Why do our cherry plants die?

Why do our cherry plants die?

By Héctor García O., Founder and General Manager, Diagnofruit Laboratories Ltd., hgarcia@diagnofruit.cl

Despite Chile being one of the world’s main producers and exporters of cherries, and with it constantly advancing in more plantations all along the country, there are still shortcomings in the process that make many cherry plants die.

A plant that is poorly developed in the nursery, where there is inferior soil preparation, mistakes with herbicide management, environmental stress (e.g. periods of high temperatures that have become common in Chile these days and the lack of water) etc., are counted as the main problems that threaten the longevity and therefore the sustainability of plantations. All these factors end up causing what we call ABIOTIC stress and can mean their finish.

If there is ABIOTIC stress, in contrast, there is the one named BIOTIC stress, which includes all those whose origin is from some type of interaction with living organisms, primarily microorganisms that also may be the cause of the plants’ death.

Definitely a plant under ABIOTIC stress is more susceptible to BIOTIC stress. In direct connection to this premise, the search of precocity in the production, high yield and fruit quality encourages us to increase the density of the plants, use of conduction technologies, powerful fertilization plans and plantations that are outside of the historical “habit”; all these factors, which may result in the appearance of new phytopathological problems or other old ones that come back because we pressure the productive system without enough local information.

High demand for plants allows the sale of low quality plant material; low tracing results in contaminated plants. At the same time, due to the world globalization effects we are living under the biggest state of sanitary emergency in the last 100 years, this factor has increased the risk of contamination in humans, so why not for plants, even some that only decades ago we would not have been exposed.

In conclusion, all that has been described is of vital importance to define for sure the pathogens that are affecting plants under new productive conditions in the different areas where cherries are being planted today. 

The Project

In order to respond to said quandary, to perfect and extend the knowledge that we’ve got, we have created a research project financed by exporters and agrochemical companies, which started to study in further detail the knowledge of Pseudomonas syringae and especially the pathovar morsprunorum (Psm), recently discovered in the southern zone of Chile, affecting cherries.

The project has evolved and furthermore than the effects of abiotic stress, today we present the following question: Why do our cherry plants die?

With the recent permission from SAG, Diagnofruit is authorized to carry out certain types of trials upon Psm (Res. Exenta n° 1805_2020), in effect, we have already begun working on the complete genome sequencing of this pathogen, with the purpose of knowing its initial genetics (founder strains) and in this way in the future detect adaptive processes, communication with other local Pseudomonas, gene bases with resistance to Copper and Antibiotics, among other key aspects.

Probably Pseudomonas, in particular P. syringae pv. syringae (Pss), as far as we know, is the pathogen of greatest importance in Chilean cherry orchards, producing symptoms of larger importance in Chilean cherry orchards, producing symptoms of less impact with necrotic foliar spots up to the denominated bacterial cancer, characterized with canker on the trunk and branches which can end up with the death of the plant.

However, as we can suppose, Pss and Psm are not the only nemeses that our plants have, and there is another kind of cancer that exists which we have seen with concern and prevalence in certain orchards and we are not aware if this could be a larger problem in orchards located in the southern zone; this type of canker is caused by fungi of the genus Cytospora, which is a group that will also be studied in our project.

Some of the canker details caused by Cytospora.

The genus Cytospora was described for the first time in 1818, and at first they were defined as endophyte fungi, degraders of dead wood, without any pathogenic character. Later they were described as the cause of disease, which was called in its generic form Cytospora but also is known as Perennial Canker, Valsa or Leucospora; and they have been described in more than 85 woody fruit plants as a host to this type of pathogen, including cherries, and especially peach trees. At the same time, many pathogenic species from the genus Cytospora have been described in Poplars and Eucalyptus, that could be a reservoir for inoculum, which many times are inside of our orchards (attention with this first piece of data).

In Washington, USA the reports started to become important towards the end of the 80’s and they were described as an disease associated to the twigs, damaged trunks in the winter (freezing weather) or injured mechanically (certain conductions could make the plant susceptible, second piece of data to save). 

In this way, the fungus colonizes in injured tissue and then begins its growth in the adjacent healthy tissue. The most common symptoms are a brown rot blossom blight  (Photo 1), canker on the trunk, gummosis and changes in the wood’s color (Photo 2).

Photo 1. Brown rot blossom blight in cherry trees. Maule Region. Photo: Carlos Tapia.

Photo 2. Rotting of the wood and bark with pits that indicate asexual fruit-bearing underlying bodies in the trunk of the affected cherry tree. Maule Region. Photo: Carlos Tapia.

CANKER is dark and sunken; it may grow at a point where it surrounds the main branches and trunk, resulting in the death of the branch or definitely the whole tree.

Minute spore producing structures the size of a pin head called pycnidia may form on the sick tissue (Photos 2 & 3) and generally when they appear, the branch is doomed to die within the next 4 to 6 weeks.

Under wet conditions, the pycnidia can exude millions of spores in small red to orange rings (Photo 3). Splatters of rain or irrigation water can disperse the spores to other injury sites and generate cycles of pathogenic infection.

Foto 3. Bark with pitting that indicates asexual fruit-bearing underlying bodies in a cherry tree branch and trunk affected by Cytospora. Maule Region. Photo: Carlos Tapia.

A recent phylogenetic study done on cherry, almond, apricot, pistachio, grenadine and poplar trees in California (2018) on 150 isolations, revealed 15 species of Cytospora associated with canker and dieback in different species. From these, 10 were described for the first time, which implies a future investigation challenge upon the etiology of the illness.

Literature in Chile is scarce with regards to Cytospora and cherries, it mentions Cytospora leucostoma as the causative agent, but probably is not the only species involved according to what has been described around the world. A characteristic that not only appears in national literature about the fact, is that evidently Cytospora and Pseudomonas, do not share the same tree at the same time, they would have different periods of colonization. If we add to this last record that Copper does not control Cytospora in an efficient manner, a poorly diagnosed orchard in the presence of canker, its management will not have any effect upon the fungus and the illness will continue to advance, contaminating neighboring plants easily.

Returning to the project and Cytospora, the objective would be to make a registry of the partnering orchards for export, generating a strain characterized at the genetic level in order to determine the species present in Chile, and we would finalize with in-vitro tests in order to determine the level of control on different fungicides. Though there may be experience with the use of active ingredients like Mancozeb, Benomil and Captan as well as the combination of normal applications of the programs; it is necessary to additionally continue studying in search of some complementary solution that allows us to prevent these types of diseases, in areas where there present favorable conditions for the fungi or susceptible varieties.

Other Diseases, Associated Companies and Investigation Equipment

Other pathogens will be covered by the investigation; Verticilosis has been a recurrent problem in samples arriving to our laboratory. There are also viruses that, as plantations grow in age, have proven effects upon productivity.

Finally, we would like to thank SAG for their technical support in the project development, and the companies that have committed themselves, among which are important exporters like Agricom, Frusan, Frutitta, Mace, Prize, Subsole and Ranco Cherries, and the agrochemical companies Agroconnexion, Anasac, Bayer, BASF, Bionativa and Syngenta; without their support it would have been impossible to carry out this investigation. At the same time, the team from Diagnofruit Laboratories will count on the technical support of renowned consultants Jessica Rodriguez, Carlos Tapia and Dr. Alan Zamorano experts in postharvest, production and virology, respectively.

The invitation is still open to participate, and soon we will be sharing news of interesting advances in the national cherry production industry.

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