By: Hector Garcia O., Co-Founder and General Manager. Diagnofruit Laboratories Ltda. [email protected]
One of the most inspiring moments for those of us who work in fruit production or for those of us who like to observe nature is undoubtedly the flowering of stone fruit, in our case cherries, since it is one of the groups of species that receive applause and photographs all over the world. Well-known examples are Japan and some states in the USA where this period is a tourist attraction, as well as a productive phase, which marks the beginning of spring and carnivals (I recommend watching the documentary “Jerte, Vida Salvaje en el Valle de las Cerezas”, 1080 WildLife Production, 2017).
In a very simple and summarized way, we can point out that the formation of the flower begins in the previous summer, being a silent process hidden from our eyes under the bracts of the bud, where there is a continuous process of development, which ends once the plant defines that the signals from the environment are sufficient to wake up, thus starting the process of sprouting and flowering. It is essential that the plant wakes up in synchrony with a benign environment, when there are pollinators, for example. Regarding this, today there is a theory about ancestral co-evolution between insects and plants, considering that some plant species are very dependent on pollinators for their reproductive process. The accumulation of cold hours is one of the parameters that the plant “sense"or measures to know when to wake up and in this sense there is still a lot of ignorance about how plants respond to temperatures, but it is even more unknown how they are able to measure temperature signals, an interesting work could give us insight in this sense that talks about the vegetative growth and the dilution of certain proteins.
But, just as various insects have co-evolved with plant species, certain fungi also “synchronize” moments of spore dispersal with susceptible phenological events of the plant, in this way vascular fungi, capable of generating wood rot, wait for the moment of leaf fall to penetrate, so that peaks of spore production occur in that period. When the flowering burst occurs, there is high humidity and cold to temperate temperatures (although the last two seasons in Chile have been rather warm and dry), these conditions are perfect for the dispersal of ubiquitous fungi of genera such as Botrytis, Monilinia, Alternaria or even yeast-like types such as Geotrichum. Within the mentioned genera, three pathogens have been described in our country as generators of Blossom Blight in cherry trees and other stone fruits., Botrytis cinerea, Monilia fructicola and M. laxaIn this way, these fungi take advantage of the climatic conditions of early spring and a huge amount of sessile tissues that they can easily colonize.
Symptomatology: Flower blight by Botrytis and Monilia The disease shares similar symptoms and signs in cherry trees. The disease begins when the pathogen attacks the stigma, anthers and petals of the flowers, the fungus advancing through these organs towards the floral tube, generally reaching the pedicel, causing it to wilt, and we can often see the development of mycelium in a mummified flower. In attacks of more advanced phenology, when the fruit has already begun to develop, a brown lesion may form where the diseased calyx touches the surface of the fruit. The lesion quickly spreads throughout the fruit, killing it when it has just set. On certain occasions, in cherry trees, gum exudation can be detected at the base of the fruit insertion into the branch.
What is the real impact of cherry blossom blight in Chile? It is not an easy question to answer, due to lack of information; there is no ad-hoc standardized monitoring program or publications with enough data to be taken as general information. Considering the model of Monilinia, this type of infection is favored by temperatures between 14° and 25°C, a range that is difficult to find in the central zone in September, but at the same time we need the humidity factor, in this way the incidence of blight is favored by the temperature and duration of periods of high humidity; it is mentioned that periods of between 3 to 5 hours of high humidity at 20°C are generators of important attacks on cherry trees. At the same time, over 20 hours of humidity, the temperature factor becomes secondary, and the infection develops without major problem. According to this, the disease is closely associated with the ecozone where our plantation is located, to name a few where we must take special care, those with a marine influence such as Las Cabras, rather cold areas of the pre-cordillera such as Romeral or Chimbarongo or directly the new plantation areas in the southern zone, Los Lagos Region, are all locations where the disease pressure is greater.
Although we do not have a particular monitoring of the disease through observation of symptoms and determination of frequencies, Diagnofruit Laboratories Ltda., has developed techniques to quantify the amount of inoculum in this period (Graph 1) and also with developing fruit, in order to establish dangerous contaminations and develop specific pre- and post-harvest management of problematic production units.
Chemical control: Probably the most important topic for technicians in charge of the economic sustainability of orchards is How, when and with what to control the disease? Before answering about control, the first thing we must consider, connecting with the first paragraphs of this article, is that everything we apply in the flowering process can affect pollinators, bees, bumblebees, among other agents that participate in this process. In general, the permitted fungicides offer security in this regard; however, it is a point that remains raised because it is an aspect where we should NEVER improvise.
The assets they control Botrytis They are perfect for controlling Monilinia, therefore, we should not think of differentiated applications. Studies of the founding populations of M. fructicola in Chile (M fructicola It is a recently introduced pathogen) indicate low levels of selection to fungicides (high sensitivity) with which we usually control Botrytis, which simplifies control, and also explains the low impact of the disease when a well-thought-out program is carried out.
Among the recommended formulations for the protection of our cherry blossoms, which we have had the opportunity to test at different levels and which are mentioned in the world literature, the following products with mixed active ingredients stand out: azoxystrobin & difenoconazole, boscalid & pyraclostrobin, fluopyram & tebuconozale and fludioxonil & fenhexamid. Pure active ingredient formulations from the carboxamide group such as penthiopyrad or boscalid or others belonging to triazoles such as difenoconazole, tebuconozale or fenbuconazole are also tools with good results. Some of a broad spectrum nature such as captan are an alternative to be inserted in the cherry blossom program in situations of less risk or as support for those already mentioned.
How many applications are necessary and what particular product? This depends on each orchard, the number should be established based on the duration of flowering (this year due to lack of cold hours we could observe longer flowering), it is also important if we have a historical presence of Black rot caused by Alternaria, which also determines the product to be used and the number of applications that will extend until the fall of the petals. In this sense, at least 2 applications should be considered between the beginning and full bloom aimed at controlling blight, then other environmental factors should be analyzed to plan applications during periods of petal fall, fall of the petals or newly set fruit, depending on whether they are necessary. The last thing we should consider when creating a base management plan and which defines which products to choose is the following question: What straw-colored shade will I apply from now on? We must rotate products, not applying carboxamides (boscalid, penthiopyrad) or hydroxyanilides (fenhexamid) more than twice a season; this is a general measure that must be included in our approaches for the design of the global phytosanitary program.
Bonus Track:There is not only cherry blossom blight caused by fungi, there is also a version bacterial, caused by organisms of the genus Pseudomonas. Many times the bud is infected and the attack is triggered at the time of flower development, or it can be infected due to favorable conditions for the dispersion of these agents, which occur in our spring anyway. Control is based on reducing the inoculum from the swollen bud onwards, essentially with copper products, and as we approach periods sensitive to metal poisoning of tissues, the recommendation turns towards the use of biological controllers.
