The last season has raised alarms regarding pathogens that cause cancer, decay and death of plants, not only in cherry trees, but also other stone fruits have been seriously affected, as well as other species such as kiwis. A wetter spring than in recent years, the presence of frosts early in the fall and others in winter, have set the perfect stage for the development of these pathogens in the central area of Chile; at the same time, hot summers, with incessant heat waves and with summer periods that seem to never end, extending into the fall, add conditions for, especially, fungi that cause cancer, such as Cytospora, begin with death processes in our orchards, considerably reducing the profitability and, by projection, the sustainability of fruit production.
Let's briefly talk about the current knowledge of the pathogen
As has happened in other genera and species, molecular analysis tools have opened the doors to new knowledge that we would have been unable to obtain through classical taxonomy, even considering seasoned experts in the field; in this way, the genus Cytospora has not been exempt from new knowledge.
In a quick review of scientific databases we can almost immediately find interesting works where new species of Cytospora, taking us out of the classic species C. leucospora (Photo 1) as a unique look.
Photo 1. Cytospora leucostoma isolated from cherry, Maule Region, 2022-23 season. Plate 25 days old.
In ChinaFor example, in a study published in 2020, 10 new species of Cytospora associated with canker and dieback of rosacea, C. cinnamomea, C. cotoneastericola, C. mali-spectabilis, C. ochracea, C. olivacea, C. pruni-mume, C. rosicola, C. sorbina, C. tibetensis and C. xinjiangensis and three known taxa, including Cytospora erumpens , C. leucostoma and C. parasitic were described. While the study analyzed isolates beyond hosts of the genus Prunus, is proof of a high diversity that was previously hidden from the common observer and also from the most educated and that we do not have many precedents in Chile.. One year later, also in China, studies in symptomatic conifers (pines, spruces and junipers) four new species were described.
On the other side of the world, in Canada, earlier this year, a new species was described affecting Prunus and was named C. paraplurivora and previously in 2018, 10 new species were described in the USA affecting almond trees, cherry trees ( Prunus avium, olives, peaches, pistachios, pomegranates, dehydrated plums, walnut trees and woody ornamental plants such as poplars, camellias and redwoods.
In Chile, the most observed species is C. leucostoma, but without a doubt, we could be facing a diversity of species that we have not been able to classify completely, essentially due to a lack of economic resources; this is why in basic genetic analysis, many times we have not been able to find known species in isolates rescued from cherry orchards, so we must delve deeper into this area, considering that each species may have different characteristics, such as virulence, adaptation to the climate, sensitivity to fungicides, among others that can affect control in the field. Apart from detections in stone fruits, we have detected attacks by this pathogen in yellow kiwis, in a rather isolated manner, without compromising the life of the plant.
Symptoms to watch for in the field
In our reality, when the pathogen is already well developed inside the plant, we can see anything from dead branches to a general decline of the plant, small leaves and early defoliation (Photo 2). Cankers and changes in the internal coloration of the wood accompany the above-mentioned symptoms, and gum exudation, as in bacterial canker, can be generated.
Photo 2. Cherry tree cv. Royal Dawn attacked by Cytospora sp. With early defoliation and general decline (Las Cabras, 2023)
In dead or living wood, the fungus can generate fruiting bodies (Photo 3), structures through which it releases spores for reproduction and colonizing healthy plants; this is probably the biggest difference with cancer caused by bacteria.
Photo 3. Fruit bodies of the fungus on dead cherry wood.
How does Cytospora infect our cherry trees?
As a general background, Cytospora is considered a secondary pathogen, why? Because it needs a wound to penetrate and colonize the plant; the big problem under our production model is that there are several ways for these entry points to be generated:
- Handling that causes injury (pruning, aggressive training systems, harvesting, etc.)
- Climate (excessive solar radiation, frost, hail, etc.)
- Insects (in the USA it has been well described for species not present in Chile; we have only detected the presence of scolytes in orchards with the presence of the pathogen, although their relationship should be studied). We have also observed that fields with rabbit attacks are susceptible to this infection at the base of the trunk.
The chances of the fungus penetrating the plant are therefore high, given the various options that are exacerbated by climate change, remember the heat waves in February this year.
Although the clouds of spores it releases Cytospora to colonize new plants, they occur more strongly in autumn and spring and/or after rain events. The presence of the fungus has been quantified in summer, which, ultimately, should keep us alert all year round, unlike bacteria that tend to disappear during dry and hot stages of the year, the summer period.
In recent studies in USA It was determined that in dehydrated plum trees C. leucostoma inoculated on twigs shows significant lesion development at 30 and 35ºC, which under our current reality could allow the disease to develop in summer without any major problem. At the other extreme, under colder temperatures, < 10ºC, the fungus develops more slowly but continues to colonize tissues without much problem.
Prevention, the most relevant cultural measure for disease control
With the background information mentioned in this article and others that we have been carrying out, we can generate a ranking of activities that must be carried out to prevent the development of the pathogen in our cherry plantations:
- Starting from healthy materialIt may sound obvious, but this is a point that is not always covered when starting a plantation. It should be remembered that a certified plant is one that does not have quarantine diseases, but it does not certify the absence of pathogens such as Cytospora or others that are present in Chile. Therefore, it is always advisable to send plants to be analyzed in search of this type of pathogens prior to the formation of the plot. It is common to find certain wood fungi, especially in the cv/pi union, however, if it is detected Cytospora either Calosphaeria It is a point of maximum alert, due to the virulence of these in cherry trees.
- Sanitary pruning. Removing symptomatic plants and branches is an activity that should be carried out every season and should not be postponed. An important point, Infected material must leave the orchardIt is customary to pile up the remains at the heads of the rows and as we have already explained, even in dead wood this fungus can continue its life cycle and continue releasing spores that will colonize healthy tissues (Photo 4).
Photo 4. Sanitary pruning material stockpiled in the row. Sanitary pruning should always be scheduled with immediate removal of diseased material from the orchard.
- Disinfection of tools. As we mentioned in a previous article, this management is essential to avoid the spread of various pathogens that cause cancer; starting with a clean cut is essential to avoid contagion between plants.
- Application of fungicides and biocontrollers by spraying, painting of cuts? Definitely, applying synthetic fungicides, biocontrollers, paints, paints plus fungicides, aimed at covering natural wounds such as falling leaves or pruning wounds, are certain alternatives to prevent infection, but within the general preventive scheme, points 1, 2 and 3 must be carried out in the orchard, so that point 4 adds to the strategy with greater power; it would be the closure, but in no way will this strategy alone serve to eliminate the risk of disease, due to the biology of the fungus, which we already explained, and that all these applications have a systemic very low is not able to control Cytospora if it has already successfully colonized the wood. We know that painting thick pruning cuts is generally inefficient in terms of the activity being poorly performed, for this it is possible to add a spray application aimed at covering the spaces where the paint did not reach or in those cuts where the paint was definitely not applied. Methyl thiophanate is a fungicide that has been effectively tested against Cytospora in the USA, but in Chile it is not registered for use in cherry trees. Captan, benomyl and triazoles are products that, although the results are dissimilar depending on the form of evaluation, would also generate control of the fungus in the field. In trials in vitro (Photo 5), which give us the first clue if a fungicide manages to control a pathogen, carried out on Chilean isolates of Cytospora, we observed that in general the EC values50 obtained are very low (<1 ppm), so we could extrapolate that low amounts of products based on pyraclostrobin, azoxystrobin&difenoconazole, even dodine, could have an interesting control effect on epiphytic populations of the fungus. On the other hand, Copper, which is the basis for the control of Pseudomonas, although it could generate control in vitro, the reality in the field is very different, and we observe development without problems of Cytospora in orchards where this mineral fungicide is applied with high frequency.
Photo 5. Mycelial growth assay towards pyraclostrobin in Cytospora sp. isolate from cherry.
Finally, and as you must be concluding after reading this article, the control of this and other wood pathogens is very complex; it is a long-term, constant job, which starts with the selection of healthy plants and then everything possible must be done to keep them that way, and the way is to generate a union of various priority preventive managements, and others that directly target the plant's ability to defend itself, for this, reducing stress, generating balanced plants (avoiding excess nitrogen), use of elicitors, among other managements, are essential for the creation of an orchard that fulfills a stable production of cherries for several years, sustainable.
