**2. Main forest pathogenic fungi**

Forest pathology deals with the diseases of forest trees, which are mainly caused by fungal and oomycete pathogens, in both their fundamental and applied aspects. The development and dissemination of effective control measures is vital to the protection of forest health. An evolution has been observed over the past few decades in terms of techniques and attitudes toward pest control. In the early 1960s, a variety of methods were used to control forest insect pests and diseases including mechanical, silvicultural, chemical and biological methods, with chemical control the most commonly used. By the 1970s, environmental concerns were being increasingly raised about the use of chemicals. As a result, research into the use of biological control agents in conjunction with silvicultural methods or pheromones began in earnest. In the last decade, integrated pest management involving a combination of control measures began to be considered the most effective way to deal with forest pests. Applications of biological control agents and microbial insecticides have become major components of pest management programs and considerable emphasis is placed on prevention and early detection as a means to avoid future pest problems [19]. There is a growing trend toward adopting more sustainable forest management strategies to contain forest pests, particularly in developed countries [20]. This movement is related to changes in the perception and role of forests, which are increasingly valued not just for economic reasons but also for their ecological and social functions. Forest insect pests, diseases and other pests are having significant impacts on forests worldwide. While the devastating impacts of indigenous forest pests are already recognized, those of introduced species are increasingly being recognized as well. Rapid transports, ease of travel, and free trade have facilitated the spread of pests [19]. A review of forest pests in both naturally regenerated forests and planted forests [19] was carried out from 2005 to 2008 in 25 countries, including a number of major forest countries (Brazil, China, Indonesia), in Africa, Asia and the Pacific, Europe, Latin America and the Caribbean, and the Near East. In this global review, the frequency of disease-causing pathogens was reported: ascomycota 59%, bacteria 3%, basidiomycota 33%, oomycota 4%, and phytoplasma 1%. In the Global Forest Resources Assessment 2010 [21], countries were also invited to list and rank up to 10 major outbreaks of insects and diseases that have occurred since 1990; the most prevalent fungal pathogens reported (in order of importance)are as follows: *Armillaria* spp., (Armillaria root disease), *Cryphonectria parasitica* (chestnut blight), *Heterobasidion* spp. (annosum root rot), *Melampsora larici-populina* (poplar rust), *Mycosphaerella pini*, (red band needle blight), *Sphaeropsis sapinea* (diplodia tip blight), *Chalara fraxinea* (ash dieback), *Gremmeniella* sp., and *Melampsora allii-populina* (poplar rust).

#### **2.1. Emerging forest fungal diseases**

of a Regulation on Plant Protection Products (91/414/EC) [5]. Integrated Plant Management (IPM) is the effort to control plant diseases with alternative methods to chemical fungicide, eliminating or controlling their use and implementing the application of alternative control methods such as natural fungicidal substances. Therefore, the industrial research aimed at the discovery and optimization of botanical fungicides needs to address the following aspects: (a) the product must overcome resistance problems to the established commercial products, (b) the product must have lower toxicity to nontarget species and acceptable levels of persistence in the environment, and (c) the product must have market and technical advantages for the

Essential oils (EOs) represent a new class of crop protectants due to their effects, short shelf-life and low toxicity to the environment [7]. In addition, the probabilities of creating new resistant strains by using essential oils as fungicidal agents are low since their constituents can act as synergists [8]. Usually, mono- and sesquiterpenes such as phenols, alcohols, ethers, carbohydrates, aldehydes and ketones are the major constituents of essential oils, which are responsible for the biological activity as well as for their fragrance [9]. In fact in recent years, researchers have reported many mono- and sesquiterpene hydrocarbons as inhibitors of microbial pathogens [10]. Compounds such as carvacrol, thymol, linalool, cymene, pinene are known to exhibit antimicrobial activity [11–14]. These are the major components of essential oils with promising antifungal applications. Many essential oils have been reported as active against animal pathogenic fungi with no side effects [15–17]. Currently, there are some reviews on antifungal activity of plant extracts, generally structured according to the botanical family of plant species source of the active EOs [16] or to the active compounds of plant extracts [18]. The present review is up to date and focused on plant essential oils with antifungal activity against plant pathogenic fungi.

Forest pathology deals with the diseases of forest trees, which are mainly caused by fungal and oomycete pathogens, in both their fundamental and applied aspects. The development and dissemination of effective control measures is vital to the protection of forest health. An evolution has been observed over the past few decades in terms of techniques and attitudes toward pest control. In the early 1960s, a variety of methods were used to control forest insect pests and diseases including mechanical, silvicultural, chemical and biological methods, with chemical control the most commonly used. By the 1970s, environmental concerns were being increasingly raised about the use of chemicals. As a result, research into the use of biological control agents in conjunction with silvicultural methods or pheromones began in earnest. In the last decade, integrated pest management involving a combination of control measures began to be considered the most effective way to deal with forest pests. Applications of biological control agents and microbial insecticides have become major components of pest management programs and considerable emphasis is placed on prevention and early detection as a means to avoid future pest problems [19]. There is a growing trend toward adopting more sustainable forest management strategies to contain forest pests, particularly in developed countries [20]. This movement is related to changes in the perception and role of forests, which are increasingly valued not just for economic reasons but also for their ecological and

agrochemical companies [6].

146 Potential of Essential Oils

**2. Main forest pathogenic fungi**

In the last 15 years, two major changes affecting forest pathology—the world movement of species with trade and the rise of plantation forestry to meet growing needs of an increasing human population—have led to an increasing number of emerging diseases [21–22]. Ghelardini et al. [23] showed seven pathways driving the emergence of diseases threatening natural and planted forest ecosystems around the world: invasions by alien pathogens, climate change, emergence of new virulent and aggressive strains or species, rise of hybrid fungal species, latent and cryptic pathogens, establishment of new associations between vectors and pathogens, and the introduction of new crops and cultivation practices.

Native forests of Europe, Asia and North America have particularly suffered from invasive alien pathogens, which in the last century have caused the decline of key tree species. Among the most striking historical examples are the destruction of chestnuts by *Cryphonectria parasitica*, the alien ascomycete responsible for chestnut blight; the devastating epidemics of Dutch elm disease (DED) caused by *Ophiostoma ulmi* and *O. novo-ulmi*, two alien and highly aggressive fungi previously unknown to science; the huge damage inflicted to white pines by *Cronartium ribicola*, the invasive agent of white pine blister rust (WPBR); and the devastation of plane trees, especially obtrusive in Southern Europe, caused by the introduction of *Ceratocystis platani,* the agent of plane canker stain. In the last years, the number of described *Phytophthora* species has dramatically increased and it is now clear that forest soils host numerous and diverse resident communities of *Phytophthora* species [24]. Recently, the introduction of *Fusarium circinatum* in Spain [25] or, as a late and worrisome case, the fastspreading epidemics of European ash dieback caused by *Hymenoscyphus fraxineus* [26–28], an anamorphic fungal pathogen with putative origin in eastern Asia [29] should be added to the list. In relation to climate change, *Phytophthora cinnamomi* is forecast to benefit from warmer winters, possibly expanding its geographic range by kilometers and reaching unaffected host populations or new host species [30]. Otherwise, [31] found that in the last 15 years, the emerging pine shoot pathogen *Diplodia sapinea* spread in France probably because of a climate shift to milder winters and wetter summers. Dutch elm disease (DED) is frequently mentioned in forest pathology reviews as the best example of a destructive disease of alien origin since it almost destroyed the elm populations of Europe, North America, and parts of Asia. DED reemerged in the 1970s in Europe as a devastating disease, which killed also elm genotypes that had been resistant in the first epidemic (at the beginning of the twentieth century). This new epidemic was caused by the emergence of the separate and highly virulent species *Ophiostoma novo-ulmi* [32] consisting of the subspecies *novo-ulmi* and *Americana* [33]. Also, [34] provided strong evidence that *Mycosphaerella populorum*, the Septoria canker of poplars, has adapted to infect, colonize, and cause mortality on poplar woody stems as a result of horizontal transfer of the necessary gene battery from ascomycete fungi associated with wood decay and from prokaryotes.

and tree microbiota in the regulation of pathogen populations and disease. In this ecosystem perspective, pathogens are no longer "enemies" but are key actors of the evolution and ecol-

Antifungal Effect of Essential Oils

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Plants in nature are constantly challenged by several harmful phytopathogens including bacteria, fungi, nematodes, or virus, producing a high and negative impact on crop productivity worldwide [51]. An uncontrolled amount of synthetic and chemical pesticides used during past decades makes necessary to adopt new strategies allowing a sustainable plant protection in crops and forest systems. The use of natural compounds as plant biostimulators of growth or biotic and abiotic stress responses in plants is getting importance in the last decade because of legal restrictions on the use of phytosanitary products on crops [52, 53]. European Union policy works upon a significant reduction in pesticide use in the short future [54]. One alternative are natural origin compounds with priming capacities, such as the essential oils (EOs) [55]. This section describes examples of recent molecular approaches studying EOs and discusses the use of EOs as an alternative of nonpollutant primers to induce plant resistance

Priming is "the physiological state that enables cells to respond to very low levels of a stimulus in a more rapid and robust manner than non-primed cells. In plants, priming plays a role in defense and development" [56, 57]. A classical priming defense strategy consists in the use of very well-conserved molecules into the phytopathogen structure called damage/pathogen/ microbe-associated molecular patterns (DAPMPs/PAMPs/MAMPs), such as the lipopolysaccharides (LPS, peptidoglycan (PGN), bacterial flagellin, fungal chitin, bacterial Ax21, or elongation factor Tu (EF-Tu). MAMPs are recognized by plasma-membrane receptors in plants called pattern recognition receptors (PRRs). PAMPs recognition activates a pattern-triggered immunity (PTI) associated with the increase in intracellular calcium, phosphorylation processes mediated by MAPKinase cascades, production of reactive oxygen species (ROS), plant protective compounds, induction of defense-related transcription factors, and corresponding plant pathogenesis-related proteins (PRs) such as glucanases and chitinases, as well as proteins and compounds involved in plant cell wall fortification, such as callose or lignin. PTI might be suppressed by host-adapted phytopathogens, producing an effector-triggered susceptibility (ETS), and adapted plants might block those effectors, activating a robust effector-triggered immunity (ETI) [53, 58–60]. In parallel to the PAMP response, each pathogen specifically triggers a cascade of signaling pathways mediated by phytohormone receptor and recognition of salicylic acid (SA), jasmonate acid (JA), or ethylene (ET). Commonly, it is well accepted that SA is induced by biotrophic and hemibiotrophic phytopathogens, while ET and JA are activated by necrotrophic ones and also by some hemibiotrophs. Those pathways are also interconnected, in order that the activation of one of them currently down-regulate the other one or vice versa [56]. A new mechanism called EMPIS (ETI-Mediating and PTI-inhibited

ogy of local communities, and more generally of the ecosystem health (e.g., [49–50]).

**3. Biotechnological approach: genomics-proteomics-metabolomics**

for environmental-friendly plant protection.

**3.1. The "priming" process**

In fungal pathogens of woody plants, emergence of new interspecific hybrids was described in *Melampsora* [35], *Phytophthora* [36], *Ophiostoma* [37], *Cronartium* [38], and *Heterobasidion* [39]. An up-to-date case of a worrisome forest pathogen that may have a latency period in asymptomatic infected plants is *H. fraxineus*, the agent of European ash dieback, which penetrates into wood tissues from infected leaves and may not produce external necroses until the next growing season [40]. The *Botryosphaeriaceae* are a classical example of a very diverse group of fungi, which comprises well-studied endophytes and latent pathogens of woody plants that typically cause disease associated with some types of stress [41]. A key factor in the spread of *Diplodia sapinea* and *D. scrobiculata* is the latency period within host plant tissues. These fungi are able to live within the host without causing any visible symptoms but rapidly shift to a pathogenic interaction when an environmental stress factor primes the host (e.g., local or large-scale climate change) [31].

An example of new association between vectors and pathogens is the spread of *C. parasitica* on chestnuts by *Dryocosmus kuriphilus*, the oriental chestnut gall wasp, in Europe [42]. *D. kuriphilus* is an invasive insect of Asian origin. Also, a new association was recently reported between *D. sapinea* and *Leptoglossus occidentalis* [43], the so-called western conifer seed bug (WCSB), an invasive coreid, accidentally introduced to Italy from the US in 1999 [44], and nowadays present in several parts of Europe [45]. This association might be beneficial for both partners: the insect enables the fungus to reach a higher number and variety of host trees, either pines or other conifers, while the fungus stimulates the tree's production of monoterpenes, signaling the status of weakness of the tree and attracting more insects [43]. Regarding the new silvicultural practices, commercial plantations of poplars may be severely damaged by emerging plant pathogens worldwide [46]. In northeastern and north-central North America, one of the most harmful poplar diseases is *Mycosphaerella populorum* (Peck). Also, the epidemics of *Phytophthora ramorum* on *Larix kaempferi* (Lamb.) Carr.) in UK might have been driven by the intrinsic fragility of clonal monocultures on great areas due to ecosystem simplification, extreme mechanization, and reduced genetic diversity [47, 23]. Looking ahead, authors of [48] propose an evolutionary ecology perspective that could provide new directions for forest research or disease management: (1) fungal evolutionary diversity (species diversity of forest pathogens and their ecological niches), (2) pathogen evolution (how forest pathogens become adapted to their hosts), (3) forest resistance to disease, especially in relation to tree breeding (trade-offs, tolerance, emerging properties in populations), and (4) the role of hyperparasites and tree microbiota in the regulation of pathogen populations and disease. In this ecosystem perspective, pathogens are no longer "enemies" but are key actors of the evolution and ecology of local communities, and more generally of the ecosystem health (e.g., [49–50]).
