Tree species of the maple genus, especially the widespread sycamore or sycamore maple (Acer pseudoplatanus), have up to now been considered a robust species and a good choice for cultivation. Rising temperatures and a lack of precipitation are however weakening the trees on the one hand, and favouring fungal pathogens on the other. This has resulted in the occurrence of various diseases in recent years, including some new ones. After the dry summer of 2003, there was an increase in the occurrence of Verticillium wilt in sycamores. Eutypella parasitica, the invasive pathogen that produces a canker on the main trunk of sycamore trees, was first detected in the Munich area in 2013. It is not as yet possible to estimate its damage potential. Since 2018, increasing damage has been caused in warm and dry areas by the sooty bark disease of maples Cryptostroma corticale.
Following serious diseases caused by the introduction of invasive fungi, such as the pathogens that cause Dutch elm disease and ash die-back, climate change is increasingly now also leading to severe damage in native tree species. In addition to the direct damage caused by drought and heat, there is an increase in the occurrence of fungal diseases. Trees are weakened by longer periods of drought and higher temperatures, and can as a result no longer support the physiological processes necessary to maintain their defence systems against pathogens. In addition, summer climate extremes such as those we have experienced in recent years can lead to irreversible damage that reduces the overall vitality of the trees and increases their vulnerability. This combination of a climate-induced susceptibility of the trees on the one hand, and imported exotic plant pathogens or more favourable conditions for these on the other, is currently leading to an increase in disease in the sycamore genus, particularly where they occur on unsuitable sites.
The globally widespread pathogen Verticillium dahliae has a large range of host plants, comprising several hundred species. Among woody plants, the maple genus is particularly susceptible to Verticillium wilt, but the ash, lime, elm and horse chestnut may also develop the disease. In southern Europe the pathogen is responsible for major economic damage in commercial olive cultivation in particular.
The fungus develops long-lived resting structures in the soil that can survive ten years or more in the absence of a host. Under favourable conditions, these microsclerotia germinate and can infect plants via the root hairs. Visible signs of an infection are leaf chlorosis (yellowing) and wilting of parts of the crown, which may in extreme cases lead to the death of the tree. The rather unspecific symptoms, without the formation of fruiting bodies, often make the disease difficult to diagnose. In an infection with V. dahliae, the fungus essentially blocks the tubes that carry water through the tree. In sycamore this leads to moss-green discolouration of individual annual growth rings, clearly visible in the cross-section of the wood (Fig. 2). This can be used to isolate and identify the pathogen in a fresh infection. As the disease progresses, cracks in the trunk may appear (Fig. 3), as well as endogenous cracks in the sapwood, often with several cracks appearing in a staggered pattern. The additional factor of drought stress is thought to be responsible for the development of cracks in the trunk in an infection with Verticillium dahliae. In intense summer dry periods, which may increase in future given the current climate change prognoses, it is more difficult for infected host plants to ward off an infection with V. dahliae. As well as acute infections, there may also be a symptomless chronic phase lasting several years.
In a study carried out on ten study sites, trunk necroses were found above the trunk base on a third of the sycamores. The majority of the trees had several cracks in their trunks. In older sycamores, these cracks, which do have a negative effect on the stability of the trees, often healed over after a few years. Cracks like this can however lead to a dramatic loss of value in the timber. In younger plants, Verticillium wilt can lead to the rapid dying off of trees when they are also subject to drought stress. Greenhouse studies have shown an additive effect of the combination of V. dahliae infection and drought stress. Given the predicted increase in summer droughts, we can expect more trunk damage and higher levels of mortality with existing Verticillium wilt infections in sycamores.
The Eutypella parasitica pathogen was first detected in central Europe more than ten years ago, and it has been spreading ever since. The fungus was probably imported from North America, and was first detected in Germany in 2013, on more than 100 maples in the urban area of Munich. In autumn 2017, Eutypella trunk canker was identified on maples in forest areas around Munich in the course of a monitoring programme carried out by the Department of Forest Conservation at the Bavarian State Institute of Forestry LWF). In the summer of 2019, another focus of infection affecting more than 150 trees was discovered in a small stand of mixed deciduous forest in the Oberallgäu region. When the disease occurs, it seems to be mainly sycamore (Acer pseudoplatanus) that is affected, although it has also been found in Norway maple (Acer platanoides) and field maple (Acer campestre). The risk of sycamore trunk canker continuing to spread is estimated to be very high in central Europe because of the wide natural distribution of maple species.
The trunk canker pathogen, which spreads via airborne ascospores, can enter the wood via broken-off branches and wounds, and spreads one to two centimetres per year in the wood tissue. The cankers develop relatively slowly, usually in the lower to middle sections of the trunk. The cankerous areas usually appear in elongated, slightly depressed elliptical form on the bark and lead to trunk deformation over the course of the years as the tree forms callus material (Fig. 5, 6). The fruiting bodies of the fungus do not develop on the bark until five to eight years after the initial infection. Once developed, they are a permanent source of infection. The spores are ejected into the air during precipitation and periods of high humidity. Because of the epidemiology of E. parasitica, areas with high levels of precipitation and near bodies of water are thus at greater risk.
As the molecular biological identification of the pathogen in the wood of 15 trunk cankers showed, E. parasitica can grow through the compartmentalising zones that the tree forms to isolate the pathogen again and again. The pathogen can thus continue to spread in a radial direction within the wood, as well as externally in the visible cankerous areas. The tissue then loses its functions as a result of the fungal colonisation and the tree’s response. When the canker develops over many years, and especially the lower down the trunk it is, the risk of the trunk breaking and falling rises considerably. These weakened trees require closer monitoring where there is a duty to ensure public and traffic safety.
The pathogen that causes the sooty bark disease of maple was first described in North America around 130 years ago. In Europe, Cryptostroma corticale was first discovered in England in the mid-1940s. In Germany there were reports of its occurrence in the Berlin city area as early as 1964, although it was only reported again from 2005. Since then the pathogen has been detected in many of the German federal states. The first proven cases of the disease to occur in Bavaria, detected in the summer of 2018, were limited to forest stands in the warm-dry region of Lower Franconia (Unterfranken). Since then the disease has spread further and an increase in the frequency and severity of its occurrence has been observed in both forests and urban areas. Striking at the moment is its occurrence in small islands of forest or in urban areas, where there is less mitigation of the effects of climate extremes than in extensive areas of contiguous forest. Symptoms of the disease have been found primarily in sycamore in pure and mixed stands of different age classes, but Norway maple, field maple and silver maple can also become infected.
The source of infection is considered to be the spores that form en masse when the sooty bark disease breaks out, initially under flakes of bark, later under long strips of bark that become detached from the trunk of the dying tree (Fig. 7). The pathogen probably enters the plant through branch stumps or wounds in the trunk. The first symptoms to be observed are often leaf chlorosis and wilting. As the disease progresses, deadwood develops in the crown, and there may be a proliferation of water sprouts in the lower trunk section (Fig. 8). The fungus spreads through the wood tissue and can be seen as greenish-brown discolouration in the cross-section of the timber (Fig. 9). If the fungus penetrates into the cambial areas, the blackish-brown spores are produced en masse under the bark. C. corticale is also described as a latent endophyte that can probably survive dormant in the plant for a long time, until an outbreak of the disease is triggered by dry and hot conditions.
In a project of the Department of Forest Conservation at the Bavarian Institute of Forestry (LWF), four permanent study areas have been established in stands damaged by C. corticale in the Lower Franconian region. In these mixed and pure stands, the formation of the sooty, blackish-brown spore deposits was to be observed in almost 50 percent of the sycamores in the spring of 2019. Of the infested trees with deposits of spores, 90 percent had already died. With the use of spore traps, a high load of conidiospores was determined both within and outside two of the study areas. The spores of C. corticale are mainly formed on the lower trunk and can also appear after felling. Once they have been dispersed by wind and precipitation, no new deposits of spores are formed. When carrying out forestry operations it is extremely important to bear in mind that the spores of the pathogen can pose a danger to human health when inhaled.
Due to changes in climatic conditions, we are currently seeing far-reaching disturbances of the stability and resilience of our forest ecosystems. Many pathogens are spreading more widely as a result, leading to both ecological and economic damage. The pathogen responsible for sooty bark disease in maple is causing damage in both forested and urban areas in warm-dry areas of Bavaria. As with Verticillium wilt, a disease that has been known for some time, the intensity of the damage caused by the C. corticale pathogen is favoured by heat and drought, and this may in future limit the cultivation of this tree species to certain sites. If the pathogens are already widespread as endophytes in the maple genus, future climate extremes can be expected to result in greater damage being caused by this pathogen. Another threat to the sycamore is on the other hand the imported Eutypella trunk canker that affects sycamore/maple in areas with relatively high precipitation. The fact that the parasite is new for the host (lack of co-evolution) can heighten the risk of an increase in the occurrence of this new disease, especially under favourable conditions for the fungus. If we are to meet the future silvicultural challenges posed by climate change and forest pathogens, we must pay great attention to the site requirements of each individual tree species and to the suitability of provenances when it comes to establishing new areas of forest or planting.