Calosphaeria pulchella, a pathogen that is here to stay

Calosphaeria pulchella, a pathogen that is here to stay

By: Héctor García, Co-Founder and General Manager Laboratorios Diagnofruit Ltda, SOCHIFIT Member.

During the 2019-20 season, after analysis of symptomatic cherry plants of various varieties, the bad news of the detection of the Calosphaeria pulchella fungus was made public, causing regressive death and cancer in this fruit species. The publication made by the work group of the University of Chile headed by Dr. Jaime Auger in January 2021, ended up making this finding official, setting off alarm bells for an industry in sustained growth, where a new pathogen can easily compromise sustainability of this precious crop.

The mushroom

Calosphaeria pulchella is a fungus belonging to the Ascomycota division, Sordariomycetes class and Calosphaeriales order; I describe these classification data with particular interest, because another fungus that generates similar problems for us, such as Cytospora, shares the same taxonomy up to the class level, which in some way connects them, we could talk about them being “cousins”. A very different situation is what happens with another fungus that is frequently mentioned as causing the death of plants of various fruit species, Chondrostereum purpureum, known to cause the disease called silvering, the latter belongs to the Basidiomycota division, which results in a fungus with very different characteristics from the first two, it would be a distant relative.

On a plate using potato dextrose agar (APD) media, we observed a very peculiar fungus, with pink to purple mycelium, tones that we can somehow also notice in wood invaded by the pathogen.

Calosphaeria pulchella colony in culture medium for fungi (APD). symptomatology

Characteristic symptoms of this disease in sweet cherry trees include dieback of the branch and main axis, as well as wood cankers and vascular necrosis (Cover Photos, 2 and 3). In general, cankers, from a wound, start around the pith and develop progressively in the xylem, cambium, phloem and cortical tissues.

In the later stages of infection, cankers are noted on the oldest branches and trunks of the axis or main trunk, and are usually associated with symptoms of branch dieback and abundant desiccation of the leaves.

Calosphaeria pulchella can produce circinated clusters of perithecia below the periderm of infected branches and trunks.

The severity of the disease is high, the loss of plants can generate the partial or total unproductiveness of the barracks as it progresses season after season.

Section of branch infected by C. pulchella, advance from the pith towards the xylem and protection towards the main axis.

«Control«

As the subtitle indicates, we speak of control in quotes, because strictly speaking, under current knowledge, successful control of the disease in an infected plant is very unlikely.

So, everything related to driving must aim at prevention. Starting with healthy plants (certified or diagnosed for this and other diseases) is probably the first big step. Effective and early diagnosis (Cytospora, Pseudomonas, among other pathogens, share symptoms with Calosphaeria), eliminating diseased plants, controlling stress, aiming at plant balance (avoid excess nitrogen) are measures that appear to be the most relevant for keep our orchards healthy. Eliminating remains of pruning or cleaning “surgeries” is essential to reduce the spread of this pathogen, since from this material Calosphaeria generates its reproductive structures that launch millions of spores when humidity and temperature conditions are favorable, which preferably they occur in spring and autumn, although they can occur throughout the year.

A study carried out in Spain answers an eternal question: can pruning tools spread the disease? Research published in 2018 indicates that pruning with non-disinfected shears increased the incidence and severity of the disease, compared to the use of disinfected shears. The results of the various treatments were consistent for both winter and summer pruning. Textually, these findings would confirm that: “frequent disinfection of pruning tools is recommended for effective management of canker caused byCalosphaeria”.

A very valuable strategy, but that in general the producers do not take the weight, is the mapping of theorchard in order to establish the progress of the disease over time; This is the only way to know if the pathogen is bending our hand or appears only sporadically and erratically, which would mean that my preventive management would be working.

Branch canker caused by C. pulchella, on a tree with an advanced stage of the disease.

Research on Chilean populations of Calosphaeria

At Diagnofruit in 2020, we carried out studies on Chilean isolates rescued from the RegionO’Higgins with the aim of establishing their levels of sensitivity to various fungicides, thinking about possible uses for prevention at critical moments, for example, spraying after pruning. Actives such as captan, benomyl, pyraclostrobin or mixed formulations such as difenoconacol&azoxystrobin, tebuconazole&fluopyram, were subjected to in vitro sensitivity tests, resulting in EC 50 lower than 0.5 ppm, which would indicate high levels of sensitivity in the populations analyzed. In the field, this type of fungus would generate few life cycles during the season, so it is more difficult for them to create resistance than other groups such as Botrytis or Alternaria where the cycles are many in a short time. However, what seems like good news loses force, because the opportunity for control is an issue that has not yet been elucidated, so the effectiveness of an application program for Calosphaeria and other wood fungi remains in question.

In-vitro fungicide susceptibility tests; mycelial growth technique.

In the last congress organized by the Chilean Society of Phytopathology (Abstract Book), 5 papers were presented whose center revolved around Calosphaeria, which is evidence of the importance of the pathogen and the associated disease not only in cherry trees but throughout the range of stone fruits that are produced in Chile.

The first of these works showed that formulations, in the prototype stage, of mixtures of different species of Trichodermas and Bacillus could have a biocontrol effect on the pathogen in controlled field trials.

A second study, set its objective in determining the sensitivity of local populations of Calosphaeria towards different fungicides considering copper. Copper Oxychloride and Copper Sulfate were analyzed, establishing average EC 50 values ​​of 716 and 583 ppm respectively, which could be interpreted as high levels, therefore, the populations would be insensitive to copper fungicides, at the same time it means that a preventive strategy for the control of Pseudomonas is not a great contribution for the prevention of this fungus.

The third investigation had the objective of characterizing the susceptibility of different cherry varieties towards Calosphaeria. Within the varieties tested, there were no significant differences in terms of the aggressiveness of C. pulchella, the progression of the disease was similar in Santina and Regina; which establishes a high risk for the sustainability of the crop in Chile, since these cultivars are the most planted in Chile, together with Lapins.

A fourth essay worked on the epidemiology of this and other cherry wood pathogens, studying aerobiological patterns of the fungi, in order to detect the moments of maximum spore release. As in studies carried out abroad, the maximum levels of inoculum in the environment were detected in rainy periods, which occurred in the studied area, Ñuble, between June and October.

A fifth and final study aimed to determine the prevalence and pathogenicity of C. pulchella in cherry and peach trees. Collections were made in commercial orchards from O’Higgins to La Araucanía, from 2020 to 2022. Of all the isolates (n=853), 48% were identified as C. pulchella; 46% cherry, 38% peach and 45% nectarine. Furthermore, they were all pathogenic.

In conclusion, cancer and dieback caused by Calosphaeria pulchella is a disease that should keep us alert, its aggressiveness and difficult control call us to continue research and take extreme care in the field to avoid contagion of a pathogen that is here to stay.

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