Introduction

Diplodia (Dothideomycetes, Botryosphaeriales, Botryosphaeriaceae) is a large genus introduced by Montagne in 1834 (Montagne 1834) with Diplodia mutila as the type species. For many years species of Diplodia were defined only on the basis of host association, which resulted in a proliferation of species names. Since host is no longer considered to be of primary importance in Diplodia species differentiation, many of the names listed in Mycobank (www.mycobank.org) are likely to be synonyms (Slippers et al. 2004). However, the advent of molecular tools for the taxonomy revealed phylogenetic relationships among species of the family Botryosphaeriaceae and helped to solve the identification issues (Phillips et al. 2005).

Species of Diplodia have a cosmopolitan distribution and have been reported as pathogens, endophytes and latent pathogens of a variety of woody and non-woody hosts (Damm et al. 2007; Slippers and Wingfield 2007; Fabre et al. 2011; Linaldeddu et al. 2014; Sosnowski et al. 2021; Del Grosso et al. 2024). In particular, some members of this genus are recognized as aggressive phytopathogens and they can affect plants at different age inducing symptoms including rot, dieback, canker and leaf spots in economically important forest and horticultural plants (Úrbez-Torres et al. 2008; Masi et al. 2018a; Bhat et al. 2023; Del Grosso et al. 2024). Among them, Diplodia corticola is one of the most studied Diplodia species due to its involvement in oak decline all over the world (Evidente et al. 2007; Félix et al. 2017; Smahi et al. 2017; Ferreira et al. 2021). Similar to most species in the family Botryosphaeriaceae, Diplodia species lack host specificity and have a proven capability to colonize and cause disease in diverse native and introduced plant hosts. For instance, Diplodia sapinea and D. corticola, which have been consistently associated with certain hosts, have occasionally been found to occur in other unrelated hosts in different regions of world (Batista et al. 2021). However, despite its relevance, we are still far from understanding the mechanisms behind host jumps and the worldwide extension of host association (Silva-Valderrama et al. 2024).

Many symptoms of plant diseases with direct involvement of Diplodia spp. can be attributed to the production of toxic secondary metabolites. In fact, although not essential for their primary metabolic processes, fungi produce various secondary metabolites with several biological activities possibly related to survival functions of the microorganism, such as competition and symbiosis (Bills and Gloer 2016; Salvatore et al. 2020b). Hence, secondary metabolites from Diplodia spp. have been widely investigated by natural product chemists which proved the production of a variety of low molecular weight metabolites with different bioactivities (e.g., antibacterial, antifungal, cytotoxic, phytotoxic, zootoxic).

This review aims at providing up-to-date information of metabolites produced by various fungal species currently belong to the genus Diplodia and their bioactivities.

Production of secondary metabolites

From the analysis of the existing literature, a total of 56 chemically characterized compounds were listed as products of Diplodia spp. (Table 1). These fungal products belong to different classes of natural products, including bisfuranoids, cyclohexanones, dihydrofuranones, dihydrofuropyran-2-ones, macrolides and α-pyrones. However, considering the great interest of researchers, the current number and variety of low molecular weight compounds produced by this fungal genus are expected to further increase in the coming years. The list of secondary metabolites reported in Table 1 contains both known metabolites and compounds which have been first characterized from Diplodia spp. According to different structural characteristics, the compounds reported in this review have been divided into 13 classes.

Table 1 List of secondary metabolites produced by Diplodia spp. gathered from the literature
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So far, 11 species of the genus Diplodia have been examined for the production of secondary metabolites (Table 2). Due to their relevance in the dynamics between host and pathogen, many studies are focused at extending the knowledge on the secondary metabolites produced by pathogenic strains of Diplodia spp. isolated from symptomatic plants (i.e., oak, grapevine, cypress). In fact, their ability to produce in vitro bioactive compounds may provide useful information regards pathogenesis process and chemotaxonomic implications. Many compounds reported in Table 1 could be considered specialized metabolites because they are often restricted to Diplodia genus suggesting a possible dedicated function related to the physiological, social, or predatory state of the fungus.

Table 2 Occurrence of secondary metabolites in Diplodia species
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These metabolites could also play a crucial role in the endophytic associations of the fungus with plants mediating communication, nutrient acquisition, and defence. Although Diplodia species can establish endophytic associations with plants (Slippers and Wingfield 2007; Phillips et al. 2012), this fungal genus has primarily been studied as a plant pathogen. Therefore, little is known on the occurrence and functions of secondary metabolites produced by endophytic strains of Diplodia spp.

It can be noticed from Table 2 that most of the investigated Diplodia spp. were isolated from the Mediterranean regions, with few exceptions.

Bisfuranoids

Bisfuranoids are a polyketide group widely distributed in nature essentially as microbial metabolites produced by different fungal species. In fact, this group of natural products is largely described from Aspergillus species which produce some representative bisfuranoids such as aflatoxins (Bennett and Lee 1979; Hsieh and Atkinson 1990). The chemical structure of bisfuranoids consists of two furan rings joined through one side. Diplobifuranylones A and B (1–2) are the first examples of monosubstituted natural bisfuranoids with the furan rings joined through a bond and, until their characterization, this chemical structure is only known for synthetic compounds (Langer et al. 2002; Chiusoli et al. 2003; Behr et al. 2004). Diplobifuranylones A and B were characterized, using spectroscopic and chemical methods, as two diastereomeric 5′-(1-hydroxyethyl)-3,4,2′,5′-tetrahydro-2H-[2,2′]bifuranyl-5-ones (Evidente et al. 2006a) (Fig. 1). Diplobifuranylones A–D (1–4), spanning four papers from the same research group, were exclusively reported as products of a phytopathogenic strain of D. corticola isolated from stem canker of declining cork oak (Q. suber) trees in Sardinia (Italy) (Evidente et al. 2006a; Cimmino et al. 2016; Masi et al. 2016; Salvatore et al. 2022). The absolute configuration (AC) of diplobifuranylones A–C (1–3) was recently determined employing electronic and vibrational circular dichroism (ECD and VCD) and optical rotatory dispersion (ORD) analysis compared with density functional theory computations. In particular, the AC of diplobifuranylones A–C has been assigned as (2S,2′S,5′S,6′S), (2S,2′R,5′S,6′R), and (2S,2′S,5′R,6′R), respectively, also applying the Mosher’s method to define the absolute configuration of the carbinol stereogenic carbon (Mazzeo et al. 2017).

Fig. 1
figure 1

Chemical structures of bifuranylones (1–4): diplobifuranylones A–D

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Cyclohexenones

Cyclohexenones are polyketides constituted by cyclohexendiole which chemical structures often contain an epoxide subunit, as in the case of epi-epoformin (7), epi-sphaeropsidone (8) and sphaeropsidone (9) produced by Diplodia spp. (Evidente et al. 2011a, 2012; Tuzi et al. 2012; Andolfi et al. 2014a; Masi et al. 2022) (Fig. 2). Two chlorinated cyclohexenones, named chlorosphaeropsidone (5) and epi-chlorosphaeropsidone (6), were isolated from the organic extracts of the in vitro culture filtrates Diplodia cupressi (Evidente et al. 2000, 2011a). Their structures and configurations were confirmed by X-ray analysis while the absolute stereochemistry was deduced by circular dichroism and NMR. Even if the authors cannot exclude that these compounds could be artefacts, they remark that their data strongly suggest the natural origin of these chlorinated cyclohexenones which could be generated from sphaeropsidone by an enzymatic opening of the epoxy ring. In addition, these compounds contain a carbon skeleton also found in other closely related fungal metabolites which supports the deduction that these compounds were produced by the fungus (Miller 1968; Sakamura et al. 1975; Nagasawa et al. 1978).

Fig. 2
figure 2

Chemical structures of cyclohexenones (5–9): chlorosphaeropsidone, epi-chlorosphaeropsidone, epi-epoformin epi-sphaeropsidone, sphaeropsidone

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Dihydrofuranones

Nine new dihydrofuranones were obtained from culture filtrates of phytopathogenic strains of different species of Diplodia (Evidente et al. 1999, 2006a, 2007; Masi et al. 2016, 2021, 2022; Salvatore et al. 2022). Dihydrofuranones are closely related to butenolides and tetronic acids, which are well known as plant, fungal, and lichen metabolites (Dean 1963; Kim and Ryu 2003; Schobert and Schlenk 2008; Hoshino et al. 2015) and, biosynthetically, they are produced by a polyketide pathway (see next section). Commonly, the dihydrofuranone moiety of these compounds is 4-monosubstituted, while the side chain shows markedly differences among the members of this class. Pinofuranoxins A and B (13 and 14) represent exceptions because their chemical structures are characterized by trisubstituted dihydrofuranones (Fig. 3). In addition, these compounds produced by D. sapinea are the first dihydrofuranones in which the presence of an epoxy moiety is reported (Masi et al. 2021). Their relative and absolute configurations were also assigned by NOESY experiments and computational analyses of electronic circular dichroism spectra (Masi et al. 2021).

Fig. 3
figure 3

Chemical structures of dihydrofuranones (10–18): diplofuranones A and B, diplofuranoxin, pinofuranoxins A and B, sapinofuranones A and B, (S,S)-sapinofuranone B, sapinofuranone C

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Sapinofuranone A (15) was isolated from liquid cultures of D. sapinea along with its C-5 epimer sapinofuranone B (16) (Evidente et al. 1999). This compound possesses two adjacent stereocenters in a 10-carbon skeleton and the absolute configuration at C-5 was determined by applying the Mosher's method. However, the full stereochemical elucidation of sapinofuranone A and related compounds was subsequently acquired by stereoselective synthesis and chemical correlation to other known natural products (Cimino et al. 2002; Nagi Reddy et al. 2016). The AC of sapinofuranone C (18) was determined by X-ray analysis combined with the advanced Mosher’s method and confirmed by measured and calculated ECD and VCD spectra of its di-p-bromobenzoate derivative (Masi et al. 2016; Mazzeo et al. 2017). The relative and absolute configuration of diplofuranoxin was determined by joining NOESY NMR experiments and computational analysis of electronic circular dichroism spectrum (Masi et al. 2022).

Dihydrofuropyran-2-ones

Dihydropyran‐2‐ones are polyketides characterized by the presence of an α, β unsaturated‐δ‐lactone produced by an extensive range of natural sources (Dickinson 1991; Collett et al. 1998). Afritoxinones A and B (19, 20) were isolated from liquid cultures of Diplodia africana, a fungal pathogen responsible for branch dieback of Phoenicean juniper in Italy (Evidente et al. 2012) (Fig. 4). Their structures were established by spectroscopic and optical methods and determined to be as (3aS*,6R*,7aS)-6-methoxy-3a,7a-dihydro-3H,6H-furo[2,3-b]pyran-2-one and (3aR*,6R*,7aS)-6-meth-oxy-3a,7a-dihydro-3H,6H-furo[2,3-b]pyran-2-one, respectively. In addition, the identification of several known metabolites was reported in the same paper, including the dihydrofuropyran-2-one oxysporone (21). Oxysporone was identified for the first time from Fusarium oxysporum in 1979 (Adesogan and Alo 1979), but its absolute configuration was subsequently assigned by computational analysis of its optical rotatory dispersion (ORD) and electronic circular dichroism (ECD) spectra (Mazzeo et al. 2013).

Fig. 4
figure 4

Chemical structures of dihydrofuropyran-2-ones (19–21): afritoxinones A and B, oxysporone

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Hydroxyfatty acids

Hydroxylinolenic and hydroxylinoleic acids (22, 23) were detected in mycelial extracts of pathogenic strains of D. corticola isolated from Quercus spp. (Salvatore et al. 2023). Fatty acids and their derivatives are common constituents of the fungal mycelium with an important role in the biomass and structures of microbial communities (Vestal and White 1989; Salvatore et al. 2018, 2020a). Moreover, fatty acids and hydroxyfatty acids could be indirectly involved in several biological functions representing the starting material for many secondary metabolites (Salvatore et al. 2020a). A dihydroxydecadienoic acid methyl ester named sapinofuranone D (24) was isolated for the first time from a phytopathogenic strain of D. corticola (Masi et al. 2016) (Fig. 5).

Fig. 5
figure 5

Chemical structure of the hydroxyfatty acid sapinofuranone D (24)

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Isochromanones

Fraxitoxin (25) is an isochromanone isolated together with (R)-mellein from liquid cultures of Diplodia fraxini. Mellein, the 3,4-dihydro-derivative of 8-hydroxy-3-methylisocoumarin (Reveglia et al. 2020), is the parent compound of a subclass of isochromanones characterized by an isocoumarin skeleton with a lactonic α-pyrone ring fused to the benzene ring at 5,6 positions. It was first isolated from Aspergillus melleus in 1933 with R configuration at C-3 (Nishikawa 1933). Isocoumarins are lactonic compounds abundantly produced by Botryosphaeriaceaus fungi (Salvatore et al. 2020a, 2021) and even some species of the genus Diplodia are producers of (3R,4R)-, (3S,4R)-hydroxymelleins and (R)-mellein (26–28) (Cabras et al. 2006; Evidente et al. 2006a, 2012; Cimmino et al. 2017; Reveglia et al. 2019; Di Lecce et al. 2021; Masi et al. 2022; Salvatore et al. 2022) (Fig. 6).

Fig. 6
figure 6

Chemical structures of isochromones (25–28): fraxitoxin, (3R,4R)-, (3S,4R)-4-hydroxymellein and (R)-mellein

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Macrolides

Macrolides are a large family of compounds characterized by a macrocyclic lactone ring with generally 12, 14, or 16 membered (Dewick 2016). Four macrolides (29–32) with a 10 membered lactone in their structures were isolated from cultures of a plant pathogenic strain of D. sapinea (Wada and Ishida 1976; Wada and Tatsuyoshi 1979) (Fig. 7) which are the first members of open-chain pentaketides. Considering that the construction of the macrocyclic core of any natural product having a macrocyclic lactone unit is a challenging task in synthetic organic chemistry, extensive studies have been recently reported on the total synthesis of these biologically important substances (Tsuji and Mandai 1978; Acta et al. 1980; Anand et al. 2003; Joyce and Makhdoum 2007; Cheng et al. 2019).

Fig. 7
figure 7

Chemical structures of macrolides (29–32): diplodialides A–D

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4-Naphthoquinones

1,4-Naphthoquinones are common metabolites, structurally related to naphthalene, which are produced by plants, animals, fungi and bacteria (Nematollahi et al. 2012; Aminin and Polonik 2020; Navarro-Tovar et al. 2023). The core 1,4-naphthoquinone skeleton is comprised of a benzene ring fused to a fully conjugated cyclic diketone with para-oriented carbonyl groups. Diploquinones A and B (33 and 34) are two tetrasubstituted 1,4-naphthoquinones isolated together with the known vanillic acid from D. mutila, a grapevine pathogen involved in Botryosphaeria dieback in Australia (Reveglia et al. 2018b) (Fig. 8). Diploquinones A and B were characterized as 6,7-dihydroxy-2-methoxy-5-methylnaphthalene-1,4-dione and 3,5,7-trihydroxy-2-methoxynaphthalene-1,4-dione using spectroscopic methods. Diploquinone B is a structural isomer of the trihydroxymethoxy-1,4-naphthaquinone which was previously isolated from the phytopathogenic fungus Cercospora melonis (Assante et al. 1977).

Fig. 8
figure 8

Chemical structures of 1,4-naphthoquinones (33, 34): diploquinones A and B

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Phenol derivatives

Besides their previous detection as products of other fungal species, diorcinol (35) and 4-hydroxyscytalone (36) were detected for the first time as Diplodia metabolites in the organic extract of D. corticola isolated from stem canker of declining cork oak trees in Sardinia (Italy) (Cimmino et al. 2016). Tyrosol (37) is one of the most frequently detected metabolites in cultures of botryosphaeraceous fungi. It was identified in cultures of phytopathogenic strains of D. corticola, D. fraxini and D. seriata (Cimmino et al. 2017; Reveglia et al. 2019; Salvatore et al. 2022) (Fig. 9).

Fig. 9
figure 9

Chemical structures of phenol derivatives (35–38): diorcinol, 4-hydroxyscytalone, tyrosol, and vanillic acid

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Pimarane diterpenes

Pimarane diterpenes, three- and tetra-cyclic diterpenes, are a very representative subgroup of the terpene family (Reveglia et al. 2018a). These compounds are frequently produced by plant and fungi (Reveglia et al. 2018a; Ye and Ai 2022). Sphaeropsidin A (40) is a well known member of this class of natural products isolated from different pathogenic fungi of forest plants including several Diplodia spp. (Evidente et al. 1996, 2000, 2006a, 2012; Sparapano et al. 2004; Andolfi et al. 2014a; Di Lecce et al. 2021; Masi et al. 2022; Salvatore et al. 2022, 2023), but also from other pathogenic and not pathogenic fungi (Wang et al. 2011; Li et al. 2017; Yang et al. 2020) (Fig. 10). Sphaeropsidin A and its analogues have been the subject of a dedicated review paper which describes their biosynthetic origin and the preparation of semisynthetic derivatives (Masi and Evidente 2021).

Fig. 10
figure 10

Chemical structures of pimarane diterpenes (39–46): diplopimarane, sphaeropsidins A–G

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α-Pyrones

Diplopyrone (47) is a monosubstituted tetrahydropyranpyran-2-one isolated as product of D. mutila and D. corticola (Evidente et al. 2003a, 2006a; Masi et al. 2016). It is the first secondary metabolite isolated from Diplodia spp. characterized by the presence of an α-pyrone moiety. The absolute configuration of diplopyrone has been recently determined using an innovative experimental-computational strategy (Fusè et al. 2019), while the absolute stereochemistry of hydroxylated carbon of diplopyrone B (48) has been determined via Mosher's method (Masi et al. 2016) and the relative stereochemistry of diplopyrone C (49) by NOESY spectrum (Salvatore et al. 2022). This structural feature is common among metabolites produced by fungi belonging to several genera including Alternaria, Aspergillus, Fusarium and Trichoderma (Tian et al. 2017; Ding et al. 2019; Gao et al. 2020) and even the total synthesis of some of them has been achieved (Yao and Larock 2002, 2003; Cherry et al. 2005). Unlike other members of this class, diplopyrone C (49) shows an epoxide group on the side chain (Salvatore et al. 2022, 2024), which is known to be highly reactive (Marco-Contelles et al. 2004; Gomes et al. 2020) (Fig. 11).

Fig. 11
figure 11

Chemical structures of α-pyrones (47–49): diplopyrone, diplopyrones B and C

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Polyalcohols

Mannitol (51) and, to a lesser extent, arabitol (50) were found to be accumulating in the mycelium and culture filtrates of Diplodia viticola (Strobel and Kosuge 1965) and D. sapinea (Moura et al. 2021) (Fig. 12). Chemically, mannitol is a very versatile compound with applications in the pharmaceutical industry, as an excipient; in the food industry as an adjuvant improving the taste of some products and in the medical field for being an osmotic diuretic. Although widely distributed, it has not been found in abundant amounts in natural sources (Oliveira et al. 2009). As a consequence, the description of D. viticola and D. sapinea as new sources of this polyalcohol turned out to be extremely relevant.

Fig. 12
figure 12

Chemical structures of polyalcohols (50, 51): arabitol, and mannitol

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Sterols

Sterols are modified triterpenoids containing the tetracyclic ring system of lanosterol without the three methyl groups at C-4 and C-14. One new pentanortriterpenoid, 23,24,25,26,27-pentanorlanost-7,9(11)-dien-3β,22-diol (53), one new triterpenoid, lanost-8-en-3β,22S,23S-triol (52), together with the known triterpenoid, 23,24,25,26,27-pentanorlanost-8-en-3β,22-diol (54), were obtained from cultures of D. cupressi isolated from healthy tissues of the moss Polytrichum commune (Liu et al. 2020) (Fig. 13). In this paper, Liu et al. (2020) describe the production of secondary metabolites from an endophytic D. cupressi and their work represents one of the very few papers on the poorly understood endophytic interactions between Diplodia spp. and plants.

Fig. 13
figure 13

Chemical structures of sterols (52–54): 23,24,25,26,27-pentanorlanost-7,9(11)-dien-3β,22-diol, lanost-8-en-3β,22S,23S-triol, and 23,24,25,26,27-pentanorlanost-8-en-3β,22-diol

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Miscellaneous

Finally, two metabolites produced by Diplodia species are placed in a miscellaneous class because they have no structural affinity with previous groups (Fig. 14). This is the case of a cleistanthane nor-diterpenoid named olicleistanone (55) and a trisubstituted 2,4-pyridione named sapinopyridione (56) (Fig. 14).

Fig. 14
figure 14

Chemical structures of compounds from the group “miscellaneous” (55, 56): olicleistanone, and sapinopyridione

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Biosynthesis

Some compounds listed in Table 1 are polyketides and, for this reason, have a common biosynthetic origin and are produced by the acetate pathway. The core biosynthesis involves stepwise condensation of a starter unit (typically acetyl-CoA) with an extender unit (typically malonyl-CoA). Figure 15 shows a proposed biosynthetic pathway which includes most of the classes (polyketides) reported in the previous section (Dewick 2016).

Fig. 15
figure 15

Biosynthetic pathway of polyketides produced by Diplodia spp

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The biosynthesis of pimarane-type diterpenoids and sterols follows the mevalonate pathway, also known as the isoprenoid pathway or HMG-CoA reductase pathway, because these compounds belong to the large family of terpenoids. This pathway produces two five-carbon building blocks called isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), which are combined in different pathways leading to more complex terpenoid structures. In particular, pimarane diterpenoids are biosynthetically formed from the conversion of geranylgeranyl-PP (GGPP) into (+)‐copalyl-PP and this in turn into sandaracopimarenyl cation while sterols are produced after a series of reactions starting by the conversion of farnesyl-PP to squalene, followed by the formation of lanosterol by squalene cyclization (Fig. 16) (Dewick 2016, Masi and Evidente 2021).

Fig. 16
figure 16

Biosynthetic pathway of terpenoids produced by Diplodia spp

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Bioactivities of secondary metabolites

Most of the secondary metabolites produced by Diplodia species have been investigated for biological activities, including antibacterial, antifungal, cytotoxic, zootoxic. The outcomes of a wide-ranging investigational work concerning compounds isolated from Diplodia strains are summarized in Table 3.

Table 3 Main bioactivities of secondary metabolites from Diplodia species
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Considering that these fungi have been associated with plant diseases, the phytotoxic properties of many products have been investigated on several plant species by cuttings, twigs, shoots and leaf puncture assays. Fungal phytotoxins could be host specific or non-host specific toxins depending on their capacity to determine a toxic effect towards plants that are hosts of the toxin-producing fungi or also on other plant species. For this reason, several Diplodia products were tested on host and non-host plants. Some of them turned out to be able to induce symptoms in plants very similar to those caused by the fungus. Sphaeropsidin A (40) and analogues are emblematic compounds produced by Diplodia spp. which are recognized as non-selective phytotoxins due to their capacity to cause symptoms on a broad host range (see Table 2). Therefore, it has been hypothesized that these phytotoxins are virulence agents in the phytopathogen-host interactions (Evidente et al. 1996; Salvatore et al. 2023). A significant number of metabolites of Diplodia spp. (e.g., diplopyrone C (49), epi-epoformin (7), diplopimarane (39), sapinopyridione (56) and sphaeropsidins (40–46)) were also tested for their antimicrobial properties, generally evidencing good effects against fungi and bacteria (Table 3). In particular, diplopyrone C showed interesting antimicrobial and antibiofilm activities against two frequently isolated nosocomial pathogens (i.e., the fungus Candida albicans and the gram-negative bacterium Klebsiella pneumoniae). In fact, this compound was capable of reducing biofilm formation and possesses a significant potential for biofilm eradication. Further insights were obtained by using a GC–MS based metabolomics footprinting approach which offers a window on the physiological state of these microorganisms in presence of diplopyrone C (Salvatore et al. 2024).

Structure–activity relationship (SAR)

Understanding how structural modifications affect the biological activity of compounds is an important task. In fact, it was reported that small modifications of chemical structures of natural products dramatically alter their biological responses and, by contrast, structurally diverse compounds can show similar activities (Peltason and Bajorath 2007).

Several studies have provided very important data regarding structure versus activity relationships of Diplodia compounds essentially through the preparation of synthetic and semisynthetic derivatives (Masi et al. 2018b).

In 2004, Sparapano et al. curried out a study on six pimarane diterpenes (i.e., sphaeropsidins A–F; 40–45) from D. cupressi, as well as eight derivatives obtained by chemical modification of sphaeropsidins A, B, and C which asses their phytotoxic and antifungal activities. The findings showed that the integrity of the tricyclic pimarane system, the preservation of the double bond C-8–C-14, the tertiary hydroxyl group at C-9, the vinyl group at C-13, and the carboxylic group at C-10 as well as the integrity of the A-ring provide these compounds with non-selective phytotoxic and antifungal activity (Sparapano et al. 2004). To better understand the antibacterial activity of sphaeropsidins, a number of derivatives of 40–42 were synthesized and their activity was evaluated in comparison to their precursors (Evidente et al. 2011b). The antibacterial data indicated which specific structural features are related to the toxic properties of sphaeropsidins, such as the presence of the C-7 carbonyl group and the hemiketal lactone. The vinyl group at C-13, the C-ring double bond, and/or the C9 tertiary hydroxy as well as the pimarane structural feature are also important in imparting activity against Xanthomonas oryzae pv. oryzae, Pseudomonas fuscovaginae, and Burkholderia glumae (Table 3). The preparation of synthetic derivatives also allowed to investigate the relationships between sphaeropsidin chemical structures and anticancer activity (Lallemand et al. 2012). The IC50 in vitro growth-inhibitory concentrations of sphaeropsidins A, B and C and of ten of their synthetic derivatives were determined using the MTT colorimetric assay in five human and one mouse cancer cell lines. Basically, the results of the SAR study on anticancer activity of 40–42 agree with the results of similar studies described above in which the antibacterial, antifungal and phytotoxic activities were tested (Sparapano et al. 2004; Evidente et al. 2011b). The results of the SAR studies conducted on pimarane diterpenes were synthetized in Fig. 17A.

Fig. 17
figure 17

Structure–activity relationship of: A pimarane diterpenes; B sphaeropsidone (9); C oxysporone (21)

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An interesting SAR evaluation was conducted comparing the phytotoxic and antifungal activities of ten compounds, either natural or synthetic analogues of sphaeropsidone (9) (Evidente et al. 2011a). It was found that the hydroxy group at C-5, the absolute C-5 configuration, the epoxy group, and the C-2 carbonyl group appear to be structural features important for the biological activity. In fact, modifications of the C-5 hydroxy group, such as acetylation, the reduction of the C-2 carbonyl group, and opening of the epoxy ring, led to compounds that were much less active and/or inactive in comparison to sphaeropsidone. Therefore, the phytotoxic and antifungal data showed a relationship between specific structural features and toxicity of the sphaeropsidones (Fig. 17B).

Oxysporone (21) and eight derivatives were tested for their phytotoxic and antifungal activities in order to establish a structure–activity relationships and eventually to generate new bioactive compounds as promising leads for practical applications (Andolfi et al. 2014b). The dihydrofuropyran carbon skeleton, the double bond of the dihydropyran ring and the hydroxy group at C-4 could be pointed out as important structural features for phytotoxic activity (Fig. 17C). Among the semisynthetic derivatives of the oxysporone, the 4-O-p-bromobenzoyl derivative, which was generated by the modification of the hydroxy group at C-4 of dihydropyran ring by a reaction with p-bromobenzoyl chloride carried out in CH3CN, turned out to be particularly active in inhibiting mycelial growth of important agriculture and forest pathogens such as A. rolfsii, D. corticola and Phythophthora spp.

The relationship between structural features and bioactivity of members of the classes of dihydrofuranones and α-pyrones were investigated by (Masi et al. 2016) who observed that both pyran-2-one and furan-2-one rings are essential features imparting phytotoxicity and that in the α-pyrones group the nature of the side chain is less important than in the dihydrofuranones group. In fact, in the tomato cuttings bioassays, diplopyrone B (48), sapinofuranones A (15) and B (16) turned out to be active, while diplopyrone (47), sapinofuranones C (18) and D (24) turned out to be inactive.

Conclusion

The present review illustrates that the genus Diplodia is a rich source of many novel and known bioactive compounds. The structures and properties of 56 chemically defined compounds produced by 11 species of Diplodia were reviewed. In particular, special emphasis was dedicated to the chemical diversity, bioactivity and implications of the metabolite production. It should be noted that most of the available information on Diplodia metabolites are restricted to phytopathogenic strains and, in general, there is a lack of data on metabolites produced by endophytes of this fungus. There is no doubt that the interest on phytopathogenic strains of Diplodia spp. is related to their capacity to induce symptoms in economically important forest and horticultural plants. However, future research should be also directed to investigate metabolites from endophytic strains of Diplodia spp. in order to explore the ecological relationships of different fungal lifestyles with the host plants.