Verticillium is a genus of soil-borne ascomycete fungi with a wide range of host plants. Plant pathogenic Verticillium species are hemibiotrophs that cause plant diseases generally referred to as
‘Verticillium wilt’ (Fradin and Thomma 2006; Inderbitzin et al. 2011b) . The disease cycle of plant pathogenic Verticillium spp. can be divided into a biotrophic and a necrotrophic phase (Figure 1.1) (Fradin and Thomma 2006) . During the biotrophic phase, Verticillium hyphae enter roots through endodermis-free zones and grow inter- and intracellularly until they reach the central cylinder (Huisman 1982; Bishop and Cooper 1983) . Within the vascular bundle, Verticillium colonizes xylem vessels and produces conidia which are carried with the xylem stream throughout the vasculature of the plant (Fradin and Thomma 2006) . At this stage, fungal biomass accumulation as well as plant defense responses such as secretion of gels and the formation of tyloses cause xylem vessel occlusion which in most cases causes wilting of the foliage (Street and Cooper 1984; Benhamou 1995) . Effectors as well as phytotoxins secreted by the fungus then induce macroscopic disease symptoms such as wilting, necrosis and chlorosis of the leaves (Pegg 1965; Pemberton and Salmond 2004; Buchner et al. 1982). This marks the transition from the biotrophic to the necrotrophic phase of the pathogen (Fradin and Thomma 2006) . During the necrotrophic phase, the fungus leaves the vasculature and uses nutrients released from senescing plant tissues to produce melanized resting structures called microsclerotia (Fradin and Thomma 2006) . Microsclerotia then return to the soil through decomposing plant material where they can rest for years until a new disease cycle is started (Figure 1.1) (Wilhelm 1955).
Figure 1.1 TheVerticilliumdisease cycle.Verticilliumis a hemibiotrophic, vascular pathogen with a biotrophic (green) and a necrotrophic (red) phase during plant infection. (A) Root exudates stimulate germination of microsclerotia in the soil. (B) Fungal hyphae (red) enter roots at endodermis-free zones. (C) Invasive hyphae enter the central cylinder. (D) Hyphae colonize xylem vessels and produce conidia. (E) The fungus colonizes the vascular tissue systemically. (F) Chlorosis, necrosis and wilting of foliage is induced. (G) Senescing tissues are colonized. (H) Microsclerotia are formed. (I) Microsclerotia return to the soil through decomposing plant material.
Figure adopted and modified from apsnet.org./edcenter/intropp/lessons/fungi/ascomycetes/Pages/VerticilliumWilt.
Using modern phylogenetic and taxonomic methods, Inderbitzin et al. (2011a) defined and characterized nine haploid and one hybrid Verticillium species (Figure 1.2). While all Verticillium species have a plant-associated lifestyle, only Verticillium alfalfae, V. nonalfalfae, V. albo-atrum, V.
dahliaeandV. longisporumare known as plant pathogens (Inderbitzin et al. 2011a).V. alfalfaeandV.
nonalfalfae, which had previously been identified as V. albo-atrum, were introduced as two new species, leading to some taxonomic confusions in the field (Inderbitzin et al. 2011a; Inderbitzin and Subbarao 2014) .Verticillium longisporum has first been described as a subspecies ofV. dahliaebut gained species rank in 1997 when Karapapa et al. discovered that Verticillium strains isolated from
longisporumis a near-diploid (allodiploid) hybrid species that arose several times independently from at least four different ancestors (Inderbitzin et al. 2011b; Depotter et al. 2016) . The parental line shared by all V. longisporum lineages has been named ‘species A1’ and forms a phylogenetic sister clade to the clade containingV. alfalfae,V. nonalfalfaeandV. dahliae(Figure 1.2). The three parental lines that gave rise to the three distinct lineages (A1/D1, A1/D2 and A1/D3) are either closely related to (D1) or contained within (D2, D3) theVerticillium dahliaeclade (Figure 1.2) (Inderbitzin et al. 2011a;
Inderbitzin et al. 2011b) . Although they are closely related, no V. dahliaeisolate has been found that can be phylogenetically grouped with theV. longisporumparental lines.
While the infection strategies of the five aforementioned pathogenic species are similar, they differ significantly in host range. Indeed, V. alfalfaehas only been isolated from one host, alfalfa (Medicago sativa), though successful interaction has also been shown with the closely related model plant Medicago truncatula (Inderbitzin et al. 2011a; Molinéro-Demilly et al. 2006) . Verticillium nonalfalfae has a more extensive host range and infects, amongst others, hops (Humulus lupulus), potato (Solanum tuberosum), tree-of-heaven (Alianthus altissima) and Pelargonium (Inderbitzin et al. 2011a;
Rebbeck et al. 2013; Kasson et al. 2014; Garibaldi et al. 2016) . Although V. albo-atrum was mentioned in studies before 2011 as agents of Verticillium wilt on various plants, the recent taxonomical developments described above suggest a more restricted host range. To this day, potato is the only clearly identified host plant ofV. albo-atrum(Inderbitzin et al. 2011a). Verticillium dahliaeis the species with the most extensive host range of the genus. Its more than 200 hosts include economically important crop plants such as cotton (Gossypium hirsutum), tomato (Solanum lycopersicum) and olive (Olea europaea) (Fradin and Thomma 2006) but notably (almost) no brassicaceous hosts (Depotter et al. 2016). Verticillium dahliaeoutbreaks have been occurring around the globe from South Africa (Ferreira 1990) over Europe (García-Ruiz et al. 2014) to China (Zhang et al. 2009), Australia (Ramsay, JR et al. 1996) and America (Johnson and Dung 2010). While no exact numbers of the economic damage caused byV. dahliaeexist, Verticillium wilt has been recognized as a serious threat to crop production worldwide (Cirulli et al. 2010; Jiménez-Díaz et al. 2012; Land et al.
2016) . V. longisporum, although also recognized as a threat to crops (Depotter et al. 2016) , has a more restricted host range.It has almost exclusively been isolated from brassicaceous hosts including oilseed rape (Brassica napus), cauliflower (Brassica oleracea var. botrytis) and sugar beet (Beta vulgaris) (Inderbitzin et al. 2011a; Depotter et al. 2016).
Figure 1.2 Phylogenetic relationship of the ten Verticillium species based on the combined actin, elongation factor 1-alpha, glyceraldehyde-3-phosphate dehydrogenase and tryptophan synthase dataset of 2658 characters and 77 taxa, withGibellulopsis nigrescensas outgroup.The Bayesian consensus tree is shown. Species are marked by vertical bars followed by species names, species in bold were described in the study by Inderbitzin et al. (2011a). The two main clades recovered are indicated on the right. Numbers at the branches are Bayesian, likelihood and parsimony support values above 70 in that order, branches in bold had maximal support in all analyses. Each isolate ofV. longisporumhas two alleles that are present in two different clades in the tree, in hypothetical Species A1, and either in hypothetical Species D1 or inV. dahliae, reflecting the hybrid origin of this species. Figure and figure legend were adopted from Inderbitzin et al. (2011a) and modified.
Verticilliumspecies used here are marked by a red rectangle.