Until a few years ago, the fungal disease Glomerella Leaf Spot (GLS) was limited solely to subtropical apple farming areas in South America, the southeast of the US, and in China. This disease was detected for the first time in the Italian region of Emilia-Romagna in 2019. Symptoms of the disease were also found for the first time in orchards in South Tyrol in 2020. The Phytopathology Working Group at Laimburg Research Centre was able to identify and detect the pathogen within a very short time (Oettl et al. 2021).

Glomerella Leaf Spot (GLS)

In early autumn 2020, a greater presence of necrotic leaf spot was observed in single orchards in South Tyrol (Fig. 1). Shortly after, a pronounced chlorosis developed on the leaves (Fig. 2) and, in affected orchards, wide-scale early leaf fall began. Almost at the same time, brown-reddish marks the size of a pin head began to appear on the fruit (Fig. 3), some of which were slightly pitted. A purple ring (also known as a halo) was often visible around the infection site (Fig. 4). No more than a month after the appearance of the first leaf spots, the trees were almost entirely stripped bare of leaves (Fig. 5). Although the symptoms were vaguely like those of the Alternaria infection, such a rapid progression of symptoms was hitherto unknown in South Tyrol. The Phytopathology Working Group at Laimburg Research Centre immediately performed detailed laboratory tests to identify the cause of this damage.

Microbiological and biomolecular identification of the pathogen

For the purpose of isolating the pathogen, the phytopathology laboratory of Laimburg Research Centre took tissue samples from the necrotic marks on the leaves and fruit, incubating them on specific culture medium. After a few days, the majority of the samples showed an even fungal morphology (Fig. 6). Microscopic analysis of the fungal spores assigned the isolates to the genus Colletotrichum sp. Sequencing of a specific DNA region, i.e., the genetic information of the fungus led to a preliminary assignment to the Colletotrichum gloeosporioides species complex (CGSC). With further specific laboratory techniques and a phylogenetic analysis with known species, it was possible to unequivocably identify the microorganism species present in South Tyrol: Colletotrichum chrysophilum.

Increased presence of Glomerella Leaf Spot (GLS) in South Tyrolean orchards

After the initial appearance of GLS in South Tyrol in 2020, in 2021, no cases of the disease were recorded in South Tyrol. In 2022, only one extremely limited, localised GLS case was recorded in orchards where the disease had already been identified two years before. However, during late summer and early autumn 2023, a large-scale spread of GLS was observed, together with a notable expansion of the affected area. The disease was observed in valley floor plots from Terlano/Terlan to Merano/Meran. Nonetheless, there were also isolated cases in the area of Bolzano /Bozen, in Oltradige and the South Tyrolean Unterland. Infestation levels varied significantly: from a few spots per fruit and tree through to plots where almost all of the fruit trees showed symptoms. Field observations saw that symptoms were more visible on Gala, Granny Smith and Cripps Pink (cpvr)-Rosy Glow(cpvr)-Sekzie(cpvr)/Pink Lady® and Golden Delicious varieties, but spots were also found on CivM49(cov)/RedPop®, MC 38(cov)/Crimson Snow®, Ipador(cov)/Giga® fruit and other varieties. In literature, descendants of Golden Delicious are described as susceptible to GLS, while varieties derived from Red Delicious or Fuji are considered to be tolerant.

The growing areas of non-European apples, which already have a greater experience with GLS, recorded that during storage, GLS marks on fruit remain limited to the infection site and that they do not develop further during cold storage (provided that the apples are stored immediately at low temperatures). The first preliminary trials in South Tyrol confirm this experience. A possible spread of the spots on fruit during conservation and shelf life of the 2023 harvest is currently the subject of scientific research. More detailed results are expected after the 2023 crop has been removed from cold storage.

Statute for GLS research in Europe

Although GLS has been present in some growing regions outside Europe for several decades now, there is no scientifically reliable information on infection conditions or local epidemiology, due to the limited spread in Europe thus far. However, reports of GLS detection in autumn 2023 in some apple growing areas in Italy, including Emilia-Romagna, Piedmont and - occasionally - South Tyrol, indicate an increasing spread of the disease. The causes for this increase during the growing season 2023 can only be hypothesised until the data can be conclusively analysed; however, there is a possibility that heavy rainfall in summer, followed by temperatures that are constantly above the average for the year, have had a significant effect on the incidence of the disease.

These climate conditions may have contributed to the fact that in late summer 2023, apple bitter rot (ABR) was observed for the first time in South Tyrol (Fig. 7). Like GLS, this disease is also caused by different species of Colletotrichum and can be seen as an evident rotten spot on apples in the field.

Although GLS and ABR can be caused by the same fungal pathogens, as current knowledge stands, these are two different pathologies that can also occur at the same time in the same plot of land but following a different epidemiology.

GLS Research at Laimburg Research Centre

Intensive research activities began at Laimburg Research Centre immediately after the first identification of GLS in 2020. The Phytopathology Working Group isolated the GLS fungus from leaves and fruit, rapidly identifying the Colletotrichum chrysophilum species. This was followed by scientific proof that this species is responsible for the disease (Deltedesco & Oettl, 2022). Precise identification of the species is a complex issue, but it is also essential in the development of targeted control strategies. Indeed, other research programmes outside Europe have shown that fungicide effectiveness depends on the Colletotrichum species to a notable extent. The first measures to control GLS in the open field, were performed by the Evaluation of Plant Protection Products working group at Laimburg Research Centre, during the 2022 and 2023 growing seasons: the preliminary analysis already provided indications on possible strategies. The Organic Farming working group at Laimburg began to perform a range of treatment tests in the field.

Tests were also performed on the susceptibility of fruit to infection with C. chrysophilum on 24 apple varieties grown in South Tyrol (data analysis is ongoing). As well as this, a method was developed to artificially infect apple seedlings in the greenhouse, so as to be able to simulate the different infection conditions in vivo (an activity required for correct, detailed scientific knowledge of the disease).

Further experiments performed by various working groups at Laimburg Research Centre are currently at the planning stage and will be implemented in the near future, also in collaboration with national and international research institutes.

Cooperation with other research institutes

Close collaboration between Laimburg Research Centre and the South Tyrolean Extension Service for Fruit- and Winegrowing is also extremely important for this emerging fungal disease. Moreover, since the first appearance of GLS, there has been a lively exchange with research and consulting staff in other apple production areas. For GLS too, a multidisciplinary approach is essential: the aim must be to combine strengths and work together to combat this challenge in the apple growing sector.