long time in the fruit starts a chain process that begins through penetration into natural microcracks and ends up generating deep macrocracks that go through the pulp, ending in cracking or splitting in cherries, which will finally be susceptible to rot or discarded in the packaging without the possibility of sale. An integrated control model, with various managements from physical barriers to cover the canopy of precipitation, eliminating water from the fruit by “blowing” to applications aimed at improving the resistance of the fruit should be considered as joint strategies to reduce the risk in problematic areas or seasons.
Local experience
In real terms, in Chile we have used a wide range of ideas to be able to minimize the problem. With conflicting opinions and diverse experiences, the control methods currently used are not replaceable with the use of protective covers.
As mentioned, there is always a position of discomfort from the point of view of the quality of fruit grown under covers: “the fruit is of poor quality”, “the fruit is softer”, “the fruit under a tent has less sugar”, etc. However, there is a certain injustice behind these comments, because roofs are implements that are designed and used to prevent cracking due to rain, therefore, when we have a major rainfall event just before harvest, the new question is: “which fruit is of lower quality, the one outdoors or the one under the tent?”… our analysis changes.
The problem of fruit under covers in terms of quality is true, but the fault does not lie with the covers, but with the technical responsibility of managing irrigation, nutrition, vigor control and phytosanitary management under the tents.
Among the techniques most commonly used by producers to prevent cracking at a local level, all of them complementary, we can mention:
1.Use of hydrophobic films (Biofilms).
With products specially formulated for this work, research in Chile has shown good performance under rains that are not very intense or of long duration.
Implemented as an application program from the beginning of painting, it has been verified that up to 50% of the total number of cracks can be protected compared to a control treatment. However, after an event of very extensive rainfall, where the amount of precipitation falls into the background, the results are more uncertain and less consistent.
2.Use of Ca Chloride (CaCl2)
Before and during rain, this strategy allows rainwater to remain off the fruit for as long as possible due to the effect of osmotic balance.
Constant and successive applications before and during rainfall at use concentrations of 0-5%-1% have been favourable when proper operation has been maintained. The operation is complicated because it requires a high level of machinery, since it should be applied at maximum intervals of two hours in the same place while it is raining, in order to avoid the washing effect of the precipitation.
3.Drying.
Ideally with helicopters and blowers, efficiently and just after the rain.
The use of helicopters is expensive, but it has proven effective in removing water from trees, preventing splits, especially those that are located in the pedicellar area known as the “half moon.”
In other countries, the use of blowers has proven to be more efficient than the use of nebulizers, since, as their name suggests, they generate nebulized air with little force but a long reach. Models used in Chile have shown similar results, even complementary to helicopters.
The problem is not just cracking
As we mentioned, beyond the split itself another effect occurs, perhaps even more complex, it is the Increased rot (Photo 1) and the possibility of their development during storage. In terms of ranking, we generally observe high frequencies of grey rot (Botrytis), green rot (Penicillium) and acid rot (Geotrichum), the latter being more related to the initial load of the fungus in the orchard. In this sense, it is very important that At least 24 hours before the event the orchard must be treated with fungicides and depending on the complete season program, we must decide whether to use a botryticide or a broad spectrum formulation. Obviously, it is always very important to check tolerances, especially considering the short time left until harvest. If we register a history of Acid Rot or monitoring in flower and/or early stages of development indicate high levels of inoculum, the application of triazoles cannot be replaced.
Lastly, and no less important, when an event of this type occurs, the inoculum load grows significantly in the process water of the packing plant, which significantly increases the risk of contamination of healthy fruit. Correct use of fungicides in post-harvest is key to ensure the success of the control, ensuring homogeneity of the application is the first step, a high-performance fungicide such as fludioxonil should be considered, and finally, strict control of the water sanitizer (chlorine, peracetic acid or other) combined with a Constant monitoring of the amount of inoculum in the water They must be carried out in parallel and constantly evaluated to support decision-making.
Photo 1. Early splitting of the stylar zone (star) colonized by multiple fungi of the genera Botrytis, Alternaria and Cladosporium.
