Introduction

In recent years, growing emphasis has been placed on increasing the area and maintenance of urban greenery. Green areas provide a wide range of environmental, social, and economic benefits to city inhabitants (Johnston and Hirons 2014). Even small green squares, thanks to the presence of healthy plants, co-create a complex system of natural elements supporting environmental processes in urbanized areas (Tzoulas et al. 2007; Shi and Woolley 2014; Karade et al. 2017). They become one of the main factors counteracting the negative effects of climate change, such as urban heat island (Livesley et al. 2016; Werbin et al. 2020) and eliminate pollution (Donovan 2017). The vegetation of green squares is a valuable natural resource with a significant contribution to biodiversity in cities (Alvey 2006; Schlaepfer 2018; Schlaepfer et al. 2020) resulted from the taxonomic diversity of plants as well as the support of other plant and animal species and their mutual interactions (Kowarik 2011). Appropriately maintained trees and shrubs are particularly valuable as the basic elements in the urban greenery structure (Stagoll et al. 2012), usually composed of many native and alien ornamental species and varieties (Nielsen et al. 2014). The role of fruit trees and shrubs is also being increasingly appreciated (Kimic 2019, 2021; Lisandru et al. 2016), as they are melliferous species providing food for insect pollinators (Hausmann et al. 2016).

Urban squares, which are public green spaces situated in or close to residential areas (Wysmułek et al. 2020), encourage people to visit them (Baur and Tynon 2010; Shanahan et al. 2015) and are gaining more importance as recreational grounds (Baur and Tynon 2010; Peschardt et al. 2012; Baur 2018). They contribute to the improvement of the physical health of city residents, as they invite to spent more time outdoor and ensure place for recreation (Aldous 2007; Baur and Tynon 2010; Pearlmutter et al. 2017), and at the same time promote various social interactions (Kazmierczak 2013). Green areas ensure contact with nature in cities (Kuo 2015; Ngiam et al. 2017; Wolf et al. 2020), which additionally improves public mental health and wellbeing (Bowler et al. 2010; Grahn and Stigsdotter 2010; Donovan 2017; Zölch et al. 2019), increases life satisfaction (Berman et al. 2012), reduces negative thinking, eliminates stress, and even alleviates symptoms of depression (Ulrich et al. 1991; Berman et al. 2012; Berto 2014). Mature and healthy trees and shrubs have high ornamental value and are important elements increasing the attractiveness of the city landscape (Troy et al. 2016), which is positively perceived by society (Tyrväinen et al. 2005). Additionally, well-maintained plants are important to ensure the safety of public greenery users (Suchocka and Kimic 2019).

Trees and shrubs, which support a number of urban ecosystem services, have a direct effect on the economic value of greenery (Seth 2003; Nowak and Dwyer 2007; Chen and Jim 2008). Failure to consider the multiple benefits provided by trees and shrubs to the functioning of cities and to their inhabitants may result in underinvestment in urban greenery (Hauer et al. 2015; Koeser et al. 2016; Donovan 2017). Modern management in that context should focus on many aspects of planning, design, maintenance, and protection as well as conservation of existing green spaces. Although the knowledge in this field has expanded considerably over the last few decades (Johnston and Hirons 2014), likewise the public awareness of the role of urban greenery (Zürcher 2017), management of urban trees and shrubs is still a huge challenge for researchers, urban greenery professionals, and city authorities due to the growing threats to green areas (Feltynowski et al. 2018).

All the aforementioned values constitute strong arguments for preservation of the good health status of urban green spaces (Baur and Tynon 2010; Pearlmutter et al. 2017). Unfortunately, it is predicted that many native tree and shrub species are likely to exhibit increasing death rates as a result of the deteriorating conditions of the urban environment, e.g., the rising air, soil, and water pollution and the escalation of extreme events as torrential rainfalls, periods of water scarcity, or thermal stress caused by periodic high temperatures and prolonged drought. These changes are associated with the progressive negative climate changes and the effects of the urban heat island, which is currently recognized as the leading cause of disturbance of urban plants (Aitken et al. 2008; Tubby and Webber 2010). Weakened trees and shrubs become more susceptible to infections by various pathogens, which reproduce and spread causing infections and further deterioration of plants’ health and their esthetic and functional values, reduction of growth rate, dieback of shoots and buds, damage to the bark, and consequent death (Marçais and Desprez-Loustau 2014). Currently, fungi are one of the most common agents of plant diseases in urban green areas, i.e., necrosis, wilt, spots, canker, and rot of various plant parts, causing various types of damage with typical symptoms. Fungal diseases generate a substantial increase in the costs of maintenance of urban greenery related to their control and removal of dead parts or even entire plants. Additionally, they raise concerns about the future of plant populations in urban green areas. Changes occurring in the ecological conditions of the urban environment have an impact on the presence and intensification of the activity of plant pathogens (Tello et al. 2005). In light of the expected global climate changes, there is a need for thoughtful management of both newly created and already existing urban green areas and their plants in the coming years (Tubby and Webber 2010). In particular, the management should be compatible with the global strategies for sustainable development, as the benefits of the presence of trees and shrubs in cities can help fulfill as many as 15 of the 17 UN 2030 Sustainable Development Goals (Turner-Skoff and Cavender 2019).

Decisions related to the management of urban greenery are crucial for the maintenance of plants and the overall urban resilience. However, in many cases, e.g., in Warsaw, there are limited numbers of investigations of fungal diseases affecting plants in urban green areas. Therefore, the aim of the study was to conduct preliminary analysis of the health of trees and shrubs growing in four squares in this city to detect the presence of fungal diseases. Since the indications for greenery management in this field are highly limited (Standardy Kształtowania Zieleni Warszawy 2016), the additional goal of the study was also to formulate main guidelines based on the various verified and recommended actions and good practices that constitute a comprehensive and sustainable strategy for integrated protection of trees and shrubs against fungal diseases. This is crucial for the health of plants growing in urban green squares.

Materials and methods

Plant materials

In 2017–2019, preliminary studies of the health status of trees and shrubs growing in four green squares located in the central districts of Warsaw were carried out:

  • Siberian Square located between Chłodna and Krochmalna Streets in Wola district (Square A),

  • Sue Ryder Square located in Ochota district, between Jan Dantyszek, Andrzej Krzycki, and Mikołaj Rej Streets (Square B),

  • Mieczysław Dawid Apfelbaum Square located in Wola district, between Pawia, Smocza, and Dzielna Streets (Square C),

  • Jan Jura-Gorzechowski Square located in Wola district, between Jan Paweł II Alley, Smocza, and Dzielna Streets (Square D).

The trees and shrubs were inventoried during field work carried out in summer 2017. A detailed list of the identified tree and shrub species with the Latin names of the species and variety (if present) is provided in the tables separately for each square with assignment of a separate number to individual trees and shrubs or groups of shrubs. The location of the identified plants in each of the green squares has been marked on a separate base map prepared in the scale of 1:500. The basic dendrometric plant parameters were measured: the trunk circumference at a height of 130 cm, the diameter of the crown, and the approximate height of trees and shrubs. The diameters and height of smaller shrubs were measured, and the surface area of shrub groups with the number of plants of a given species and their average height were determined (the data are held by the authors but not shown in this paper). This documentation served as the input material for identification of plants infected with fungal diseases. The plants (stems, shoots, leaves, fruits) were inspected for the presence of symptoms of diseases and etiological signs of pathogens. The investigations were carried out in the four seasons: autumn, winter, spring, and summer. The names of the host plants (trees and shrubs) followed the nomenclature proposed by Dolatowski and Seneta (2009). The names of the pathogens were determined based on the Index Fungorum 2017 and 2020 (the global fungal nomenclature) and the Mycobank 2017 and 2020 (an on-line database aimed as a service to the mycological and scientific community).

Mycological investigations

The mycological investigations were carried out in 2017–2019 in the Section of Plant Pathology, Warsaw University of Life Sciences—SGGW. Samples of leaves, shoots, or fruits were taken from trees and shrubs with visible symptoms of diseases.

Pathogens were identified based on their morphology using mycological keys and scientific publications (Braun 2011; Braun and Cook 2012; Majewski 1977, 1979; Marcinkowska 2012). The collected plant material was examined under an SZ11 stereoscopic microscope (Olympus). Microscopic slides were analyzed using a BX50 light microscope (Olympus). Photographs of individual fungal structures (approx. 30) were taken with the DP71 camera (Olympus) and the measurements were made in Cell F software (Olympus). Additionally, individual objects in the square were photographed.

Results

In total, 889 plants were inventoried in the four Warsaw squares during the 2017–2019 investigations. Fungal diseases were identified on the following 111 plants:

  • 68 infected plants of the 225 specimens inventoried in Siberian Square (Square A)

  • 5 infected plants of the 222 specimens inventoried in Sue Ryder Square (Square B)

  • 23 infected plants of the 203 specimens inventoried in Mieczysław Dawid Apfelbaum Square (Square C)

  • 15 infected plants of the 239 specimens inventoried in Jan Jura-Gorzechowski Square (Square D)

Powdery mildews were the largest group, as they were noted on 85 plants representing eight different tree and shrub species (Table 1). Symptoms caused by the fungus Sawadaea tulasnei (order Erysiphales) (Fig. 1 Bb, Ca, Fig. 2 Ia, Ib) were observed on A. platanoides in Square A (on 100% of plants representing this species) and in Square D (21% of this species) and on Acer tataricum ssp. ginnala (Fig. 1 Cc) in Square C (47% of this species). The powdery mildew disease caused by S. bicornis (Fig. 1 Aa) was identified on one specimen of Acer negundo tree growing in Square B (100% of plants of this species). The anamorphs of both identified fungi (S. tulasnei and S. bicornis) produced catenescent conidia (Euoidium type). Short conidiophores were observed to bear conidial chains. Macro- and micro conidiophores were observed. There were many ellipsoid-obovoid asci in the chasmothecia, usually with eight hyaline ascospores. The appendages were mainly simple and some were dichotomously branched. The appendages in S. bicornis were mainly branched, and some were trichotomously branched. The apices were tightly uncinate to circinate.

Table 1 List of representatives of trees and shrubs infected by fungal pathogens in selected green squares in Warsaw (2017–2019)
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Fig. 1
figure 1

Disease symptoms caused by fungal pathogens on plant hosts examined during survey cross-referenced to the list of infected plants in Table 1. A Acer negundo L. (Aa- Sawadaea) (Phot. AB). B Acer platanoides L.: Ba Rhytisma (Phot. MŁ), Bb Sawadaea (Phot. AS). C Acer tataricum ssp. ginnala: Ca Sawadaea, Phot. JM), Cb Pleurotus (Phot. JM). D Aesculus hippocastanum L.: Da Erysiphe (Phot. EMM). E Berberis vulgaris L.’Atropurpurea’: Ea (Phot. AS and EMM), Eb Erysiphe (Phot. EMM). F Magnolia sp.: Fa Erysiphe (Phot. EMM). G Malus x purpurea Rehd.’Ola’: Ga Podosphaera (Phot. MŁ), Gb Venturia (Phot. EMM). H Malus sp.: Ha Podosphaera (Phot. AB), Hb Venturia (Phot. AB). I Populus nigra L.’Italica’: Ia Erysiphe (Phot. MŁ), Ib Melampsora (Phot. EMM), Ic Pleurotus (Phot. MŁ). J Pyrus communis L: Ja Gymnosporangium (Phot. EMM). K Quercus robur L. ‘Fastigiata’: Ka Erysiphe (Phot. AB). L Spiraea japonica L.: La Podosphaera sp. (Phot. AS and EMM). (Authors of photographs: Ewa Mirzwa-Mróz (EMM), students: Marta Łukaszewska (MŁ), Aleksandra Borkowska (AB), Agata Skrzypczak (AS))

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Fig. 2
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Etiological signs of fungal pathogens observed during survey cross-referenced to the list of infected plants in Table 1. A Erysiphe adunca: Aa chasmothecium (on Populus nigra L.’Italica’) (Phot. MŁ). B Erysiphe berberidis: Ba chasmothecia (Phot. EMM), Bb chasmothecium with asci (magnification) (Phot. MŁ), Bc appendages (magnification) (on Berberis vulgaris L.’Atropurpurea’) (Phot. MŁ). C Erysiphe flexuosa: Ca chasmotecium (Phot. AB), Cb appendages (magnification) (on Aesculus hippocastanum) (Phot. AB). D Erysiphe magnifica: Da chasmotecium (Phot. JM), Db pseudoidium (on Magnolia sp.) (Phot. JM). E Gymnosporangium sabine: Ea spermogonia (Phot. EMM), Eb aecia (on Pyrus communis L.) (Phot. EMM). F Melampsora laricis-populina: Fa urediniospores (on Populus nigra L.’Italica’) (Phot. MŁ). G Podosphaera leucotricha: Ga mycelium with conidial stage (on Malus sp.) (Phot. EMM). H Podosphaera sp.: Ha mycelium with conidial stage (on Spiraea japonica L.) (Phot. EMM). (Authors of photographs: Ewa Mirzwa-Mróz (EMM), students: Marta Łukaszewska (MŁ), Aleksandra Borkowska (AB), Justyna Muszyńska (JM))

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The symptoms of Erysiphe alphitoides (Fig. 1 Ka) as the causal agent of powdery mildew were identified on one tree of Quercus robur L. ‘Fastigiata’ growing in Square B (100% of plants of this species). The fungus E. magnifica (Fig. 1 Fa, Fig. 2 Da, Db) was observed on all 5 magnolia trees growing in Square C (100% of plants). At the same time E. berberidis (Fig. 1 Ea, Eb, Fig. 2 Ba, Bb, Bc) was identified on all Berberis vulgaris 'Atropurpurea' plants in Square A and on 8% of B. vulgaris plants in Square D. Powdery mildews, such as E. alphitoides, E. magnifica, and E. berberidis, produced Pseudoidium-type anamorphs. In turn, their chasmothecia had several-fold dichotomously branched appendages. In each chasmothecium, there were several asci with colorless ovoid or ellipsoid ascospores. The other two species of powdery mildew with crozier-shaped secondary appendages, i.e., E. flexuosa observed on one specimen of Aesculus hippocastanum growing in Square B (12,5% of those species) (Fig. 1 Da, Fig. 2 Ca, Cb) and E. adunca identified on all trees of Populus nigra ‘Italica’ (Fig. 1 Ia, Fig. 2 Aa), also produced Pseudoidium-type anamorphs and numerous asci in the chasmothecia. However, the chasmothecia in E. flexuosa had undulate helicoid secondary appendages with closely circinate tips, whereas E. adunca had a slightly circinate apex only.

Powdery mildew caused by Podosphaera leucotricha was observed on leaves of 62,5% of apple trees (Malus sp.) growing in Square A (Fig. 1 Ga) and on 50% of this species growing in Square B (Fig. 1 Ha, Fig. 2 Ga). They were covered by a fine white coating formed by the mycelium, conidiophores, and conidia. The conidia were cylindrical–ellipsoidal and formed chains on short conidiophores (Euoidium type). As in the case of P. leucotricha observed on Malus sp., a similar type of conidial sporulation was observed on leaves of 100% of infected Spiraea japonica L. shrubs, which were covered by a white coating. Unfortunately, there were no chasmothecia of this pathogen in the coating. As shown by literature reports, this species is mainly infected by powdery mildews from the Podosphaera genus (Braun and Cook 2012; Mieslerová et al. 2020); hence, the powdery mildew agent on S. japonica plants growing in square D was identified as Podosphaera sp.

The causal agent of apple scab, i.e., the fungus Venturia inaequalis (Pleosporales) with the anamorph stage Fusicladium pomi (Fr.) Lind (syn. Spilocaea pomi Fr.), was identified on various ornamental apple species and cultivars including 97.5% of Malus x purpurea ‘Ola’ trees growing in Square A and 50% of Malus sp. trees from Square B. The disease symptoms were observed on leaves and fruits (Fig. 1 Gb, Hb). The leaves were covered by a brown-olive coating consisting of short conidiophores and pear-shaped conidia, while the fruits had small scab-like spots.

The other pathogens observed on other fruit trees (Pyrus communis) were the causal agents of rust diseases: pear rust caused by Gymnosporangium sabinae (100% of trees representing this species) (Fig. 1 Ja, Fig. 2 Ea, Eb) with orange–red spots with black spermogonia and Roestelia-type aecia (conical) developed underneath these spots. The poplar rust caused by the fungus Melampsora laricis-populina syn. Melampsora populi (Sowerby) M. Morelet were observed on 100% of P. nigra ‘Italica’ trees) (Fig. 1 Ib, Fig. 2 Fa).

Fruiting bodies of Pleurotus ostreatus were observed on only one specimen of Amur maple (A. tataricum) growing in square C and on the trunk of one tree of a Black poplar (P. nigra ‘Italica’ growing in Square A (Fig. 1 Cb, Ic).

Leaves of the all Norway maples (Acer platanoides L.) growing in Square A had large dark spots (mycelial stroma) characteristic of the fungus Rhytisma acerinum, i.e., the cause of tar spot disease of maple leaves (Fig. 1 Ba). The disease commonly affects maples in urban green areas.

Discussion

Occurrence of fungal diseases

In the present study, powdery mildews were the most common pathogens as they had optimal conditions for development on plants growing in urban agglomerations (higher air temperature than that in non-urban areas and almost always sufficient humidity at the leaf blade surface—transpiration). They have been identified based on the anamorph type, characteristics of the teleomorph (shape of appendages on the chasmothecium, number of asci in the ascocarps, and partly the size of ascospores) (Braun and Cook 2012; Marcinkowska 2012; Mieslerová et al. 2020). Symptoms of powdery mildew by the fungus S. tulasnei (order Erysiphales) observed on A. platanoides and on A. tataricum ssp. ginnala included the mycelium developed mainly on the upper side of the leaves. The morphology and sizes of fungus structures (conidia, chasmothecia, appendages, asci, and ascospores) were similar to those described in literature (Braun and Cook 2012; Marcinkowska 2012; Mieslerová et al. 2020). In Poland, this pathogen was observed on the Norway maple by Kochman (1960) and Sucharzewska (2010). In a study carried out by Sucharzewska (2010), S. tulasnei developed abundantly on the Norway maple during a dry and warm summer. A similar situation was observed in the present study, i.e., the very high temperatures with little rainfall recorded during the growing season may explain the infection of 100% of A. platanoides trees in all the analyzed squares (A, B, C, and D), which are located virtually in the center of Warsaw.

The powdery mildew disease identified on Acer negundo trees was caused by S. bicornis, whose morphological features were consistent with literature data (Braun and Cook 2012; Kochman 1986; Mieslerová et al. 2020). The pathogen formed a fine white coating on both sides of the leaves, gradually covering their entire surface. In contrast to those observed in S. tulasnei, the appendages formed on the chasmothecia were 1–threefold branched (Kochman 1986; Heluta et al. 2016). This fungus (referred to as Uncinula bicornis (Wallr.) Lév.) was observed on A. negundo in Poland in 1999 (Dynowska et al. 1999).

Powdery mildew caused by P. leucotricha observed on many Malus sp. is the second-most common apple tree disease after scab (Cimanowski 1969). In our investigations, we did not observe the teleomorph of this pathogen. As demonstrated in literature, one globoid ascus with eight hyaline ascospores develops in each chasmothecium. Secondary appendages are stiff to flexuous, with mostly simple apices (Braun and Cook 2012; Marcinkowska 2012). In Poland, as shown by Cimanowski (1969), the perfect stage does not play a major role in the development of this pathogen, as the ascospores of this fungus do not exhibit the ability to germinate and infect apple trees. The symptoms of powdery mildew and scab on ornamental apple trees are also a problem in Polish Botanical Gardens (Dziubiak 2007).

E. alphitoides, i.e., the causal agent of powdery mildew identified on a Q. robur L. ‘Fastigiata’ tree was previously classified into the genus Microsphaera due to the structure of its teleomorph (the shape of appendages and the number of asci in chasmothecia). The morphological features of the pathogen were consistent with those reported in the literature (Braun and Cook 2012; Mieslerová et al. 2020). This powdery mildew was imported to Europe from North America and spread very quickly across the continent. In Poland, it is mainly a threat to oaks grown in nurseries (Mańka 2005). In the urban environment in Olsztyn, E. alphitoides was observed each year on mature and young Q. robur plants (Sucharzewska 2009). The fungus E. magnifica was observed on all magnolia trees growing in Square C. Infections of magnolias by this pathogen were also reported from Wales (Chater and Woods 2019) and Slovakia (Ondruskova et al. 2014).

Following the description provided by Braun (2011) and Braun and Cook (2012) related to E. berberidis identified on B. vulgaris 'Atropurpurea' plants, powdery mildew symptoms were observed on B. vulgaris 'Atropurpurea' plants analyzed in Poznan (Werner and Andrzejak 2008). Given the structure of appendages and the number of asci in the chasmothecium, this fungus was previously referred to as Microsphaera berberidis (Braun and Cook 2012). The pathogen was also observed on B. vulgaris in the Czech Republic (Mieslerová et al. 2020).

The fungus E. adunca was observed on all P. nigra 'Italica' trees in Square A. Its previous name was Uncinula adunca due to morphology of appendages and the number of asci in the chasmothecium (Braun and Cook 2012; Mieslerová et al. 2020). It is a common pathogen of the black poplar in Poland (Mułenko et al. 2008). The morphological features of this fungus were in line with the description provided by Braun and Cook (2012) and (Marcinkowska 2012).

Chestnut powdery mildew caused by E. flexuosa was identified in chestnut trees growing in Poland. This pathogen attacks both A. hippocastanum and A. x carnea (Piątek 2002; Zimmermannová-Pastircáková et al. 2002; Werner and Gołębniak 2010) and exhibits a clear preference for the latter host (Sucharzewska et al. 2018). In the present study, this pathogen was observed only on selected A. hippocastanum trees growing in Square B.

Apple scab caused by V. inaequalis (Pleosporales) which was identified according to Kochman (1986) and Marcinkowska (2012) on Malus x purpurea and other specimens of Malus sp. is one of the most dangerous diseases of those species (Bowen et al. 2011; Marcinkowska 2012), especially in areas where apples trees are grown commercially (MacHardy 1996; Bowen et al. 2011). As in the present study, a high prevalence of the disease was noted in Malus x purpurea in Poznan (Werner and Andrzejak 2008).

The fungus G. sabinae causing pear rust was identified on the basis of the structure of aecia and aeciospores (Majewski 1979; Marcinkowska 2012). The fungus which was noted on all pear trees in Square C, was also observed in Poland on various species representing the genera Pyrus and Juniperus by Werner and Gołębniak (2008). Only spermogonia and aecia were found to develop on the pear trees, as the pear is an intermediate host of pear rust. The thelia of this pathogen develop on its thelial host, i.e., Juniperus sabina L. (Kochman 1986; Marcinkowska 2012).

The rust fungus M. laricis-populina (Pucciniales) was noted on all black poplar trees growing in Square A. The morphology and average size of its urediniospores (38.3 × 14.5 µm) were characteristic for this species of rust (Klebahn 1902; Pei and McCracken 2005).

P. ostreatus which was detected on black poplars in Square A and Amur maples in Square C, was identified based on the structure of its fruiting body, i.e., the cap, hymenophore, and stipe. It is an edible fungus with fleshy annual shelving conks, sessile or with a short eccentric stalk (Pavlik and Pavlik 2013; Grzywacz and Staniszewski 2003; Szczepkowski 2012). P. ostreatus is a saprotrophic fungus accelerating wood decomposition (Pavlik 2005).

The fungus R. acerinum which was recognized on all A. platanoides trees growing in Square A, was also observed in green areas in Poznan as well (Werner and Andrzejak 2008). As shown by Łuszczyński (2002) and Kosiba (2007), the fungus can be used as an indicator of air pollution, as it is highly susceptible to the content of sulfur dioxide (SO2) in the air. Since all the A. platanus trees showed symptoms of this disease, we can suppose that the air in the area of this Square is not polluted with SO2.

Compared to the observations made by the authors in previous years (unpublished data), relatively few cases of diseases of the trees and shrubs growing in the squares were observed in the 2017–2019 study. Most likely, this was related to the high temperatures recorded during the growing season and the long period of drought.

Management recommendations

Management of urban greenery is associated with the necessity to make a number of difficult and often costly decisions, as the maintenance of trees and shrubs is expensive. Additionally, progressive deterioration of the greenery health status and failure to maintain plant fitness are accompanied by an increase in financial expenditures (Nowak and Dwyer 2007). The deterioration of urban plant health caused by fungal diseases is a resultant of the combined effects of negative climate changes and human activity, and modifications of the natural environment contribute to faster growth of pathogens (Fisher et al. 2012). This tends to increase the expenses associated with the maintenance of green areas, and each financial decision must be justified (Hauer et al. 2015).

The management of urban greenery is a process that begins before planting and continues throughout plant life. The range of management activities that must be adjusted to the health status of trees and shrubs expands with the intensification of diseases, especially those caused by fungi. At every stage of plants’ life (immature to semi-mature, mature, senescent), care treatments can significantly improve their health status, and thus increase the benefits of green areas to the city and its inhabitants (Vogt et al. 2015). In the case of fungal diseases, the recommended comprehensive management of greenery is based on solutions minimizing the spread and negative effects of infections caused by fungal pathogens. The main points of management recommendations are listed in Table 2.

Table 2 Management recommendations for trees and shrubs growing on green squares in Warsaw
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Conclusion

The importance of trees and shrubs growing in urban green spaces, even in small green squares, is likely to increase in following years, given the benefits they bring to cities and their inhabitants. The negative phenomena related to climate change, resulting in the deterioration of plants, may predispose trees and shrubs to the incidence of fungal diseases, which will further worsen their health. Such a scenario is confirmed by the results of the preliminary studies of fungal diseases affecting trees and shrubs growing in four squares in Warsaw carried out in 2017–2019. These investigations were conducted in very warm and dry spring and summer periods. Although the conditions were unfavorable for the spread of pathogens, Sawadaea tulasnei, S. bicornis, Podosphaera leucotricha, Podosphaera sp., Erysiphe alphitoides, E. magnifica, E. berberidis, E. flexuosa and E. adunca (causal agents of powdery mildew), Gymnosporangium sabinae, Melampsora laricis-populina (rusts), Venturia inaequalis (apple scab), Rhytisma acerinum (tar spot of maple), and Pleurotus ostreatus (oyster mushroom) were identified. They are common fungal pathogens in these urban green areas.

However, they had substantially lower intensity than in typical years and attacked a smaller number of trees. Favorable conditions promoting the development of fungal diseases may support faster growth and spread of pathogens, which in turn will become a major threat to plant health. Therefore, it is necessary to continue and expand research aimed at diagnosis of fungal diseases on trees and shrubs in urban green areas to ensure early identification of emerging threats posed by fungal pathogens and to take comprehensive measures to eliminate these disease causal agents. Protection of plant health against the effects of fungal diseases requires conscious management of urban greenery. The formulation of guidelines based on the good practice of multifaceted plant monitoring and application of the recommendations in a comprehensive and sustainable strategy for counteracting fungal diseases will help to preserve more healthy trees and shrubs in urban green squares and other green areas.

Author contribution statement

KK, EMM and MSS contributed to the conception and design of the study. Plant inventory in selected green squares was conducted by KK. Recognition of fungal diseases symptoms on plants during site visits were performed by EMM and MSS. Plant materials collection and selected photographic documentation of pathogens were taken by students under the supervision of EMM and MSS. Data analysis was performed by all authors. Literature review and recommendations were performed and developed by KK, and supplemented by EMM and MSS. The first draft of the manuscript was written by KK and developed together with EMM and MSS. All authors read and commented the manuscript.