A key piece of knowledge that we are probably taught on our first physiopathology class, is the so called triangle of disease, which presents in a schematic way the three critical components that affect the development of a plant’s pathology. Environment, Pathogen, and Host form the triangle, where if environmental conditions are suitable, pathogen inoculum is virulent, and host is susceptible, giving way to DISEASE, anywhere from plant wilting to death, including fruit rotting.
In order to direct the focus on this article we will elaborate on one of the critical points mentioned above: the susceptible host. Certain plant species are susceptible to certain pathogens, in many cases because of a pathogen-host coevolution. A classic example of this is Pseudomonas syringae pv syringae (Pss) which causes bacterial canker in cherry trees. The same bacteria could hardly cause said disease in grape vines, or its symptoms or damage level. This susceptibility towards Pss in cherry trees presents itself in different degrees if we focus on different varieties, as research in USA has shown that Bing and Sweetheart are the most susceptible cultivars, while Regina and Rainier proved to be more resistant, whereas rootstocks can also present higher or lower tolerance to disease.
We know the behavior of varieties towards rotting; Bing or Santina rot less than other cultivars such as Sweetheart or Brooks, but if we consider this example: Can a Regina variety fruit tree get severely sick? Or, why are there nearby Bing orchards that are less aggressively affected by the attack of Pss? We now enter, beyond any specie’s intrinsic given susceptibility that each species, variety or mix cv/pi have to a susceptibility that we increase or reduce through certain agronomic management, such as mineral nutrition. For example, plants out of balance are more susceptible to pathogen attacks. This is where we begin proposing the idea of Zen fruit or plants, considering ancestral concepts as shown on the image in this article.
Over a decade ago, we started the process of understanding the connections between disease and nutritious balance, being kiwifruit and grapevines where we could integrate more knowledge. In some important work collaborations with the Kiwi committee, we were able to establish connections between the nitrogenous levels in plants and fruit that could influence upon premature fruit softening, which is related to higher presence of gray mold (Botrytis) after prolonged refrigerated storage. When we have a fast moving industry, assured sales, or benign climate conditions, said studies, or more precisely the results of these, rarely cause any impact, because there are not many complaints, and sustainability is not threatened (even when not producing it in optimal levels). However, under the constant threat of climate change, growing prices of supplies and transportation, and logistical problems, NOW everything that involves production based on DETAILS becomes a critical point that we must address urgently for the 2022-23 season and onwards.
If we go back to the kiwifruit and grapevines example, the first thing we learned is that the foliar fertilization standards were excessive in Nitrogen (I will explain why I started with this subject later). This is because said standards were built in USA, Italy or New Zealand, where markets are different; shorter storage and transportation periods, and the detail of the unbalanced nutrients cannot be noticed.
We then interiorized another concept, we don’t export leaves, only the fruit, so on that thought: Should we analyze the nutrient concentrations on the fruit to separate (short term tool) and to regulate fertilization thinking about the next season (mid-term tool)? The answer is an absolute YES, these analyses should always be done together with foliar analysis, because the final product, the fruit, must be a part of the equation, almost in as such an important way as the plant itself.
Even though mineral accumulation curves were known mainly because of international investigation, we lacked information about local dynamics. With this information we were able to determine when to take samples, and the truth is it was a complex point to determine, because N accumulates until the harvest in fruit; yet there is a plateau we use as a limit, considering also the possibility of having the result on time and to have any insight before we separate this fruit into a storage space (average time of response of a lab = 15 days).
As an alternative conclusion, an important discovery is that an extended nitrogen fertilization during the season, when too close to harvesting, increases the possibilities of unbalanced fruit production, so in troubled orchards we start fertilizing more frequently and with less amount of nitrogen. Another detail with nitrogen is that the source of this element is also crucial to generate Zen state fruit; nitrates and ammoniums can make a substantial difference, considering similar amounts of nitrogen units applied in a year.
Is nitrogen the only important element?
You might wonder why I’m writing mostly about N in this central part of the text, considering that there are other important elements that we know are of importance in plant and especially fruit balance. The answer is simple, and it has to do with the local experience and technical logistical reality. Nitrogen is, in a certain way, the most “extrovert” element when it comes to signals of nutritious unbalance, and that has much to do with the importance of this element within the plant functioning and the magnitude in which it is concentrated in different organs. At the same time, Kjedahl lab analysis is very standardized and mechanical, therefore, differences between labs are minimal.
But there is another important element: Calcium. This mineral locks out pathogens by giving the cell wall its structure. As a general fact, at least 60% of Ca in annual plants is in the cell walls, 7% in membranes, and the other 33% is dissolved within the cell. Ca and N are very related, an orchard with high N values presents lots of vegetative growth, which generates more shade, delays in fruit ripening, lower fruit quality, leaves that channel Ca to their own structures and development of sprouts, and therefore, the resulting fruit is a poor sink for Ca and a strong one for N. The result of this: cherries susceptible to rotting, and low quality fruit.
Calcium is present in a lower proportion in plant tissues, therefore, the range that we must look at is small and the sampling technique and the chemical analysis itself may end up generating a result with little strength as a marker. So, reality tells us that we carry out few analyses, due to the costs that may be involved, therefore, we have few bullets to fire and we have to aim them well, for this reason, primarily at least, we must have N readings in our fruits, and secondarily Ca readings; and if we have both, we can be more accurate with the N/Ca ratio, the lower it is, the more resistant it is to fruit rotting.
In reality, adding other elements such as Mg or K to the analysis ends up confusing us, so for now my recommendation is to work with N at least. An important detail, the result must be requested from the laboratory in Concentration (%) and Content (mg/ 100 g of Fresh Fruit), the first one explains the behavior of the fruit in post-harvest better, because it measures tissue concentration without the amount of water involved at the time of sampling, which as we know varies with the level of maturity of fruit. The content is a value closer to the nutrition field of study, and it tells us how many mg. of Ca or N we are consuming if we eat 100 grams of cherries, integrating the water content, information that for these purposes is convenient to eliminate.
Rotting susceptible cherries, what do we know today?
Unfortunately, local information is still extremely limited and globally it has not been studied much either, due to what was mentioned above in relation to how fast cherries from other places of production sell.
For two seasons, including the recent one, we have been carrying out exploratory studies together with the Cherrie Committee in order to identify those factors that have the most influence on the generation of fruit susceptible to rot, where applied nitrogen and nitrogen in fruits set the standard. However, it is difficult to find thresholds, because the varieties have quite different ranges of mineral accumulation, which indicates that we must work in a differentiated way, in general the early varieties have higher N values.
Progress has been made on the effect of N and Ca over fruit quality, and this information helps us in having an idea taken out of context. In an Australian study, increases in the rates of N applied in pre-harvest, resulted in direct increases in the amount of nitrogen in fruits, and losing the balance can trigger quality to drop and a decrease in the perception of quality by the consumer. We also observe these types of results in the exploratory studies that we have carried out in Chile, with more nitrogen, more pitting, bruising, less firmness, among other measured parameters.
In figure 1, we note that the more N available the plant has, the more it accumulates it in the fruits. If we have more N in the fruit, there is more food source for fungi, thus increasing susceptibility. Without N application, the plant also suffers in quality, for this reason we must know the balance points of our orchards. As a side note, in Chile we can find N levels that easily double the 75 gr. per plant, without even taking into account the contributions of water and organic fertilizers, many times exceeding 1% of N in fruit.
Graph 1 . Firmness of average post-harvest pulp and Total Nitrogen in fruits after the treatments with Nitrogen (Ca(NO 3 ) 2 ) applied by fertigation (Modified from Swarts et al., 2017) in plants cv. Lapins/F12-1, 10 years old, Grove Research Station, Tasmania, Australia.
As has been demonstrated, not only in the production of cherries, but in all agriculture, we must make technical efforts to know and apply the fertilizer demands of our orchards correctly, deficiencies and excesses can be important sources of susceptibility to diseases such as bacteriosis or wood fungi, which find a host with little resistance due to imbalances derived from the inappropriate use of mineral nutrition. Currently our goal is to win the China race in good shape, and for this, when thinking about rotting and fruit quality, we must decrease N and increase Ca.
The new sustainability departments that are being created in the exporter companies, in charge of monitoring the water and carbon footprint of the production process must integrate the Nitrogen footprint as an absolute priority, in order to generate plants and fruit that are more resistant to biotic and abiotic stress conditions, also resulting in production with less impact on the environment.
