Verticillium dahliae
(verticillium wilt)

It is not possible to conclusively compile host ranges for individual Verticillium species from literature surveys (Walker, 1990; Inderbitzin and Subbarao, 2014) because of inconsistent application of species names due to past taxonomic disagreements (Hawksworth and Talboys, 1970) and the inherent unreliability of species identification based on morphological characters (Inderbitzin and Subbarao, 2014). But it is clear, that V. dahliae, by far has the widest host range among Verticillium species, affecting plants in at least 14 different families, and may be more than 200 hosts (Bhat and Subbarao, 1999; Pegg and Brady, 2002). Verticillium longisporum, in contrast, has a more restricted host range limited to members of Brassicaceae (Subbarao et al., 1995).  Verticillium alfalfae has only been associated with disease on alfalfa (Inderbitzin and Subbarao, 2014). The host range of other newly described species of Verticillium is only now becoming apparent with many new recent studies (Subbarao et al., 1995; Gurung et al., 2015; Jing et al., 2018; Li et al., 2019). Verticillium nonalfalfae prefers environments that are cooler (20℃) than V. dahliae (25℃) and apparently is also only recovered from production fields located above 1800 m (Jing et al., 2018).

PHYTOSANITARY RISK

RISK CRITERIA CATEGORY

ECONOMIC IMPORTANCE High
DISTRIBUTION Worldwide
SEEDBORNE INCIDENCE Low
SEED TRANSMITTED Yes
SEED TREATMENT None

OVERALL RISK Low

EPPO currently lists V. dahliae and V. albo-atrum as A2 quarantine pests (in association with hop planting material) (EPPO, 1982; Anon., 1993). In general, the pathogen is so widespread that it is not regarded as a quarantine risk. However in Australia, where cotton is widely affected by moderately pathogenic strains of V. dahliae, the dangers of importing highly pathogenic defoliating strains have been recognized (Allen, 1995). Moreover, the experience of the Israeli potato-production areas of the Negev (Nachmias and Krikun, 1985) underlines the fact that the pathogen is not universal and thus steps to limit its further spread may be worthwhile. A lack of differentiation between populations of V. dahliae from spinach seed sources and the coastal California population, in addition to the evidence of high migration rates, suggest that imported infested spinach seed has contributed a large proportion of genotypes affecting lettuce (Atallah et al., 2011) and this introduced population from spinach is capable of infecting lettuce (Short et al., 2015a). Subsequent analysis of the population structure of V. dahliae on a global scale indicated that V. dahliae is highly clonal, consisting of at least seven major lineages that are not linked to host or geography. Prevention of pathogen movement is thus essential for Verticillium wilt management. The data also showed that the potential origin of V. dahliae as a pathogen might lie in northern Europe (Short et al., 2015).

The distribution of V. longisporum (Karapapa et al., 1997) is still comparatively limited although there is evidence of recent long-distance spread from north-west Europe to Japan and North America, where it is now an important pathogen of cauliflower in California (Koike et al., 1994). This pathogen is a potential threat to many areas where cruciferous crops are grown.

V. dahliae has a very wide host range among economically important crops and ornamental plants, native species and weeds, including both woody and herbaceous plants (Koike et al., 2000; Inderbitzin and Subbarao, 2014).

The primary economic hosts of V. dahliae include: artichoke, aubergine, bell pepper, cotton, hop, lettuce, mints (Mentha spp.), oilseed rape, olive, potato, strawberry and tomato.

Other significant hosts include ash (Fraxinus americana, F. augustifolia, F. excelsior, F. pennsylvanica), Brussels sprouts, cauliflower, cocoa, grapevine, groundnut, horseradish, lucerne, maple (Acer palmatum, A. platanoides, A. saccharinum), pistachio nut, rose (Rosa spp.), stone fruits: almond, apricot, cherry, nectarine, peach, plum (Prunus spp.) and sunflower.

Other host plants reported to be infected by V. dahliae follow:

- fruit plants including raspberry and blackberry, blackcurrant, gooseberry, black pepper (Capsicum annuum var. conicum), blueberry, melon, paprika, quince, watermelon and winter melon (Benincasa hispida).

- perennial crops including avocado, mango, absinthe (Artemisia absinthium) and cassava;

- agronomic crops including false flax (Camelina sativa), jojoba (Simmondsia chinensis), linseed/flax (Linum usitatissimum), safflower, sugarbeet and tobacco. Unexpectedly, V. dahliae can be isolated from root and crown tissues of cereals including barley, ryegrass and winter wheat (Mathre, 1989; Celetti et al., 1990; Krikun and Bernier, 1990), although it does not appear to cause disease on cereal crops.

- vegetable crops including artichoke, cabbage, Chinese cabbage (Brassica chinensis), chicory/endive, cucumber, garlic, horseradish, okra, perennial mint (Agastache rugosa), radish, spinach and turnip;

- various legumes including chickpea, Egyptian clover (Trifolium alexandrinum), faba bean, grain lupine (Lupinus angustifolius, L. albus), milk vetch (Astragalus adsurgens), mungbean, pea and soyabean;

- numerous woody and herbaceous ornamental species, such as Agastache, Ageratum, Anthemis, Antirrhinum majus, Aster spp., Aucuba japonica, Banksia victoria, Basella rubra, Berberis spp., Callistephus chinensis, Carpobrotus spp., Centaurea cyanus, Chrysanthemum spp., Cistus spp., Corchorus olitorius, Cosmos sulphureus, Cotinus coggyria, Cytisus scoparius, Dahlia variabilis, Daphne mezereum, Echinacea purpurea, Fuchsia, Geranium, Gerbera, Hedera, Helenium, Helichrysum, Hibiscus cannabinus, Hippophae rhamnoides, Hoheria populnea, Impatiens walleriana and I. balsamina, Koelreuteria, Lathyrus, Laurus, Lavatera olbia, Leucospermum cordifolium, Liatris spp., Ligustrum vulgare, Lonicera, Lupinus, Mahonia spp., Malva spp., Malvaviscus arboreus, Matthiola incana, Nicotiana, Olearia paniculata, Osmanthus, Papaver spp., Pelargonium domesticum, Perilla ocymoides, Peperomia obtusifolia, Phlox, Phlebalium billardieri, Platycodon grandiflorum, Poinsettia, Prostranthera spp., Protea compacta, Rhus cotinus, Ribes sanguineum, Salvia, Senecio spp., Tagetes spp., Valeriana officinalis and Viburnum spp.

Tree species attacked include: Ailanthus spp., Aralia cordata, elm (Ulmus spp), linden (Tilia spp.), pedunculate oak (Quercus petraea), pyramid tree (Lagunaria patersonii), redbud (Circis canadensis, C. chinensis, C. yunnanensis) and sweet chestnut (Castanea sativa).

V. dahliae is also known to colonize a number of dicotyledonous weeds, such as Abutilon theophrasti, Amaranthus spp., Ambrosia spp., Atropa belladonna, Calendula arvensis, Capsella bursa-pastoris, Chenopodium album, Chrysanthemum spp., Cirsium arvense, Datura spp., Echinops spp., Galium album, Geranium dissectum, Lappula echinata, Malva spp., Matricaria chamomilla, Melandrium noctiflorum, Picris hieracioides, Plantago lanceolata, Polygonum persicaria, Portulaca oleracea, Senecio vulgaris, Solanum nigrum, Solanum sarrachoides, Sonchus oleraceus, Taraxacum officinale, Torilis arvensis, Tripleurospermum inodorum, Veronica persica and Xanthium spp. (McKeen and Thorpe, 1973; Thanassoulopoulos et al., 1981; Mesturino, 1990; Vallad et al., 2005) with or without producing symptoms.

Key references on host range of V. dahliae include Mesturino (1990), Fitt et al. (1991), Lawrence et al. (1991), Rijkers et al. (1992), Powelson and Rowe (1993), Blanco-Lopez et al. (1994), Chen (1994), Koike et al. (1994), Luisi et al. (1994), Worf et al. (1994), Dobinson et al. (1996), Bao et al. (1998), Wei et al. (1998), Bhat and Subbarao (1999), Sink et al. (1999), Heale (2000), Ma et al. (2000) and Douhan and Johnson (2001).

The fungus can survive for 14 years in soil as microsclerotia (Wilhelm, 1955), either free or embedded in plant debris, which are stimulated to germinate in response to root exudates (Mace et al., 1981; Mol, 1995). Infection of plants, both individuals and populations, is directly related to the inoculum density of microsclerotia in the soil (Pullman et al., 1982; Harris et al., 1996; Mol et al., 1996; Xiao et al., 1998; Khan et al., 2000). The hyphae or germinating conidia produced by microsclerotia infect at or just behind the root tips, then progress into the cortex and grow towards the developing vascular tissues (Huisman, 1988; Bowers et al., 1996). Various host responses may occur in tissues as a defensive response to infection (Mace et al., 1981; Mueller et al., 1993; Bowers et al., 1996). Once in the xylem vessels, the pathogen spreads by mycelial growth, and also by the production of conidia which become transported in the transpiration stream. In this way the pathogen rapidly becomes systemic in susceptible crops (Vallad and Subbbarao, 2008). The major disease effects are believed to result from occlusion of vessels and the production of toxins (Mace et al., 1981; Cooper, 2000). Microsclerotia are formed in senescing diseased tissues (Rijkers et al., 1992; Mol, 1995; Mol and Scholte, 1995). The microsclerotia are long-lived and, under suitable conditions, can remain viable in soil for more than a decade; they survive well over a range of soil moisture and temperature conditions, but lose viability most rapidly in wet, warm soil (Green, 1980). Both bacteria and fungi attack and degrade the microsclerotia in soil (Baard et al., 1981; Tjamos, 2000). Wilt diseases caused by Verticillium spp. are monocyclic, i.e. they have only one cycle of infection, colonization and production of microsclerotia per season (see disease cycle in Rowe (1985)).

The colonization of dicotyledonous weeds (McKeen and Thorpe, 1973; Busch et al., 1978; Vargas-Machuca et al., 1987; Mesturino, 1990; Naser et al., 1998), legume and cereal rotation crops (Mathre, 1986, 1989; Skipp et al., 1986; Celetti et al., 1990; Krikun and Bernier, 1990; Vallad et al., 2005) and 'volunteer' potato plants may contribute to bridging the period between susceptible crops. The pathogen can be disseminated with infected planting material of strawberry, hop, liatris (Gilad et al., 1993), olive (Thanassoulopoulos, 1993; Naser et al., 1998), stone fruits and woody ornamentals (Chen, 1994); with leaves and petioles of olive (Naser et al., 1998) and several shade trees (Rijkers et al., 1992); with potato seed tubers, either internally or as surface contaminants (Easton et al., 1972; Tsror et al., 1999; Omer et al., 2000); with seed of cotton, principally as microsclerotia associated with the lint but also internally (Evans et al., 1966; Sackston, 1983), safflower (Goethal, 1971; Klisiewicz, 1975), sunflower (Ataga et al., 1996; Sackston, 1980); aubergine (Porta-Puglia and Montorsi, 1982), spinach (Park and Kim, 1986), chickpea (Maden, 1987), linseed (Fitt et al., 1992) and oilseed rape (Heppner and Heitefuss, 1995); and on manure (Lopez-Escudero et al., 1999). Much of the national and international movement of V. dahliae has probably occurred with contaminated planting material (Short et al., 2015; Short et al., 2015a). V. dahliae may also spread naturally on weed seeds (Evans, 1971).

Diseases caused by V. dahliae are favoured by moderate to high temperatures, although temperatures above 30°C are inhibitory, which probably explains why the pathogen is largely absent from lowland tropical areas (Mace et al., 1981). In the USA, potato early-dying syndrome is caused predominantly by V. dahliae in those areas where average temperatures during the growing season frequently exceed 25°C; V. albo-atrum is the main cause in the cooler production areas (Rowe et al., 1987; Powelson and Rowe, 1993). In New Zealand, V. dahliae predominates in the warmer northern districts and is effectively replaced by V. albo-atrum in the cooler southern areas (Smith, 1965) and this is identical to the situation in China (Jing et al., 2018). The apparent paradox of cotton wilt being more severe in irrigated crops results from a reduction in soil temperature after irrigation (Karaca et al., 1971). The relationship between soil moisture status and the severity of Verticillium wilt seems to vary with climate and cropping systems (Arbogast et al., 2000; Xiao and Subbarao, 2000). The disease is often more severe in irrigated crops (El-Zik, 1985; Cappaert et al., 1992; Xiao and Subbarao, 2000) and salinity of irrigation water may exacerbate disease (Nachmias et al., 1993). In potato, detailed studies have shown that high soil moisture early in the season enhances Verticillium wilt possibly by slowing root growth and/or increasing the rate of microsclerotial germination (Gaudreault et al., 1995). A link has also been established between tolerance of potato cultivars to moisture deficit stress and resistance to Verticillium wilt (Arbogast et al., 2000). Disease may be exacerbated in the presence of plant-pathogenic nematodes (Tchatchoua and Sikora, 1983; Evans, 1987; MacGuidwin and Rouse, 1990; Grontoft et al., 1992; Santamour, 1992; Powelson and Rowe, 1993; Bowers et al., 1996). Synergistic interactions with root-lesion nematodes, particularly Pratylenchus penetrans, are well documented (Rowe et al., 1985, 1987; Wheeler et al., 1992, 1994; Powelson and Rowe, 1993; Bowers et al., 1996; Saeed et al., 1997; Hafez et al., 1999; Wang et al., 1999) and may be specific to particular species of Pratylenchus (Wheeler et al., 1994; Bowers et al., 1996) and strains of the fungus (Botseas and Rowe, 1994). On potato, relationships have also been noted between V. dahliae and soft rot bacteria in the genus Erwinia (Rahimian et al., 1984; Powelson, 1985; Tsror et al., 1990) and the fungus Colletotrichum coccodes (Johnson and Miliczky, 1993; Powelson and Rowe, 1993). Vesicular-arbuscular mycorrhiza may reduce disease in cotton (Liu, 1995).

V. dahliae has a very extensive world distribution and affects a very large number of economic and other plants. There is growing evidence that the present distribution is the result of dissemination from one or more centres in comparatively recent times. Investigations indicate that it is a variable and probably very mutable species. It has a basic capability of invading the roots and colonizing the vascular systems of higher plants. This invasion may be comparatively benign, or may result in severe damage or death. The V. dahliae population in any one area may respond locally to selection pressure imposed by vegetation and cropping patterns (Tjamos, 1981). This microevolution is imposed on a base population whose character may be more or less variable and reflects the origins of the parent population. This has resulted in populations of different degrees of variability and pathogenic potential in different parts of the world (Heale, 2000; Katan, 2000; Omer et al., 2000).

V. dahliae strains attacking cotton are present in most areas where cotton is grown. In the USA, cotton wilt is caused mainly by a pathotype of enhanced virulence to cotton (Schnathorst and Mathre, 1966; El-Zik, 1985; Bell, 1992), which defoliates susceptible cultivars but has limited pathogenicity to other common hosts of V. dahliae. Cotton-defoliating isolates (not always isolated from cotton) are further distinguished by belonging to one vegetative compatibility group (Puhalla and Hummel, 1983; Joaquim and Rowe, 1990; Daayf et al., 1995; Katan, 2000). Cotton-defoliating isolates have been reported from several countries including Spain (Bejarano-Alcazar et al., 1996), China (Quingji and Chiyi, 1990; Ma and Shezeng, 2000), Mexico and Peru (Mathre et al., 1966), Israel (Bao et al., 1998), Iran (Hamdollah-Zadeh, 1993) and the former USSR (Daayf et al., 1995). They may have evolved locally or may be the result of dissemination with seed, although there is evidence that the most pathogenic strains in the former USSR are closely related to the North American defoliating strain (Daayf et al., 1995). Strains capable of attacking cotton cultivars selected for resistance have appeared in several areas, having evolved from the local 'cotton' population; and so-called Race 2 types capable of attacking tomato cultivars with the Ve resistance gene have appeared in the USA, Australia, South Africa and Europe. However, in other crops resistance to V. dahliae appears generally to have been durable. Two pathogenic races of V. dahliae were also described on lettuce (race 1 and race 2), similar to what occurs in tomato and sunflower (Alexander, 1962; Bertero and Vazquez, 1982; Vallad et al., 2006). These two races were described in California, USA, following the identification of the Ve gene (Grogan et al., 1979) and currently 55% of strains recovered in coastal California are characterized as race 1, and the remaining as race 2.

V. longisporum has evidently spread from Scandinavia to north-west Europe and to Japan (Horiuchi et al., 1990) and the USA (Subbarao et al., 1995). Recently described as a new species (Karapapa et al., 1997) (see Notes on Taxonomy), it affects mainly cruciferous hosts. It is a major pathogen of oilseed rape in Sweden, Germany, France, Poland, Russia and Ukraine (Heale, 2000; Karapapa et al., 2000), cauliflower in the USA (Subbarao et al., 1995) and other Brassica species in Japan (Horiuchi et al., 1990).

Incidence

V. dahliae has been found to be associated with seed of several crops. Seed transmission of V. dahliae was described in lettuce (Vallad et al., 2005) and other members of Asteraceae family such as safflower and sunflower (Sackston and Martens, 1959; Schuster and Nuland, 1960) and also in members of Solanaceae (Kadow, 1934), Amaranthaceae (Snyder and Wilhelm, 1962 and Fabaceae (Isaac, 1946). In cotton, it is found principally as microsclerotia associated with the lint, but also internally at up to 10 microsclerotia/seed (Evans et al., 1966; Sackston, 1983). One report found the pathogen in 2% of cotton seed  (Karaca et al., 1973). Shtok and Idessis (1974) showed that V. dahliae penetrates cotton seeds, infecting mainly the seed coat, then the husk and embryo. In sunflower, V. dahliae was detected in up to 30% of seeds harvested from plants inoculated with the pathogen under greenhouse conditions (Sackston, 1980). Abundant microsclerotia were found on infected sunflower seed, but the fungus was also isolated from seed without microsclerotia (Bruni, 1970). In safflower, black microsclerotia of V. dahliae were formed on pericarps of infected seeds which were germinated on a moist base (Klisiewicz, 1975). In spinach, V. dahliae infection in several randomly selected seed lots was found to vary between 0 and 10% (Spek, 1972) and more recently over 75% of the seed in individual seed lots have been recorded (du Toit et al., 2005). More than 250 microsclerotia within individual spinach seeds has also been observed (Maruthachalam et al., 2013). It was also found at 10% in seed of linseed (Fitt et al., 1992). The pathogen has also been recorded on seeds of aubergine (Porta-Puglia and Montorsi, 1982), chickpea (Maden, 1987), lettuce (Vallad et al., 2005) and oilseed rape (Heppner and Heitefuss, 1995).

Pathogen Transmission

Seed

Seed transmission of V. dahliae was demonstrated in 13% of seeds from infected safflower plants (Goethal, 1971). In spinach, V. dahliae can remain viable in seed from one season to the next and can thus infect new plants and infest soil. This has only been demonstrated in the case of steamed soil, but it is probable that seed transmission also occurs in non-steamed soil. In samples from 34 commercial lots of spinach seed, seed infection with V. dahliae was found to be fairly low, but enough to serve as a source of inoculum for subsequent crops (Spek, 1973). Infested spinach seed planted repeatedly in the same soil can quickly augment the soil inoculum levels to render even tolerant crops such as lettuce susceptible (Short et al., 2015). Results from a simulation model developed by Wu and Subbarao (2014) suggested that even with a low seed infestation rate, the pathogen would eventually become established if susceptible lettuce cultivars were grown consecutively in the same field for many years. A threshold for seed infestation can be established only when two of the three drivers of the disease: low microsclerotia production per diseased plant, long tail dispersal gradient, and low microsclerotia survival between lettuce crops, are present. Infected cotton seeds are thought to be a primary way in which V. dahlia, particularly severe defoliating strains, have been introduced into new geographic areas (Bejarano-Alcazar et al., 1996; Ma and Shezeng, 2000).

Planting stock

The pathogen can be disseminated readily in infected planting stock of vegetatively propagated crops such as mints (Douhan and Johnson, 2001), strawberry, hop, liatris (Gilad et al., 1993), olive (Thanassoulopoulos, 1993; Naser et al., 1998), stone fruits and woody ornamentals (Chen, 1994). Potato seed tubers have been shown to carry the pathogen either internally or as surface contaminants (Easton et al., 1972; Tsror et al., 1999; Omer et al., 2000). In recent surveys in North America and Israel, V. dahliae was detected in about one-third of commercial lots of certified seed tubers that were examined (Tsror et al., 1999; Omer et al., 2000).

Seed Health Testing

Standard techniques for isolating V. dahliae from host tissues are described by Melouk (1992). Individual seeds or tiny pieces of vascular tissue excised from surface-disinfested basal stem tissues are rinsed in sterile distilled water and then plated on streptomycin sulfate-alcohol-agar (SAA) or NP-10 (Kabir et al., 2004) medium. Alternatively, tissues or seed can be surface disinfested, and then squeezed or homogenized in a blender. The sap or extract is then dilution plated on SAA or NP-10. This medium is prepared by adding 20 g agar to 1000 ml distilled water, autoclaving for 15 min and allowing the agar suspension to cool to 50°C. At that point an antibiotic solution is added. This is prepared by dissolving 300 mg of streptomycin sulfate (75%) and 30 mg of tetracycline HCl in 6 ml of 95% ethanol. Inoculated plates are incubated in the dark at 22-25°C for 5-7 days, at which time they are examined for typical sporulation of V. dahliae.

A detailed step by step description of two agar plate methods to detect V. dahliae in spinach seeds, standardized by the USA National Seed Health System (NSHS), can be accessed at their website (National Seed Health System, 2004).

A diagnostic protocol for V. dahliae is given in EPPO (2007).

The pathogen will usually grow from vascular tissue excised from suspect plants and incubated under moist conditions at 22-25°C, producing typical conidiophores. The most reliable diagnosis is to excise vascular tissue (basal stems or petioles are best) aseptically and plate them on streptomycin sulfate-alcohol-agar (SAA) medium or NP-10 semi-selective medium (Kabir et al., 2004) (see Seed Health Testing). Plates are incubated at 22-25°C in the dark and examined for typical conidiophores and the presence of microsclerotia. For further identification and isolation, spore balls at the tips of conidiogenous cells can be touched with a sterile needle and then streaked onto SAA or NP-10 media. Internal colonization of host tissues by V. dahliae can be quantified by extracting plant sap and dilution plating on a selective medium (Hoyos et al., 1991). Bioassay methods have found some use in screening breeding materials (Madhosingh, 1996) and serological methods for detection of Verticillium in host tissues have also been developed for several crops (Auger et al., 1995; van de Koppel et al., 1995; Plasencia et al., 1997).

Molecular diagnostic techniques have been developed that utilize PCR amplification and RPLP or RAPD markers to identify species of Verticillium infecting plant tissues or even specific strains of the pathogen (Hu et al., 1993; Li et al., 1999; Koike et al., 2000; Typas, 2000). In some cases correlations have been made between molecular diagnostic procedures and vegetative compatibility groups (Dobinson et al., 2000), races (Dobinson et al., 1998) or pathotypes such as defoliating strains in cotton (Wang et al., 1999; Encarnacion et al., 2000). PCR assays to identify each species of Verticillium are available and are being used worldwide (Inderbitzin et al., 2013).

Soil assays are commonly used to estimate the population of V. dahliae microsclerotia in soil. Prior to assay, soil samples are air-dried at 22-25°C for 4-6 weeks, to kill conidia and mycelia of V. dahliae and other fungi, and then finely sieved. In all assays, soil is plated onto a semi-selective NP-10 agar medium and colonies are counted following an incubation period. Standard methods include direct dilution plating of soil samples, wet-sieving soil samples and then plating only one fraction in which microsclerotia are concentrated, and dry plating of soil samples using an Anderson Sampler (Melouk, 1992; Harris et al., 1993; Wheeler et al., 1995; Termorshuizen et al. 1998; Kabir et al., 2004). The most effective semi-selective medium for soil analysis is sodium polypectate-NPX agar. The medium is prepared by adding 5 g of polygalacturonic acid sodium salt to a flask containing 500 ml distilled water, autoclaving for 15 min and allowing to cool to 50°C. In another flask, 15 g of Bacto agar, 1 g of KNO3, 1 g KH2PO4, 0.5 g MgSO4.7H2O, and 0.5 ml tergitol NP-10 are added to 500 ml distilled water. The contents are then stirred and autoclaved for 15 min. This solution is allowed to cool to 50°C before adding 50 mg of streptomycin sulfate, 50 mg of chlorotetracycline HCl and 50 mg of chloramphenicol. The two parts of the medium are then combined, stirred, and then poured into Petri plates immediately (Kabir et al., 2004). Molecular  quantification of Verticillium microsclerotia in soil have been developed but require sophisticated equipment not easily available in every laboratory (Bilodeau et al., 2012).

Cultures of V. dahliae can be stored at 10°C for 1-2 years on several standard agar media or in dried host tissues or in soil (Melouk, 1992).

Wilt diseases caused by V. dahliae are indistinguishable by symptoms from those caused by other Verticillium spp., and it is necessary to isolate the pathogen before the cause can be verified. Species of Verticillium share several common hosts; they also co-exist in some temperate regions, where they are may be isolated from the same host in the same field. The precise environmental conditions under which several newly described Verticillium spp. cause disease are currently unknown. In general, V. nonalfalfae occurs in more northern areas where soil temperatures generally do not exceed 25°C (Jing et al., 2018) whereas V. dahliae occurs in temperate areas where soil temperatures may reach 30°C. V. nubilum and V. tricorpus are weakly pathogenic on some hosts and may be associated with wilt-like diseases, particularly in potatoes and tomatoes. Verticillium tricorpus may be confused with V. dahliae in plate assays of soil samples. An excellent discussion of differentiation among Verticillium species can be found in Heale (2000).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Introduction

Diseases caused by V. dahliae can be controlled through the use of disease-resistant cultivars when they are available. For effective management, an integrated approach is necessary. This usually involves a combination of cultural practices which minimize disease, such as crop rotation and manipulation of fertility and irrigation, planting pathogen-free seeds or stock, use of available resistant cultivars and sometimes pre-plant soil treatments such as soil fumigation or solarization that reduce the viability of microsclerotia in soil (El-Zik, 1985; Shen, 1985; Bell, 1992; Powelson and Rowe, 1993; Jeger et al., 1996; Xiao et al., 1998).

Host-Plant Resistance

Resistant or tolerant cultivars have been useful in controlling Verticillium wilt in many crops worldwide. Tomato cultivars containing the Ve resistance gene (Diwan et al., 1999) are widely employed to combat Verticillium wilt caused by race 1, although strains of the pathogen capable of overcoming this resistance (Race 2 strains) have appeared in North America (Dobinson et al., 1996), Australia (O'Brien and Hutton, 1981), Japan (Nagao et al., 1997), South Africa (Ferreira et al., 1990) and several European countries (Tjamos, 1980; Montorsi, 1986; Ligoxigakis and Vakalounakis, 1992). The development of resistant cultivars has been the mainstay of wilt control in cotton, particularly where cotton defoliating strains have become established. The usefulness of some resistant types has been curtailed by the evolution of strains able to overcome their resistance (Bell, 1992; Melero-Vara et al., 1995; Ma and Shezeng, 2000). In potato, truly resistant commercial cultivars are not yet available, but considerable work is underway to identify disease-resistant germplasm (Powelson and Rowe, 1993; Pavek et al., 1994; Corsini et al., 1996). Host resistance is also an important wilt management factor in strawberry (Shaw et al., 1996), horseradish (Atibalentja and Eastburn, 1988), hop (Romanko et al., 1996), aubergine (Cirulli et al., 2000), lettuce (Hayes et al., 2007) and sunflower (Sackston, 1992). The application of genetic engineering techniques may increase this potential greatly (McFadden, 2000), particularly approaches aimed at activating natural defence mechanisms (Li et al., 1996). Resistance to V. dahliae in transgenic potatoes has recently been achieved by transfer of an antifungal defensin peptide from lucerne (Gao et al., 2000).

Grafting wilt-susceptible cultivars to resistant tomato rootstocks has been used to manage Verticillium in tomato (Granges et al., 1996; Lazarovits and Subbarao, 2009) and aubergine (Ginoux and Dauple, 1982). Grafting to Verticillium-resistant rootstocks is also used in woody plants such as pistachio (Morgan et al., 1992).

Cropping Systems

Although there are many reports of the value of crop rotation in controlling wilt diseases, there is considerable disagreement as to its overall effectiveness. Rotation with paddy rice has proven particularly effective in eliminating V. dahliae from soil as a result of flooding (Pullman and DeVay, 1981) and reducing wilt in subsequent cotton crops. Cotton wilt has been reduced by rotations with cereals, legumes and crucifers (Egamov, 1976; Mannapova, 1976; Butterfield et al., 1978) and with lucerne (Sezgin et al., 1982). However, there are reports that rotation systems have not been effective in providing control (Huisman and Ashworth, 1976). Studies of the effects of various rotations on potato early dying also have shown variable results (Busch et al., 1978; Joaquim et al., 1988; Easton et al., 1992; Powelson and Rowe, 1993; Chen et al., 1995). The inclusion in the rotation of crops intended to be ploughed down, so called green manures, has shown some effectiveness in Verticillium management (Lazarovits et al., 2000), particularly with sudan grass in potato (Davis et al., 1996) and broccoli and other crucifers in several crops (Xiao et al., 1998; Subbarao et al., 1999; Subbarao et al., 2007; Inderbitzin et al., 2018). However, results with these treatments in various locations and cropping systems have been highly variable. Removal of infected crop debris at harvest has been advocated in some cropping systems (Mol et al., 1995) but deemed ineffective in others. The highly variable results from modifications in cropping systems is probably a reflection of intrinsic differences in strains or pre-plant populations of the pathogen in different localities, major differences in soils or their microflora (Inderbitzin et al., 2018) or the degree to which weed hosts have been excluded from rotation crops (Minton, 1972; Busch et al., 1978).

Pathogen-free Planting Material

Seed transmission of V. dahliae is significant in several crops, but mostly as a means of introduction of the pathogen into new areas (Goethal, 1971; Spek, 1973; Bejarano-Alcazar et al., 1996). Acid delinting of cotton seed effectively removes V. dahliae (Shen, 1985). Fungicidal seed treatments may have some value, but only in eliminating the pathogen on seed surfaces.

Transmission of V. dahliae in vegetative planting stock is significant, both as a means of introduction of highly aggressive strains and as a source of inoculum to infect crops directly (Thanassoulopoulos, 1993; Chen, 1994; Omer et al., 2000). The implementation of inspection and certification schemes to reduce the distribution of infected planting stock is an important disease management practice in some crops (Tsror et al., 1999) and should be given a higher priority. Hot-water treatment of tubers may have potential as a means to eliminate the pathogen in stocks of some high-value ornamental crops (Gilad et al., 1993).

Biological Control

Although not widely used at present, considerable research is being aimed at the development of specific biocontrol agents that may be useful against V. dahliae (Tjamos, 2000). Both fungal and bacterial biocontrol agents are being examined that can be applied to seed and roots of planting stock. Examples of fungi that have some efficacy include Pythium oligandrum (Al-Rawahi and Hancock, 1998), Heteroconium chaetospira (Narisawa et al., 2000) and Talaromyces flavus (Fravel et al., 1995; Madi et al., 1997; Nagtzaam et al., 1998). Effective bacterial antagonists include plant growth promoting rhizobacteria such as Pseudomonads (Sharma et al., 1998) and other genera obtained from plant rhizospheres (Berg et al., 1994).

Fertility and Irrigation Management

Studies with nitrogen and phosphorus management have shown in some cases that providing optimal amounts of these nutrients can minimize Verticillium wilt (Pennypacker, 1989; Davis et al., 1994). The fertilizer source of nitrogen may also be significant (Elmer et al., 1994; Lazarovits et al., 2000). The addition of various organic soil amendments, both plant and animal derived, has shown to be effective in reducing disease in some cropping situations (Conn et al., 1999; LaMondia et al., 1999; Lazarovits et al., 1999, 2000). This occurs primarily by affecting the survival of V. dahliae microsclerotia in soil and increasing populations of other components of the soil microflora. A limitation of this approach is that it is often only effective in certain soils, locations or cropping systems and may be totally ineffective elsewhere (Lazarovits et al., 2000). In irrigated crops, manipulation of soil moisture offers potential as a management technique. In potato early dying, it has been found that disease is increased by excessive soil moisture during the first half of the growing season and that reducing early season irrigation may be a viable option to minimize disease losses (Cappaert et al., 1994). Similar findings have been made with cauliflower production (Xiao et al., 1998).

Pre-plant Soil Treatments

Metham-sodium is widely used in potato production as a preplant soil fumigant for control of both Verticillium and nematodes (Powelson and Rowe, 1993; Saeed et al., 1997). However, its efficacy against V. dahliae is low (Duniway, 2002). Pre-plant soil disinfestation with a range of biocidal chemicals is used throughout the world to control wilt in the production of many high-value crops. Control of Verticillium wilt following fumigation may also be related to the reduction of soil populations of nematodes, which act synergistically in diseases caused by V. dahliae.

In production areas with warm, arid climates, plastic tarps have been applied to moist soils, allowing them to heat with solar energy. This process, referred to as pre-plant soil solarization, has proved a successful management technique for Verticillium by reducing the viability of miscrosclerotia in soil (Katan, 1985; Melero-Vara et al., 1995; Jimenez-Diaz et al., 1996). It has been particularly effective for production of crops grown in ground beds under glass or polythene (Tjamos et al., 1989; Bourbos et al., 1996), although the use of hydroponic culture or artificial growing substrates will generally avoid the disease entirely. Solarization also has been used successfully to control wilt in perennial crops by the application of tarps at the time of planting or even to established fruit or nut trees (Ashworth and Gaona, 1982; Tjamos et al., 1991; Stapleton et al., 1993).

Quantitative disease forecasting systems have been developed that can be used to assess the potential risk of disease losses to Verticillium wilt based on pre-plant populations of microsclerotia in soil (Wheeler et al., 1992; Khan et al., 2000). In some situations, these are used as effective tools to assess whether to plant susceptible crops in particular fields or whether there may be an economic benefit from soil fumigation, solarization or other pre-plant treatments.

25/01/20 Review by:

Krishna Subbarao, University of California, Davis, USA