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Although V. dahliae causes a disease syndrome often referred to as a wilt, wilting may not be the main symptom seen or may be absent in many hosts. The combination of visible symptoms depends on the host, on the resistance of the cultivar and on the environmental conditions. The most extreme form of disease is an irreversible wilting or total defoliation of the whole plant followed by death. However, wilting may affect only some shoots, leaves or even parts of leaves, so called 'one-sided wilt'. Not uncommonly, sectorial chlorosis and/or necrosis of leaf tissue may be the only external symptom of disease. Sometimes the only symptom is a chlorosis of lower leaves or an overall stunted growth of the plant. In tomato and aubergine, a tan discoloration of the vascular tissues may be seen in a sectioned stem. This is much less evident than the chocolate-brown discoloration characteristic of Fusarium wilt. In herbaceous hosts, symptoms are usually not evident until 4-8 weeks into vegetative growth and often develop only when fruit or tuber production begins. In the 'early dying' disease of potato, the pathogen causes premature senescence and there are no characteristic symptoms (Powelson and Rowe, 1993). In woody hosts, poor growth and early leaf senescence may be the only symptoms. Vascular staining may be present in woody xylem tissues, but may be restricted to certain growth rings.
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).
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, 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-Vera 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 et al., 1998), hop (Romanko et al., 1996), aubergine (Cirulli et al., 2000) and sunflower (Sackston et al., 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) and aubergine (Ginoux and Dauple, 1982). Grafting to Verticillium-resistant rootstocks is also used in woody plants such as pistachio (Morgan et al., 1992).
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 (Subbarao et al., 1999). 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, 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).
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). 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-Vera 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.
V. dahliae affects many important crops worldwide and causes economically significant losses in many countries. History shows that V. dahliae has the potential to evolve new strains that can overcome the resistance in commercial cultivars, particularly in cotton. It has also shown that the pathogen can be taken to new areas and cause serious losses (Nachmias and Krikun, 1985). Thus, for many vulnerable crops, selection for wilt resistance remains a major criterion in breeding programmes.
Verticillium wilt is the most important disease causing losses to the cotton crop in the three major cotton-producing countries (China, the former Soviet Union and the USA) and eight of the other top 20 cotton-producing countries (Turkey, Australia, Greece, Syria, Zimbabwe, Peru, South Africa and Spain). In certain states of India, Brazil, Mexico and Argentina it is also the major pathogen (Bell, 1992).
In the USA, losses from potential cotton production from 1952 to 1990 ranged from 1.46% to 3.48% or 521,600 to 1,238,800 bales. The total number of bales lost for the period was 11,077,300. The highest loss occurred in 1967 since when losses have decreased due to a decline in the use of nitrogen fertilizer and irrigation water linked with the production of resistant cultivars (Bell, 1992).
The most severe losses occur in the former Soviet Union. Annual crop losses of 25-30% occur in many potentially high yielding farms. In 1996, losses of 500,000 metric tonnes of raw cotton were estimated (ca 760,000 bales); 80% of the losses occurred in Uzbekistan (Mukhamezhanov, 1966). Losses have since diminished in this region with the cultivation of resistant cultivars (Bell, 1992). In China, wilt losses for cotton for 1982 (Shen, 1985) and 1993, 1995 and 1996 (Ma and Shezeng, 2000) were estimated at 100,000 tonnes. Al-Hamidi (1985) reported that more than 50% of the cotton-growing area in Syria is infected with V. dahliae and another wilt pathogen, Fusarium oxysporum f.sp. vasinfectum. In New South Wales, Australia, Verticillium wilt appeared late in the season and with a disease incidence of 4.1%, only small economic losses were recorded (Anon., 1987).
Losses in cotton fibre quality should be added to total yield losses. Yarns from fibre produced on infected plants are of a lower grade and have an inferior appearance. This is because of an increase in number of immature fibres which are shorter and weaker than normal fibres (Bell, 1992).
In potato, losses in the USA as high as 30-50% of specific crops have been recorded (Powelson and Rowe, 1993). Johnson et al. (1987) also reported individual losses of 31% in potato yields in the USA. Yield losses in Israel in the spring and autumn due to V. dahliae were 32 and 46%, respectively. Potato yields in the region were 50 t/ha and 30 t/ha in the spring and autumn, respectively, so the potential loss due to the pathogen was 16 t/ha (Nachmias et al., 1988). The economic impact of Verticillium wilt across the potato industry is very significant because expensive soil fumigation has become a routine disease-preventative practice, particularly in irrigated production areas (Rowe, 1985). Significant losses in potato, tomato and stone fruits occur worldwide in temperate production areas.
In California, USA, black heart caused by V. dahliae is an occasionally serious problem in almond orchards where severe economic losses of $9000-11,000/ha have been recorded (Stapleton, 1997). Economic losses were recorded over a 5 year period and were primarily due to tree removal, replacement costs, extra pruning and lost production from weakened trees (Asai and Stapleton, 1994).
In Greece, Verticillium wilt reduced the early commercial yield of aubergines by 40.8% and the final commercial yield by 39.4%. Infection also spoiled the fruit quality (Bletsos et al., 1999). In southern Italy, Verticillium wilt caused a yield decrease of 54.7% and 62-85% in the susceptible aubergine cultivars Bari-12 and Florida Market, respectively, compared with healthy plants grown in non-infected soil. Less severe decreases in yield were observed in resistant cultivars. In susceptible cultivars, the disease reduced both weight and number of fruits, while in resistant cultivars only one of these factors was reduced (Ciccarese et al., 1994).
V. dahliae caused wilting, stunting and early dying of paprika (Capsicum annuum) plants in Israel, resulting in a 22% reduction in yield. The disease caused height reduction of 43-62% in three individual cultivars (Tsror et al., 1998).
A comprehensive survey of Verticillium wilt in olives was carried out in nine provinces of Syria over 7 years. The percentage infection ranged from 0.85 to 4.5% in different provinces and newly planted groves in lowland areas had more infection than older groves in hilly areas. The disease caused a loss of between 1 and 2.3% of total olive production annually (Al-Ahmad and Mosli, 1993).
Significant wilt losses occur in oilseed rape in northern Europe. In Sweden, Verticillium wilt can be common in some rape-growing areas. Infected plants ripen prematurely and considerable seed scattering can occur. In a small plot experiment, V. dahliae reduced seed yield by 50% (Svensson and Lerenius, 1987).
In 1976, 108 tomato fields in western North Carolina, USA, were surveyed for Verticillium wilt. The disease was confirmed in 56% of fields and the estimated disease incidence was 9.2%. Both race 1 and race 2 isolates were recovered. In field tests, mean yields were reduced by race 1 isolates of V. dahliae in the susceptible cultivars Manapal and Walter and the race 1 resistant cultivars Flora-Dade and Monte Carlo by 39.9, 47.1, 3.5 and 6.5%, respectively. Race 2 isolates reduced yields by 10.3, 31.2, 19.3 and 22.8% in the four cultivars (Bender and Shoemaker, 1984).
Significant wilt losses can also occur in hop, mint and strawberry in Europe and the USA.