Glomerella leaf spot

Until a few years ago, the Glomerella Leaf Spot (GLS) disease was restricted to some subtropical apple-growing regions in South America, the south-east of the USA and China. In 2019, the fungal disease was detected for the first time in the Italian region of Emilia-Romagna. In 2020, symptoms of the disease were also observed for the first time in South Tyrolean apple orchards. Within a very short time, the Phytopathology working group at the Laimburg Research Centre succeeded in detecting and identifying the causal agent of the disease (Oettl et al., 2021).


The disease Glomerella Leaf Spot (GLS)

In the early fall of 2020, an increased occurrence of necrotic leaf spots (dead leaf tissue) was observed in some apple orchards in South Tyrol (Fig. 1). A pronounced yellow discoloration of the leaves (chlorosis, Fig. 2) also developed very quickly, and massive, premature leaf fall started in the affected orchards. Almost at the same time, pinhead-sized, reddish-brown spots appeared on the fruit (Fig. 3), some of which were slightly sunken. A purple-coloured ring (also known as a halo) was often observed around the infection site (Fig. 4). This was followed by almost complete defoliation of the affected trees within a month after the first leaf spots appearing (Fig. 5). Although the symptoms vaguely resembled an infection with Alternaria, such rapid symptom development was previously unknown in South Tyrol. The Phytopathology working group at the Laimburg Research Centre immediately carried out extensive laboratory tests to get to the bottom of the cause of this damage.


Microbiological and genetic identification of the pathogen

In the Phytopathology laboratory at the Laimburg Research Centre, tissue samples were taken from necrotic leaf and fruit spots and incubated on specific culture media in order to isolate the pathogen. After a few days, fungal growth with uniform morphology was recognizable in most of the samples (Fig. 6). The isolates were assigned to the genus Colletotrichum sp. by light microscopic examination of the fungal spores. The sequencing of a defined DNA region of the fungus led to a preliminary assignment of the pathogen to the Colletotrichum gloeosporioides species complex (CGSC). Through further, extremely specific laboratory techniques and phylogenetic analyses with previously described Colletotrichum species, the fungal species causing the GLS damage in South Tyrol was determined: it is Colletotrichum chrysophilum.


The increased occurrence of Glomerella Leaf Spot (GLS) in South Tyrolean apple orchards

After the first occurrence of GLS in South Tyrol in 2020, there was no disease incidence in South Tyrol in 2021. In 2022, occurrence of GLS was observed only extremely limited in the apple orchards where the disease had already been detected two years earlier. In contrast, the late summer and autumn of 2023 were characterized by a massive occurrence of the disease and a considerable expansion of the affected area: GLS was observed in several orchards in the bottom of the valley from Terlan/Terlano to Meran/o. However, there were also isolated cases in the Bozen/Bolzano area, the Überetsch/Oltradige and the Unterland/Bassa Atensina. The level of infestation varied considerably from a few spots per fruit and plant to isolated orchards in which almost 100 percent of the fruit showed symptoms. In the field observations, the symptoms were most visible on Gala, Granny Smith and Cripps Pink(cpvr)-Rosy Glow(cpvr)-Sekzie(cpvr)/Pink Lady® and Golden Delicious, but fruit spots were also found on CivM49(cov)/RedPop®, MC 38(cov)/Crimson Snow®, Ipador(cov)/Giga® and other varieties. In the literature, descendants of Golden Delicious are described as susceptible to GLS, whereas varieties derived from Red Delicious or Fuji are considered tolerant.

Apple-growing regions overseas, which already have more experience with GLS, have reported that GLS fruit spots remain limited to the original infection site during storage and that no further development of the spots takes place in the cold store or shelf life, provided the apples are stored promptly at lower temperatures. Initial preliminary trials in South Tyrol confirm this experience; more in-depth findings can be expected once the trials from the 2023 harvest have been removed from storage.


State of research on GLS in Europe

Although GLS has been present in subtropical overseas apple growing regions for some time, there is no scientifically reliable information on local infection conditions and epidemiology due to its limited occurrence in Europe to date. However, reports of GLS detections from other Italian apple-growing regions such as Emilia Romagna, Piedmont and occasionally from Trentino in autumn 2023 indicate a further spread of the disease. And while only hypotheses can be made about the causes of the increased incidence in the 2023 growing season until the concluding data evaluation, it can be assumed that the massive rainfall events in late summer, followed by consistently high temperatures, had a significant influence on the incidence of infection.

These weather conditions may have contributed to the fact that in late summer 2023, apple bitter rot (ABR) was also observed in the field for the first time in South Tyrol (Fig. 7). Like GLS, this disease is caused by various Colletotrichum species and develops severe fruit rot in the field.

Although GLS and ABR can be caused by the same fungal pathogen, according to current knowledge they are considered as two different diseases. Nonetheless, both GLS and ABR can occur simultaneously in the same orchard but have a different epidemiology.


Research activities on GLS at the Laimburg Research Centre

Immediately after the first detection of GLS at the Laimburg Research Centre in 2020, intensive scientific activities started: the Phytopathology working group isolated the fungus from the leaf and fruit spots and identified the species Colletotrichum chrysophilum within a very short time. This was followed by scientific proof that this species is the causal agent of the disease (Deltedesco & Oettl, 2022). The exact identification of the species is time-consuming, but essential in order to develop targeted control strategies: Indeed, other research showed that the efficacy of fungicides is significantly dependent on the several Colletotrichum species. In vitro screenings on the effectiveness of fungicides, both for integrated cultivation and organic production methods, are currently being performed in the laboratory. The first trials in the field were carried out by the Laimburg Research Centre’s working group for Testing Plant Protection Products in the 2022 and 2023 growing seasons, and the preliminary evaluation has already provided indications of possible strategies. The Organic Farming working group at the Laimburg Research Centre has also carried out various treatment trials in the field.

Tests on the susceptibility of fruit to an infection with C. chrysophilum were carried out on 24 apple varieties grown in South Tyrol (data currently being analyzed). In addition, a method for the artificial infection of apple seedlings in the greenhouse was developed so that various infection conditions can be simulated in vivo.

Possible changes or developments in GLS fruit spots during storage and shelf life with fruit from the 2023 harvest are currently under scientific investigation. A whole series of further trials by the various working groups at the Laimburg Research Centre are being planned at the moment and will be implemented timely - also in cooperation with national and international research institutions.


Cooperation and scientific exchange with other research institutions

The close cooperation between the Laimburg Research Centre and the South Tyrolean Extension Service for Fruit and Wine Growing is also a matter of course for this newly emerging fungal disease. In addition, since the first detection there has been a lively exchange with scientists and advisors from other apple-growing regions. A joint approach to research and testing issues is also essential for GLS: the aim is to join forces and work together to counteract this further challenge for apple cultivation.

Abb. 1 Fig. 1: The first symptoms of GLS are purple-brown necrotic leaf spots.
Abb. 2 Fig. 2: At an advanced stage, chlorosis of the leaves may become more pronounced.
Abb. 3

Fig. 3: The typical GLS fruit spots have a diameter of approx. one to three millimetres, are reddish-brown and can be slightly sunken, here on the variety Golden Delicious.


Abb. 4 Fig. 4: Before developing the red top colour, a purple ring (halo) often appears around the infection site, here on the variety Rosy Glow (cpvr).
Abb. 5 Fig. 5: Within a few weeks of the appearance of the first leaf spots, a premature but almost complete defoliation of apple trees is typical of GLS.
Abb. 6 Fig. 6: An isolate of Colletotrichum chrysophilum on culture medium.
Abb. 7 Fig. 7: In autumn 2023, apple bitter rot (ABR), also caused by Colletotrichum species, was also observed for the first time in the South Tyrolean apple orchards.