By: Héctor García O., Co-Founder and General Manager of Diagnofruit Laboratories Ltda, SOCHIFIT and AMICH Member. hgarcia@diagnofruit
A key piece of knowledge that we are probably taught in the first class of plant pathology is the so-called disease triangle, which schematically proposes the critical axes that influence the development of a disease in a plant. Environment, Pathogen and Host form the triangle, where if the environmental conditions are met, there is inoculum of the pathogen and the host is susceptible, giving way to DISEASE, from wilting to death of plants, through fruit rot.
To direct the focus of this article we will concentrate on one of the critical points stated: susceptible host. Certain plant species are susceptible to certain pathogens, often due to a pathogen-host coevolution; a classic example is Pseudomonas syringae pv. syringae (Pss) that causes bacterial canker in cherry trees, the same bacteria could hardly cause this disease in grapevines, neither symptoms nor level of damage. This susceptibility to Pss in cherry trees also occurs in degrees if we observe varieties; thus, research in the USA indicates that Bing and Sweetheart are the most susceptible cultivars, while Regina and Rainier turned out to be the most resistant, likewise the rootstocks offer greater or lesser tolerance to the disease.
We know the behavior of varieties towards rots; Bing or Santina rot less than other cultivars such as Sweetheart or Brooks, but taking the previous example, can a Regina plant get severely sick? Or why are there nearby Bing orchards that differ in aggressiveness to Pss attack? We then enter, beyond the intrinsic susceptibility that each species, variety or mix cv/pi has, into a susceptibility that we increase or decrease through certain management, such as mineral nutrition; for example, plants in imbalance are more susceptible to
pathogen attacks, this is where we begin to raise the idea of plants or fruits in a Zen state, taking ancestral oriental concepts as illustrated by the image in the article.
More than a decade ago we began the work of understanding the connections that existed between diseases and nutritional balance, and it was in kiwis and table grapes where we were able to integrate the most knowledge. In important work with the Kiwi Committee we related how the levels of nitrogen in the plant and fruit could influence premature softening, which went hand in hand with increases in the presence of gray rot (Botrytis) after refrigerated storage.
prolonged. These studies, or rather their results, when we have a fast-moving industry, assured sales or benign weather, the truth is that they do not make much noise and the impact is less, because the claims are not abundant and sustainability is not threatened (even if it is not produced optimally). However, under the constant threat of climate change, rising input prices, transportation and logistical problems, TODAY everything that involves production based on DETAILS becomes a critical point that we must urgently address for the 2022-23 season and in the long term.
Returning to the example of kiwis and grapes, the first thing we learned was that the foliar standards were way over the top with respect to Nitrogen (later I will explain why I take this element as a spearhead for this topic). This is because these standards were built in the USA, Italy or New Zealand, where the sales markets are different; shorter trips and storage times, so the detail of the imbalance could not be noticed.
Then, we internalize another concept, we do not export leaves, but fruit, so, from that perspective, should we analyze the concentration of nutrients in the fruit to segregate (short-term tool) and to regulate fertilization probably already thinking about the next season (medium-term tool)? The answer is resounding and is YES, these analyses must accompany the foliar ones; the final product, the fruit, must be part of the context, almost as important as the plant in general.
Although the mineral accumulation curves were known from essentially international works, we were unaware of the local dynamics. With this information we could determine when to sample, and the truth is that it was a complex point to determine, because the N accumulates until the fruit is harvested; however, there is a kind of plateau that we use as a limit, also considering the possibility that the result is on time and gives us insight before segregating that fruit to a storage facility (average lab response time = 15 days).
As an alternative conclusion, an important finding is that the extension in time of nitrogen fertilization during the season, very close to harvest, increases the possibilities of generating unbalanced fruit, so in problematic orchards we begin to advance the timing of application and decrease the nitrogen units. A final detail with nitrogen is that the source of this element is also crucial when generating a fruit in Zen state, nitrates and ammonium can make a substantial difference, considering similar units of total nitrogen applied
in the year.
Is Nitrogen the Only Thing Important?
You may wonder why I have focused this text on N, considering that there are other elements that we know are important in the balance of vegetables and especially fruits. The answer is simple and has to do with experience and the local technical-logistical reality. Nitrogen, in a certain way, is the most “extroverted” element when it comes to marking imbalances and this is closely related to the importance of this macroelement within the general functioning of the plant and the magnitudes that it is concentrated in the different organs. At the same time, the Kjeldahl laboratory analysis is very standardized and mechanized, therefore, the differences between laboratories are minimal.
But there is another important element: Calcium; this mineral, among other functions, closes the entry to pathogens by giving structure to the cell wall. As a general fact, in annual plants, at least 60% of Ca is in the cell wall, 7% in the membranes and the remaining 33% is in the soluble fraction of the cell. Ca and N are closely related, an orchard high in N has a lot of vegetative growth, which generates more shade, delays in ripening, lower quality of fruits, leaves that divert Ca towards their own structures and development of shoots, in this way the constructed fruit is a weak sink for Ca and strong for nitrogen, the result: a cherry susceptible to
rotting and poor quality.
Calcium is found in lower proportions in plant tissues, therefore, the range we must look at is small and the sampling technique and the chemical analysis itself may end up generating a result with little power as a marker. So, reality tells us that we carry out few analyses,
Because of the costs that may be involved, therefore, we have few bullets and they must be directed well, for this reason, in a primary way, at least, we must have N readings in our fruits and secondarily of Ca; and if we have both, we can fine-tune with the N/Ca ratio, which the lower it is, the more representative of a fruit that is more resistant to rot.
Adding other elements such as Mg, K, the truth is that they end up confusing us, for now my recommendation is to work with at least N. Important detail, the result must be requested from the laboratory in Concentration (%) and Content (mg / 100 g of Fresh Fruit), the first better explains the behavior of the fruit in post-harvest, because it measures the concentration in the tissues without the amount of water involved at the time of sampling, which as we know varies with the degree of ripeness of the fruit. The content is a value closer to nutrition, it tells us if we eat 100
grams of cherries how many mg of Ca or N are we consuming, integrating the water content, noise that according to our studies is convenient to eliminate.
Cherries susceptible to rot: what do we know today?
Unfortunately, local information is still extremely limited and has not been studied at a global level either, due to the above-mentioned reasons regarding how quickly cherries from other production areas are sold.
For two seasons, including the last one, we have been carrying out exploratory studies together with the Cherry Committee in order to identify those factors that have the greatest influence on the generation of fruit susceptible to rot, where applied nitrogen and nitrogen in fruit 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, early varieties have higher N values.
There has been progress in the effect of N and Ca on fruit quality and this information helps us to generate an extrapolated idea. In an Australian study, increases in the ranges of N applied in pre-harvest, result in direct increases in the amount of nitrogen in fruits, and losing the balance can trigger quality impairments and a decrease in the perception of quality by the consumer. We also observed these types of results in the exploratory studies we have carried out in Chile, the more nitrogen, the more pitting, bruises, less firmness, among other quantified parameters.
If we look at graph 1, we can see that the more N the plant has available, the more it accumulates in the fruit. If we have more N in the fruit, there is a greater source of food for fungi, thus increasing susceptibility. Without the application of N, the plant also suffers in quality, which is why we must know the equilibrium points of our orchards. As an aside, in Chile we can find N contributions that easily double 75 g per plant, not counting the contributions of water and organic fertilizers, often exceeding 1% of N in the fruit.
In conclusion, not only in cherry production, but in all agriculture, we must make technical efforts to know and apply the fertilizer demands of our orchards in an exact manner. Deficiencies and excesses can be marked sources of susceptibility to diseases such as bacteriosis or wood fungi, which find a host with little resistance due to imbalances derived from the inadequate use of mineral nutrition. Today our goal is to win the race in good form in China and for this, thinking about rots and fruit quality, we must reduce N and increase Ca.
The new sustainability departments that are being created in exporters, responsible for 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.
