April is the most important month in the transition of cherry orchards to dormancy, however the first signal that the plants receive to prepare for this winter lethargy occurs several months before, specifically in December with the beginning of summer and the shortening of the photoperiod.
“The last day of spring is the longest day of the year and from the first day of summer the photoperiod begins to shorten and that signal is a primary and very important signal for the plant to understand that at some point, in the next 3 or 4 months, it will have to enter the latency stage and it has to have formed and completely lignified structures, and this signal is associated with a physiological concept called cold tolerance and clearly coincides with some physiological and phenological aspects in the species”, says Carlos Tapia, specialist advisor in cherry production and Technical Director of Avium.
The shortening of the photoperiod occurs around 70 days after full flower, in relation to the shortening of the length of the day according to the summer solstice, and coincides perfectly with the cessation of the growth of the annual shoots (annual vegetative development), and the beginning of the induction of flowers the following year. This induction is a biochemical process in which a vegetative bud receives a signal that allows it, based on some subsequent determinations, to transform into a floral bud without yet having morphological changes.
But what should cherry orchards show during this time of year? In addition to a significant percentage of leaf fall (yellow leaf = leaf fall), which ideally reaches at least 50 percent by May 1, the fruit centers should show very closed spurs and bases of twigs, very well lignified, with their bracts very well formed or, in the worst case, with some outline of green tips that can be seen at this time, but which do not represent a major problem.
What is it that NoWhat would we like to see at this time? Open buds or green buds with a good percentage of green tips or almost ready to open, which not only presents a problem from the point of view of the production potential that could be lost due to some early frosts, but also from the phytosanitary point of view and I do not want to focus only on bacterial canker, but also on some wood fungi that are present and that we in the cherry world were not used to seeing or detecting; one of them is Cytospora, which has been more present in the fields lately, especially in colder and southern areas and which is very well explained in the article by Héctor García entitled “Why are our cherry trees dying?”, Tapia says.
At this time of year it is necessary to set goals and the main thing is to try to make physiological dormancy in cherry plants begin to appear naturally.
How do we detect physiological dormancy? It is the moment when the plant crosses the 50 percent line of leaf fall, always considering yellow leaves as fallen leaves, and that somehow marks us and tells us that the plant is sleeping, not deeply yet because the phases of dormancy are different, but that it has already entered a clear period of latency or dormancy. Consider that this stage is the condition of autumn, and in no case would early defoliation due to other effects represent this state,” says Carlos Tapia.
However, it may happen that the orchards are still very green in terms of leaf colour and vegetative expression, although not necessarily growing, and this may be due to various reasons, from not having given the plants a correct signal in terms of nutrition until very late in the season, suspension of irrigation, very hot days for the season, among others.
In these cases, it is necessary to make decisions and invite the plants to shed their leaves, unfortunately, not naturally.
“In this case, one of the practices that we are most accustomed to doing is the application using the Sulfate poisoning model, which in the most classic case is Zinc Sulfate mixed with urea; this mix makes the Sulfate together generate the poisoning effect and the urea, for its part, enhances the decomposition of the leaf and in this way the leaf begins to fall, not naturally, but fulfilling what we want,” says Carlos Tapia.
It is between April 15 and 20 that the right time to analyze and make decisions is, considering that the ideal is that the beginning of dormancy coincides with the first week of May, when, climatically, the hours of cold that are useful for the plants can be counted; of course, these will only be useful if the orchards have started this physiological dormancy, that is, they have crossed the 50 percent line of fallen leaves, which marks the beginning of winter lethargy.
What comes after entering dormancy?
Once the plants begin their physiological dormancy, whether naturally or artificially, other fundamental aspects for cherry orchards must be taken into account.
“From then on, my concern is twofold: one is the management of fall-winter phytosanitary programs, the use of products based on fungicides, copper and/or biological agents. They are involved in the first stage, covering all the natural wounds that occur as a result of leaf fall, so we have two or three applications that we have to do as the percentage of leaf fall increases, and then through some type of rigorous program thinking about getting through the winter.” says Carlos Tapia, specialist advisor in cherry production and Technical Director of Avium.
On the other hand, it is necessary to understand that although the plants will enter a period of dormancy, they are not dead, but rather in a stage in which their metabolism is practically zero, not only vegetatively, but also from the point of view of root growth.
“We have extensively studied background information that under 7ºC soil temperature the roots do not have a significant development, almost none, between 7ºC and 14-15ºC the roots generate a slight movement, but above 15ºC soil temperature the roots generate or develop in terms of accelerated growth, therefore although the roots are not active in winter, they are alive; one of the important aspects in the maintenance of these structures is that the roots cannot dry out”, warns the specialist advisor on cherry production.
Unfortunately, the drought that the country has been experiencing for more than a decade has caused much drier winters, with less rainfall, which is why the decision must often be made to irrigate again in the winter season.
“My proposal in super extreme terms, and I put myself in the worst case scenario where it will not rain in winter or it will rain very little, is that we have to consider a model of soil moisture maintenance to avoid, mainly, that the roots that were generated in the post-harvest stage just past, in January and February particularly, do not dehydrate and do not die,” Tapia adds.
This large number of new root structures are very fine and generally contain a large part of the nitrogen reserves from the last post-harvest and are the most effective and efficient in absorbing water and nutrients in the first stage; however, they are the thinnest and have the greatest dehydration power.
“How much do we have to water in winter when faced with a climatic catastrophe due to a lack of rain? Nothing has been studied very well, but from my point of view I think that we have to try to maintain a certain soil humidity from the point of view of survival, and a recovery rate of 1 mm per day or 30 mm per month as a minimum makes sense to me; this is to the extent that through some information search model, test pits, sensors or a mixture of both, we can establish that we have a soil humidity that, although it is not so useful for the plant, does allow us to maintain the water condition of these roots and avoid dehydration.” explains Carlos Tapia.
It is essential to monitor, predict, analyze and see the possibilities of resuming irrigation that exists in each orchard.
It should be noted that not only the amount of water incorporated is important, but also its distribution.
“At the end of winter we have to plant, depending on what happens, if there was very little rainfall, we must somehow end the season with a long irrigation, prioritizing, more than the amount of water in the soil, the distribution of water in the profile, simulating the effect of a heavy rain,” recommends Carlos Tapia.
In past seasons, specifically in 2019, important information was generated regarding the fact that we were going through a year of low caliber; this occurred mainly in Bing, Royal Dawn and Santina varieties, but in specific areas. Precisely where the most information was collected regarding a lower caliber curve in relation to previous years or the average, were sectors that had 100 to 150 mm less rainfall than the previous year or compared to the average of the last 10 years.
“Definitely dehydration or low rainfall affected dehydration and this had effects not necessarily on production, but on size; in order not to go through a similar situation again, we have to understand that in the face of low rainfall, we must ensure that we maintain minimum humidity levels so that these roots do not dehydrate,”concludes Carlos Tapia, Technical Director of Avium.
From now on, the immediate objective is to search for and find the physiological dormancy of cherry plants; once this dormancy has started, to carry out a complete phytosanitary program, in addition to seeing and verifying what the soil moisture monitoring model is. This is in order to adequately respond to a need in the face of a possible new winter with low rainfall and to prevent water shortages from affecting the next cherry season.
