2019-20 Season Analysis
By Hector Garcia O.
Director-General Manager Diagnofrut Laboratories Ltda.
When we talk about cherries, we think of a very delicate fruit, with a lot of water and which by default is very susceptible to attacks by pathogens causing different types of rot. Although this reasoning is largely true, the truth is that if we consider modern cultivars and a climate without rainfall, the frequencies of rot are quite low, as long as we keep certain critical points in order; on the other hand, species such as table grapes or blueberries present many more difficulties in controlling rot due to characteristics that we will analyze later.
Attention to the weather: In all the fruit species that we cultivate, the climate plays a key role. We know that cold in winter, degree days in spring, luminosity, precipitation, among other factors, can be the basis for a successful season or one to be forgotten. From the point of view of rots (I will refer to those produced by fungi, which are the ones we observe in Chile), there are certain differences in terms of climatic conditions for their establishment, and they depend on the causal agent. First, to put things into context, the rots that we most frequently find, at the field and packing level in our country are, in order of importance:
- Gray Rot, caused by Botrytis cinerea
- Black Rot, caused by Alternaria spp.
- Acid Rot, caused by Geotrichum candidum
- Green Rot, caused by Penicillium spp.
- Brown rot, caused by Monilina spp. (very rarely observed, only in very precarious management situations).
All of these rots are favored by rainfall, the fruit is damaged, split, and each of these pathogens has the gateway to the generation of the disease. However, there are some subtleties, springs of high humidity, not necessarily rain, result in high inoculum values of G. candidum. Early flowering periods, generally driven by high temperatures in winter, have generated the development of black rot more frequently, probably explained by high humidity + temperature during the shedding period and early stages of fruit development, as we will see later. Grey rot is more closely linked to precipitation events from straw colour onwards; Botrytis, unlike what occurs in table grapes or blueberries, does not generate latent infections during the flowering period that later transform into grey rot at harvest or in storage.
What is the reality of the 2019-20 season in relation to the climate?
In general, we are facing an extremely warm and very dry spring, with only light rainfall in some areas during the flowering period. Flowering is 5 to 10 days earlier than the previous season. Low fungal inoculum load, lower than in the 2018-20 season (Chart 1). In short, a “quiet” season in terms of the presence of pathogens in the fields.
Despite the good omens, it is never good to be complacent. Producing fruit under thermal stress is also a factor that affects susceptibility to diseases. Calcium flows are interrupted, the result is softer fruit, turgor problems, which result in fruit with a greater chance of being infected; a risk that is at least doubled in orchards under fixed cover. Orchards with a roof or cover that is not removed during the season generally present similar amounts of inoculum as the control without a roof, calculated through qPCR analysis; however, their rot frequencies after refrigerated storage under identical storage conditions are considerably higher in fruit produced under cover, thus, this behavior would be essentially attributable to the degree of susceptibility of the fruit. Considering what has been described, the next step is not to neglect the phytosanitary program.
Chemical Control
When we design a phytosanitary plan, the first thing we need to do is define whether my orchard has historically presented a more frequent rot, or whether it is simply more susceptible than the average in the area. Once we have defined the “weak point”, we choose the active ingredients to be used in a time-specific manner. If my records are poor, we can use information from the exporter, essentially, packing receipts, which in general in Chile are very precise. For this, as a producer, I must demand this information every year and generate a database in order to establish improvements year after year.
A basic program designed to prevent rot should consider at least a couple of applications during the flowering period, one or two between the fall of the jacket and the first millimeters of fruit development, and at least two between straw color and harvest. This depends on the variety, area and climate behavior during the season.
If my problem is Botrytis, the specific control should begin in flowering, favoring the use of active ingredients with botryticidal characteristics such as fenhexamid, fludioxonil exclusively for full flower (>80% exposed flowers). At the beginning of the flower, with the idea of lowering inoculum of this and other pathogens, we can use active ingredients from the triazole group, such as tebuconazole and difenoconazole. Although high concentration of inoculum in flower does not correlate with high incidence of pre- and post-harvest rots in cherries, it is important to cover the flower for the effects of blight, another disease caused mainly by botrytis and secondarily by moniliniaIf we have an orchard that has problems with grey rot in fruits, we must take care of the system from the beginning, because it tells us about a general susceptibility of the orchard. From straw colour onwards, it is important to keep the amount of inoculum on the fruits under control, therefore, we must consider the mandatory use of some botryticide applications. Now, what to apply? The answer is not simple, because another consideration comes into play with greater force than in flowering, which is the limitation of residues on the fruit and the number of active ingredients according to the destination market. In addition to the aforementioned fenhexamid, fludioxonil, tebuconazole, we add to this stage active ingredients from the carboxamide group, such as boscalid. The idea should always be to generate a program alternating between active ingredients and chemical groups and avoid the generation of resistance. The number of applications should be established based on the climate and historical susceptibility, with the culture of avoiding excessive use.
Another increasingly common problem in orchards is the so-called Black Rot (Photo 1), caused by several species of the Alternaria genus. According to studies carried out in 2018-19 and which we are repeating this season, the pathogen would colonize the fruit suture, in sectors where the ovary was not completely sealed during the differentiation period, in early stages of fruit development, close to the fall of the jacket and the first mm of fruit growth, when the fruit is exposed to the environment. The conditions of the current season have been unfavorable for the presence of Alternaria, but we must not let our guard down based on the aforementioned factors that have a multivariate nature. Although colonization would occur early, the development of the lesion becomes visible from straw color onwards. Therefore, disease control must be directed towards the end of flowering and the beginning of ripening, covering the poorly sealed suture and then preventing the spread of the disease. Most botryticides exhibit in-vitro control, however, the carboxamide group works at very low doses in a homogeneous manner across the different species that are present in the disease, essentially A. alternata and A. arborescens. So, if the problem in the orchard is Black Rot, I should consider carboxamides in any of the critical periods mentioned, which as noted will also control gray rot.
Less common, but when they appear they tend to be quite aggressive, they are the Brown and Acid Rots. The first, caused by species of the genus Monilinia, is very rare, we have only observed it in situations of poor management; studies carried out on Chilean populations of M. fructicola They point out that it has very little selection pressure towards the fungicides that we normally use to control Botrytis, therefore a good control program for gray rot will work well against brown rot. Acid Rot, caused by G. candidum, is more frequent, but is closely associated with wet springs; control is based almost exclusively on active ingredients from the triazole group; if there is a risk, pre-harvest applications should be included, in addition to one during flowering.
According to what we have observed Green Rot It is associated with problems of fruit condition, cracks, microcracks, over-ripeness, in pre-harvest are part of the predisposing factors; these are increased by bad practices in post-harvest such as poor cooling. Formulations that include fludioxonil should be used to control and prevent attacks by fungi of the genus Penicillium.
In conclusion, the dry weather of the 2019-20 season has resulted in less inoculum load in the orchards, healthy fruit has been observed in the first harvests and we hope that there will not be major complications if the meteorological panorama does not change. Obviously, monitoring and recording the incidence and classification of rots is crucial to generate the necessary adjustments in phytosanitary programs with the information provided in this article. One detail to note is fruit quality, thermal stress can generate susceptible fruit, accelerated maturity (high rate of accumulation of soluble solids and decreased acidity), can shorten the “legs” of our cherries, therefore, the support of a good basic phytosanitary program is crucial for successful production and export.
