By: Hector Garcia O., Co-Founder and General Manager of Laboratorios Diagnofruit Ltda, SOCHIFIT and AMICH Member. [email protected]
Dr. Cecilia Ramos B., Full Professor at the University of the Americas (UDLA), Co-Founder of Diagnofruit Laboratories Ltda, Member of SOCHIFIT and AMICH. [email protected]
At least four previous seasons with dry and hot springs have marked the production of cherries in recent times, this has led, in general terms, to good results in pre- and post-harvest conditions; however, there is a phytopathogen called Botrytis which we should never underestimate as it can manifest itself and respond to small environmental changes, causing problems from the orchard to the final sale.
Botrytis, the super pathogen
Probably, Botrytis cinerea It is the most important pathogen in national fruit growing, a large part of the costs of the phytosanitary management program in table grapes, blueberries, strawberries and cherries is aimed at controlling this fungus. The versatility it has in terms of host colonization is one of the characteristics that leads us to classify Botrytis as a SUPERPATHOGEN, with polyphagia being the first superpower to highlight.
Although in Chile, as in other countries, sexual reproduction of this fungus has not been observed, asexual reproduction is more than enough to achieve a very high colonization rate due to the great capacity to produce conidia (spores).
The pathogen's ability to adapt or "fitness" is another characteristic that we consider a super-power. Although it has an optimal temperature range for its growth, which goes from 15 to 25ºC (Chilean populations between 18 and 20ºC), it can also develop at temperatures as low as 0ºC, which allows attacks on cherries in refrigerated storage. Obviously, poorly cooled fruit, poorly stowed or faulty containers are breeding grounds for this fungus, always complying with the rule that the higher the temperature, the greater the development of rot.
A hidden weapon that it possesses is its enzymatic battery (pectinases essentially), which allows it to penetrate healthy epidermis, installing the infection in the fruit. The indiscriminate use of growth regulators, unbalanced fruit (high nitrogen and low calcium), bruises, among other factors, predispose the pathogen-fruit interaction to be more effective and in this way the fungus colonizes more easily, even at refrigerated storage temperatures, as already mentioned.
As can be seen from the pathogen risk list of Fungicide Resistance Action Committee, better known as FRAC, Botrytis cinerea It has been assigned the highest risk rating according to its pathogenic nature, corresponding to level 3, sharing space with other important known ones such as Penicillium either Venturia (Table 1); which, combined with the type of fungicide to be used according to its inherent risk, can in certain cases generate the highest combined risk of resistance, classified with a grade 9 (Table 1). This means that, in addition to the attributes already described, this superpathogen has a high ease to generate changes in its genome, turning it into a fungus capable of generating resistance to the fungicides that we normally use for its control.
This inherent ability that it possesses Botrytis of “changing” by generating specific mutations in its genome that cause the target protein of the fungicide to modify its structure and, therefore, there is no activity of the fungicide or there is an overproduction of certain proteins of the membrane of the fungus that are capable of expelling the fungicide from the cell limiting its action, are strategies that the fungus uses in a very “intelligent” way. to withstand and adapt to a hostile environment. But if the environment is too hostile, temperatures are generally too low, food is scarce, or another limiting factor, this fungus has the ability to resist, and what does it do? It forms the so-called “sclerotia”, which are resistance structures that are formed by compaction of the mycelium, where the fungus lowers its metabolic activity in what would be a period of “hibernation”, waiting for better conditions to become active again and continue its life cycle.
As a good supervillain, he also has his own multiverse, being Botrytis cinerea the version that we know best, but there are cryptic species (that we can only distinguish through genetic analysis) that coexist, with some frequency, in the same "dimension". Although we have no precedents in cherries, species such as B. pseudocinerea in table grapes and B. prunorum in kiwis and stone fruits living with B. cinerea, but be careful, the latter is always the dominant one and it is to this that we must direct control, at least in the immediate future; perhaps climate change in the future may modify this reality.
The disease
Botrytis It presents two versions of the disease in cherry trees, in phenological order it would be:
i) Flower blight: occurs more frequently during temperate and humid springs. Conidia colonize stamens and pistils and from there an infectious process begins that ends with the rotting of the entire flower. The last seasons in Chile have not provided environmental conditions for the expression of this disease.
ii) Gray rot in fruits (pre and post-harvest): once the fruit increases its maturity, therefore generating more dry matter, more soluble solids and less acidity, it becomes more susceptible to infections by Botrytis; in this way, conidia that fall on the epidermis of the fruit are able to degrade the tissues and begin to feed on the carbohydrates that are abundant at that stage in the pulp, allowing a profuse generation of conidia and grey mycelium at room temperature and white if this process occurs during refrigerated storage. Pre-harvest rains generate ideal conditions for the development of this fungus, with each crack being an easy entry point for Botrytis and other secondary fungi, to prevent it is always better to apply before the weather event, at least 24 hours before the rain. Although the last seasons have had little inoculum of Botrytis In the field (Chart 1), we must always be alert to changes in this reality, continuously monitoring and acting preventively.
Control
What has already been described gives us insight into the critical moments where we must carry out applications to control the pathogen:
i) Flowering, in general, two applications are recommended to avoid the dreaded blight, the beginning of flowering and then full flowering are the moments that best prevent the development of the disease. Synthetic fungicides, plant extracts and/or biocontrollers can be used, alternating between them. In complex situations, orchards with a history, wet spring, two applications of botryticides (carboxamides, hydroxyanilides or phenylpyrrole) should be scheduled as a base and products of biological origin should be considered as support.
ii) Pre-harvest: the critical stage begins a few days after straw colour until harvest. Depending on the variety, early or late, the application window is different, considering this, a base program considers at least a couple of applications up to four in those with more extended development. Again the base must be built with specific botryticides and successive applications of extracts or biocontrollers. Although the populations of Botrytis In cherry trees, in general, they do not present resistance to fungicides, it is important as a general anti-resistance measure to try not to repeat fungicides from the same chemical groups more than 2 times in the season.
iii) Postharvest: the peak moment for control. Due to the possibility of immersion of the fruit, applications at this stage are very effective. Fludioxonil is the standard fungicide for the control of this pathogen in postharvest and the amount of residue on the fruit must be constantly monitored. As the calibration process is carried out in water, it is easy for the inoculum of the fruit to be absorbed. Botrytis and other fungi continue to contaminate the system, then to prevent infections some sanitation method must be used, among the most efficient is chlorine.
So, today we learned a little more about Botrytis, this super pathogen that we already know why we should not underestimate, even when the environmental conditions seem not to be the most optimal for this fungus, and in that sense, we must be cautious and proactive not leaving spaces for it to sneak in through some fissure in the system; at least, we must ensure the fulfillment of minimum actions such as observing the springs and environmental conditions that may be predisposing for the development of the fungus and ideally have data on the amount of inoculum present in the field; carry out adequate and timely cultural management that opens the way to a successful control program that should consider, at least, the implementation of the base fungicide program both in flowering and pre-harvest and not leave the hard work only in the hands of extracts or biocontrollers, even if it is an apparently "benign" season; and finally, do not forget the use of anti-resistance measures and monitoring that gives us the field history necessary to avoid selecting the population of Botrytis, and maintain the balance to avoid generating resistance to fungicides in our gardens.
