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Fusarium Crown and Root Rot of Tomato and Control Methods



Nusret Ozbay and Steven E. Newman
 
ABSTRACT

F. oxysporum is a widespread soilborne plant pathogen, which causes diseases such as vascular wilt and crown and root rot. The species delimitation has been defined according to morphological and physiological characteristics; with strains of F. oxysporum classified into formae speciales on the basis of pathogenicity on a particular host plant and races based on differences in virulence to given host cultivars. There are over 120 described formae speciales and races. One of these formae speciales is Fusarium oxysporum Schlect f. sp. radicis-lycopersici Jarvis and Shoemaker, which causes Fusarium crown and root rot (FCRR) of tomato. FCRR is one of the most damaging soil-borne diseases of tomato and becoming more common in greenhouse and field tomato production. The disease occurs in both the greenhouse and the field on tomato worldwide and causes significant losses in tomato production. In closed systems, with recirculation of nutrient solution and rock wool as a growing medium, crown and root rot of tomato can cause serious problems. This review discussed biology, ecology and epidemiology of the fungus and management options; cultural control, chemical control, biological control and integrated control in managing FCRR.

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  How to cite this article:

Nusret Ozbay and Steven E. Newman, 2004. Fusarium Crown and Root Rot of Tomato and Control Methods. Plant Pathology Journal, 3: 9-18.

DOI: 10.3923/ppj.2004.9.18

URL: https://scialert.net/abstract/?doi=ppj.2004.9.18

INTRODUCTION

Fusarium crown and root rot (FCRR) caused by Fusarium oxysporum Schlect f. sp. radicis-lycopersici Jarvis and Shoemaker (FORL) is one of the most damaging soil-borne diseases of tomato and becoming more common in greenhouse tomato production[1]. The disease was initially reported in 1974 in Japan[2] and soon afterwards was identified in North America[3]. FCRR also has been reported in Canada, Mexico, Israel, Japan, many countries in Europe[4] and other states in the U.S. including California, New Jersey, New York, New Hampshire, Ohio, Pennsylvania and Texas[1]. The disease was identified in Colorado in 1998. The disease occurs in both the greenhouse and the field on tomato worldwide and causes significant losses in tomato production[5,6]. In closed systems, with recirculation of nutrient solution and rock wool as a growing medium, crown and root rot of tomato can cause serious problems[7,8]. It is reported that FCRR affects 40% of the surveyed acreage in Florida[3]. Commercial yields have been reported to be reduced 15-65%[9,10,3].

Symptoms: Wide ranges of symptoms are associated with FORL. The fungus invades susceptible plants through wounds and natural openings created by newly emerging roots[1]. Early symptoms caused by FORL in tomato seedlings include stunting, yellowing and premature abscission of cotyledons and lower leaves. A pronounced brown lesion that girdles the hypocotyls, root rot, wilting and seedling death are advanced symptoms[11]. Typically, the first symptom in the mature plants is a yellowing along the margins of the oldest leaves when the first fruit is at or near maturity. Yellowing is soon followed by necrosis and collapse of the leaf petiole. Symptom development progresses slowly upward on successively younger leaves. Some plants may be stunted and wilt quickly and wither. Older plants may wilt slowly and still be alive at the end of the harvest[6]. Wilting first occurs during the warmest part of the day and plants appear to recover at night. Infected plants may be stunted, totally wilt and die, or persist in a weakened state, producing reduced numbers of inferior fruit[1].

As disease progresses dry brown lesions develop in the cortex of the tap or main lateral roots and taproot often rots away[1]. Chocolate brown lesions develop at or near the soil line and extend into the vascular system. When diseased plants are sectioned lengthwise, extensive brown discoloration and rot are evident in the cortex of the crown and roots. This brown vascular discoloration typically does not extend more than 25-30 cm above the soil line, which helps to distinguish this disease from Fusarium wilt, where discoloration may extend 1 meter high. Stem cankers may develop at or above the soil line. Following rains and during fogs, the pink sporulation of the pathogen can be profuse on exposed necrotic lesions[12,13,6]. The fungus produces masses of white mycelium and yellow to orange spores in necrotic stem lesions on dead and dying plants[1].

Biology, ecology and epidemiology: The causal fungus produces three types of spores: macroconidia, micro conidia and chlamydospores. Two of these spore types figure prominently in the survival and spread of FORL. Microconidia form in great abundance in necrotic tissue and have been implicated in the recolonization of sterilized soil in greenhouses through aerial dispersal[12,14]. They readily reinfest soil sterilized by heat or broad-spectrum biocides such as fumigants. Chlamydospores have thicker walls and enable the fungus to survive in the soil and wooden stakes for long periods in the soil[15,1].

FCRR severity varies widely by site and season and is favored by cool temperatures[16]. The pathogen grows best from 10 to 20°C, which is lower than the optimum for the fungus that causes Fusarium wilt on tomato[17,1]. The disease affects the plants in early crops and those located in the cooler areas of the greenhouse. The optimum temperature for disease development is 21°C. Low soil pH, ammoniacal nitrogen and waterlogged soil also exacerbate the disease[17,1].

FORL is a polycyclic soilborne pathogen[18]. Lateral spread of the pathogen from plant to plant during a growing season is via root-to-root contact[1]. Large numbers of FORL macroconidia are produced on stem surface of diseased plants and aerially disseminated in fields and greenhouses. Infection occurs either via soil infestation and subsequent root infection or by a direct infection of the foliage[18-20]. Movement of the fungus in the soil in the absence of roots is minimal (< 2.5 cm)[1]. Long-range dissemination of FORL can occur through infected transplants and perhaps via chlamydospores in soil particles on contaminated shoes, plant stakes, farm machinery, transplant trays and other equipment[12,1].

The host range of the pathogen includes some non-solanaceous plants, including spinach (Spinacia oleracea L.), beets (Beta procumbens L.), legumes, cucurbits and other solanaceous plants, such as pepper and eggplant (Solanum melongena L.), but not potatoes[12,1]. The fungus also has been isolated either naturally or experimentally from the roots of a number of weeds, including Brazilian pepper (Schinus terebinthifolius), carpet weed (Mollugo verticillata), hickweed (Stellaria media), corn spurry (Spergula arvense), cudweed (Gnaphatium sp.), curly dock (Rumex crispus), narrow leaved plantain (Plantago lanceolata), redroot pigweed (Amaranthus retroflexus), Scoparia sp., Shepard's purse (Capsella bursa-pastoris) and wild buckwheat (Polygonum convolvulus)[1] and saltcedar (Tamarisk nilotica)[21]. Monocots, such as corn (Zea mays L.) are not susceptible.

MATERIALS AND METHODS

Management options: Several control procedures have been attempted for managing FCRR in the greenhouse and field, including use of resistant varieties[9,22,79], cultural practices[5,23,24], pesticides[25,26], fumigants[27,10], etc. but losses are still substantial. Control programs should be put in place before planting, because there are no rescue treatments. Once the disease is present, avoiding stress is an important part of minimizing losses[23]. Four management options; cultural control, chemical control, biological control and integrated control will be discussed in the following sections for managing FCRR in a greenhouse.

Cultural control: Aggressive sanitation programs are very important, starting with a very clean house and cleaning or disinfecting. The worse cases of this disease have been associated with attempts to reuse items without sanitizing them, especially items that come in direct contact with the soil mix[23]. Crown rot is likely to occur with a higher frequency where direct seeding is used, instead of healthy transplants and where the soil contains high levels of chloride salts[17]. The utmost sanitation production scheme for transplants in greenhouses should be used so that individuals or equipment used within or around the transplant site do not become contaminated with disease-causing organisms from the field. Finally, transplants should be transported, pulled and set without tissue damage, as damaged tissues are likely to be sites for infection[17]. Transplanting should be done when soil or media is 20°C or above[28]. The selection and application of fertilizers can significantly influence disease development. For example, increasing soil pH and minimizing use of ammoniacal nitrogen help controlling FCRR[17]. Dead tomato plants need to be completely removed[5].

One of the most important components in an integrated disease control program is the selection and planting of cultivars that are resistant to pathogens. The term resistance usually describes the plant host’s ability to suppress or retard the activity and progress of a pathogenic agent, which results in the absence or reduction of symptoms[29]. Some resistant varieties are available for FCRR for mostly greenhouse production. The use of crown rot resistant cultivars is increasing, but is currently not widely accepted due to horticultural characteristics that make these varieties less competitive than standard varieties[9]. The following are some cultivars with resistance: Trend, Trust, Medallion, Match, Switch and Blitz for greenhouse production; Charleston and Conquest for field production[28,30]. Fusarium crown and root rot resistance is conferred by a single dominant gene and already has been incorporated into commercially available cultivars[22].

Crop rotation is a historical method of crop production that reduces soil pest problems by removing susceptible plants from an infested area for a period of time long enough to reduce pest populations to tolerable levels[31]. Rotation away from tomatoes may be necessary on fields with a recurring crown rot problem. Avoid rotation with susceptible crops, such as eggplant or peppers; use non-hosts, such as lettuce instead[5,24]. Crop rotation as a control strategy may be limited in controlling FCRR because the fungus can survive in soil for many years. Capital field improvements such as irrigation systems and the availability of suitable land also limit adoption of crop rotation as a pest control strategy. Once a grower has invested in an irrigation system for a piece of land, the grower is less likely to rotate to a lower value crop[5].

Another cultural method in controlling FCRR is soil solarization. Solarization is a non-chemical soil disinfestation method, first developed for soilborne disease control in Israel and California during the 1970s[76,33]. Soil solarization, is defined as heating the soil by solar energy resulting in both physical and biological processes to control pathogens and other soil pests[32,77]. Solarization depends on solar energy to heat the soil to temperatures that are lethal to pathogenic organisms. This is accomplished by covering moist soil with a clear plastic film or mulch during a 2 to 8 week period with plentiful solar radiation. Most soilborne pests and plant pathogens are mesophilic and are killed at temperatures between 40 and 60°C. At these elevated temperature, disfunction of membranes and increased respiration are responsible for death. However, death depends on the thermal dose, a product of temperature and exposure time[34]. Exposure to long periods of sub-lethal temperatures may effectively control diseases by reducing the ability of propagules to geminate, increasing the susceptibility to biological control organisms and decreasing the ability to infect the host[34].

Soil solarization has been demonstrated to control FCRR. In stydies with different solarization methods, soil solarization reduced populations of Fusarium oxysporum f.sp. radicis-lycopersici down to a depth of 5 cm[35].

Crown rot incidence was significantly reduced by Metam Sodium (29%), solarization + Metam Sodium (51%) and by Methyl bromide+chloropicrin (50%), while disease severity was significantly reduced (74%) by both the latter two treatments. No significant differences in marketable yield were observed among the treatments[36]. Preliminary studies carried out in the open field showed that 12 days of soil solarization reduced survival of FORL propagules significantly. The effectiveness of pathogen control was improved by combing solarization with manure, or extending the solarization treatment to 27 days. In a closed greenhouse, solarization and biofumigation with bovine manure proved effective in reducing the viability of FORL chlamydospores, reducing disease incidence and in increasing commercial yield[37].

Grafting also is used in control of crown and root rot of tomato. Resistant rootstocks provide excellent control of many tomato soilborne pathogens and particularly Fusarium oxysporum f. sp. lycopersici., F. oxysporum f. sp. radicis-lycopersici, P. lycopersici and Meloidogyne spp.[38]. This technique, which initially was considered too expensive, is now widely used at a commercial level in many Mediterranean countries and North America[38]. In general, without grafting, the tomato plant density per hectare is about 18,000 plants. When grafted plants are used, the same yield could be obtained with half plant population (9,000 plants ha-1)[38]. In addition to controlling some soilborne pathogens, tomato grafting promotes growth, increases yield, increases plant tolerance to low temperature, extends the growth period and improves fruit quality[38]. Susceptible tomato plants grafted on FCRR-tolerant hybrid rootstock (He-man), even cropped in a severe FORL infested soil, remained healthy during the growing season and gave a profitable yield[39].

Chemical control: Chemical control of FCRR in steam sterilized soil by using a captafol drench proved effective in preventing reinfestation by airborne FORL conidia[40]. Mihuta-Grimm[26] reported that the application of benomyl at 0.090 g a.i./L on a 21-day schedule to plants growing on rockwool productions slabs resulted in optimum FCRR control. Yield from infected transplants treated with benomyl, however, was not significantly different from that of control plants. Other candidate fungicides proved to be phytotoxic at levels needed to control FCRR[41]. Although fungicides such as benomyl or captafol have been demonstrated[25] to be effective, they have some drawbacks. They are expensive, cause environmental pollution and may induce pathogen resistance. Fungicides added to seeds can also cause stunting and chlorosis of young seedlings and results may vary as fungicides are absorbed or inactivated by components of the soil or planting medium[41]. Captafol is no longer labeled for usage, thus it can not be used commercially.

Fumigation with methyl bromide (MBr)+chloropicrin formulations have been the most commonly used pre-plant practice for control of Fusarium crown and root rot in tomatoes[36,10]. Application of MBr+chloropicrin significantly reduces the incidence and severity of the disease[10]. Mbr: chloropicrin (67:33, by volume) reduced populations of FORL to a depth of 35 cm[35]. However, Fusarium crown and root rot incidence is still very high[22]. Even with the use of methyl bromide as a preplant fumigant, epidemics of Fusarium crown and root rot have occurred in commercial production fields[35]. MBr is a powerful soil fumigant providing effective control of a wide range of soil-borne pathogens and pests, including fungi, bacteria, nematodes, insects, mites, weeds and parasitic plants[42]. Despite these major advantages, the use of MBr has been associated with major problems, including the depletion of the ozone layer[43]. Because of this, its production and use will be phased out on a worldwide scale, by 2005 in the U.S. and E.U. and other developed countries and by 2015 in the developing countries[44]. An estimated 22.2 million kilograms of methyl bromide are applied annually for preplant soil fumigation in the United States[45]. Many strawberry and tomato growers have depended on MBr for control of nematodes, weeds and fungal pathogens[46]. The elimination of MBr in accordance with the U.S. Clean Air Act poses a critical challenge to these growers, whose crops are valued at more than $2.5 billion annually[46].

The ban on MBr production and use has prompted the study of new chemical alternatives for the control of soil-borne pests. However, these materials tend to provide a narrower spectrum of control than MBr, have less predictable efficacy and may have their own problems with environmental pollution and safety[47]. Five soil fumigants (1,3-dichloropropene, chloropicrin, dozamet, fosthiazate and sodium methyldithiocarbamat, a contact nematicide and several combinations with pebulate herbicide were compared to MBr/chloropicrin (98 and 2%, respectively) for control of nutsedge, Fusarium wilt and crown rot and nematodes in tomato[48]. Fusarium crown rot was reduced by methyl bromide and 1,3 dichloropropene + chloropicrin in the spring, but in the fall all chemical treatments, except those containing SMDC, provided better crown rot control than MBr[48]. McGovern et al.[38] reported that Crown rot incidence (defined as percentage number of plants infected by the pathogen) was significantly reduced by Metam Sodium (29%), solarization + Metam Sodium (51%) and by MBr + chloropicrin (50%), while disease severity (defined as percentage crown discoloration) was significantly reduced (74%) by both the latter two treatments. No significant differences in marketable yield were observed among the treatments.

A fresh market tomato study comparing metam sodium and MBr fumigation to an untreated control reported that yields and fruit quality obtained with metam sodium were equivalent to those achieved with methyl bromide fumigation[49]. Metam sodium has been demonstrated to significantly reduce crown rot incidence and when combined with solarization, control was equivalent to MBr+chloropicrin[36]. Metam sodium could reduce Fusarium crown rot only when thoroughly incorporated in the planting bed, such as through application to the soil prior to bed formation[36].

Plantpro45TM, a new “low risk” iodine-based compound, was investigated as a potential alternative in controlling FCRR[50]. Plantpro45TM provided significant control of Fusarium crown rot of tomato in naturally infested fields. Under greenhouse conditions, soil drench with Plantpro 45TM at 80 ppm a.i. followed by planting 21 days later and a foliar application at 80 ppm one week after planting increased root and shoot weight and improved root condition of tomato when grown in field soil naturally infested with FORL. Final disease incidence ratings revealed that plots pretreated with Plantpro45 were comparable to MBr for control of FCRR[50,51].

Biological control: Research has demonstrated that biological control of FCRR has been successful in some instances under greenhouse and field conditions. The fungus Trichoderma, a natural soil-inhabiting genus, has been used successfully to control Fusarium crown rot and root rot of tomatoes[27,52]. The potential of T. harzianum, Aspergillus ochraceus Wilhelm and Penicillium funiculosum Thom in controlling FCRR of field grown tomatoes was shown[53]. Sivan and Chet[54] used T. harzianum in combination with soil sterilization and reduced rates of MBr to obtained significant control of tomato crown and root rot in the field with transplants colonized by T. harzianum during greenhouse propagation.

Datnoff et al.[27] conducted field experiments in Florida to evaluate commercial formulation of two fungi, T. harzianum and Glomus intraradices Schenck & Smith, for control of FCRR of tomato. Compared to untreated controls, significant reduction in disease incidence was obtained with treatment by biocontrol agents. The interaction between G. intradices and FORL and effect of G. intradices on tomato plants were investigated. Caron et al.[55,56] reported that tomato crown and root rot was decreased with G. intradices. However, there was no growth response of tomato plants to inoculation with the biocontrol agent. T. harzianum applied, as a peat-bran preparation to the rooting medium at the time tomatoes were transplanted. Such an application resulted in significant decrease in Fusarium crown rot through the growing season in field conditions. Yield increased as much as 26.2% over the controls in response to the treatment[52]. Nemec et al.[57] evaluated some biocontrol agents; T. harzianum, G. intraradices and Streptomyces griseverdis, for controlling root diseases of vegetable crops and citrus. At the end of the study, they found that all biological control agents reduced FCRR of tomato in the field. T. harzianum and B.subtilis were the most effective biocontrol agents. Paenibacillus macerans and T. harzianum were evaluated for promoting plant growth and suppressing FCRR under fumigated and non-fumigated field conditions[58]. Trichoderma harzianum and P. macerans significantly reduced severity of FCRR. Trichoderma harzianum reduced the severity of FCRR by 12% and P. macerans 9% in comparison to the untreated control in the nonfumigated treatments. No differences were observed between the biologicals and the untreated control in the MBr treated plots[58]. Datnoff and Pernezny[58] also reported that T. harzianum and P. macerans alone or in combination significantly increased the growth of tomato transplants in the greenhouse and after outplanting into the field 30 days later.

The potential of T. harzianum as a biocontrol agent in a soilless culture system was investigated with tomato plants infected with FORL. The application of Trichoderma reduced the incidence and spread of FCRR in tomatoes on an artificial growing medium[59]. Marois and Mitchell[25] reported that in greenhouse and growth chamber experiments, the fungal biocontrol amendment significantly reduced the mean lesion length and the incidence of FCRR. Under greenhouse conditions, the incidence of crown rot of tomato was reduced by up to 80% 75 days after sowing when T. harzianum T35 was applied as either a seed coating or a wheat bran-peat preparation applied to soil[52].

Much research has been done on the potential of nonpathogenic F. oxysporum for control of FCRR. Louter and Edgington[60] and Brammall and Higgens[61] used isolates of avirulent F. oxysporum and isolates of F. solani, respectively, to reduce the effects of FORL on tomato plants. It was suggested that the fungi acted through either cross protection (Louter and Edmington, 1985) or competition for infection court sites[61].

Alabouvette and Couteadier[62] studied the efficacy of nonpathogenic F. oxysporum strain Fo47 and the fungicide himexazol for of control the root diseases of tomato in the greenhouse. Both treatments gave a good control of FCRR; the yield was slightly higher with the biological treatment and the cost of the biocontrol treatment was less than the cost of the chemical product. Under greenhouse conditions, control of FCRR of tomato can be achieved by introduction of either strain Fo47, or fluorescent Pseudomonas strain C7, or by the association of both into the growing medium[63]. Four nonpathogenic isolates of F. oxysporum (26B, 43A, 43AN1 and 43AN2) and an isolate of T. harzianum (Th2) were found to be effective in protecting tomato seedlings from FCRR. However, the T. harzianum isolate was less effective than the F. oxysporum isolate at reducing disease[7].

There are also some bacteria, especially Pseudomonas spp., that have been shown to be effective in controlling FCRR[64]. Pseudomonas fluorescens strain CHA0 suppressed crown and root rot of tomato[64,65]. M’Piga et al.[66] reported that P. fluorescens colonizes and grows in the outer root tissues of whole tomato plants and sensitizes them to respond rapidly and efficiently to FORL attack in addition to exhibiting an antimicrobial activity in planta[65].

Streptomyces griseoviridis strain K61 (MycostopTM) has been tested against F. oxysporum-induced crown rot in Israel and in the UK[67]. A clear reduction of the disease was observed, but complete control was not achieved by using MycostopTM. MycostopTM is a live formulated strain of the bacterium S. griseoviridis that was discovered in Finnish peat[67]. It is labeled for use on greenhouse tomato and is available from at least two suppliers in the U.S.[67]. Streptomyces sp. Di-944, a rhizobacterium from tomato, suppressed Rhizoctonia damping-off and Fusarium root rot in plug transplants when applied to seeds or added to potting medium. Antibiosis was suspected as a key mechanism of biocontrol[68].

Among the most promising bioactive oligosaccharides is chitosan (poly-N-glucosamine), a mostly deacetylated derivative of chitin occurring in the cell walls of several fungi, which is readily extracted from the chitin of crustacean shell wastes[78]. Oligomers of chitosan, which are likely to be released by the action of plant encoded-chitinase from walls of invading fungi, can protect tomato roots against F. oxysporum f.sp. radicis-lycopersici when applied to the seed or roots[69,70]. Chitosan, derived from crab-shell chitin, was applied as seed coating and substrate amendment prior to infection with the fungus F. oxysporum f. sp. radicis-lycopersici. Experiments were performed either on a mixture of peat, perlite and vermiculite or on bacto-agar in petri dishes. In both cases, a combination of seed coating and substrate amendment was found to significantly reduce disease incidence.

The potential of Bacillus pumilus strain SE 34, either alone or in combination with chitosan, for inducing defense reactions in tomato plants inoculated with the vascular fungus, FORL, was studied by light and transmission electron microscopy[71]. Treatment of the roots with B. pumilus alone or in combination with chitosan prior to inoculation with FORL, substantially reduced symptom severity of FCRR as compared with untreated controls. Although some small, brownish lesions could be occasionally seen on the lateral roots, their frequency and severity never reached levels similar to those observed in control plants[71].

Chitosan, oligandrin and crude glucan, isolated from the mycoparasite P. oligandrum, were applied to decapitated tomato plants and evaluated for their potential to induce defence mechanisms in root tissues infected by Fusarium oxysporum f. sp. radicis-lycopersici[72]. A significant decrease in disease incidence was monitored in oligandrin- and chitosan-treated plants as compared to water-treated plants whereas glucans from P. oligandrum cell walls failed to induce a resistance response. In root tissues from oligandrin-treated plants, restriction of fungal growth to the outer root tissues, decrease in pathogen viability and formation of aggregated deposits, which often accumulated at the surface of invading hyphae, were the most striking features of the reaction. In chitosan-treated plants, the main response was the formation of enlarged wall appositions at sites of attempted penetration of the reaction.

Lettuce residue soil amendments and lettuce intercropping were considered for biological control[73,74]. Co-planted lettuce and T. harzianum strain Th2 provided protection from naturally occurring FCRR in a commercial tomato crop[7].

Integrated management: At present, Fusarium crown and root rot is difficult to control in field-grown tomatoes because the pathogen rapidly colonizes sterilized soil and persists for long periods. However, an integration of the following management procedures may help to reduce the impact of crown and root rot[1]:

1. Use disease-free transplants. Transplant houses should not be located near tomato production fields. Avoid over watering, which makes the transplants more susceptible to crown and root rot. Disinfect transplant trays by steaming before reuse.
2. Use a pre-plant fumigant. The soil should be of good tilth and adequately moist for at least two weeks prior to fumigation. Use an appropriate chisel spacing and depth and immediately cover the bed with plastic mulch following fumigation.
3. Optimize cultural practices in the field. Avoid injuring transplants when they are set in the field. Physical damage and injury from excessive soluble salts may make young plants more susceptible to crown and root rot. The use of water drawn from wells rather than ditches for watering-in transplants may help to prevent recontamination of fumigated soil. Avoid ammoniacal nitrogen and maintain the soil pH at 6 to 7. Rapidly plow in crop debris following final harvest. Disinfest tomato stakes before reuse, or use new stakes.
4. Rotate with a non-susceptible crop. Incomplete knowledge of the host range of FORL makes precise recommendations in this area difficult. Current research data suggests that leguminous crops should be avoided in favor of corn and similar crops. Rotation and intercropping with lettuce had reduced FORL in greenhouse-grown tomatoes.
5. Significant progress has been made in breeding for resistance to Fusarium crown and root rot in field-grown tomatoes. Although the commonly used commercial cultivars do not have resistance, some resistant cultivars, such as Conquest, are available for field use and Trust for greenhouse use.
6. Additional management strategies under investigation include the use of biological control, cover crops and soil solarization alone or in combination with fumigants.

Integration of different management methods has been shown to be effective in controlling FCRR. McGovern[30] indicated that Fusarium crown and root rot of tomato can be effectively managed by integrating the use of pathogen-free transplants and stakes, resistant cultivars and pre-plant fumigation. A commercial tomato field in southwest Florida, naturally infested with FORL was used to compare the effectiveness of methyl bromide: chloropicrin, 67%:33% (Terr-O-Gas 67, 336 kg ha-1), metam sodium (Vapam, 935 l ha-1), composted sewage sludge (Florida Organix, 5.5 MT ha-1), soil solarization and combinations of solarization and Vapam or Florida Organix in reducing FCRR. Fusarium crown rot incidence was significantly reduced by Vapam (-29%), solarization plus Vapam (-51%) and by Terr-O-Gas (-50%), while disease severity was significantly reduced (-74%) by both the latter two treatments[36].

Minuto et al.[75] reported that the combination of soil solarization with reduced dosage of Dazomet and of MB controls Fusarium and Verticillium wilts and Fusarium crown rot on tomato. Preliminary studies carried out in the open field showed at 12 days of soil solarization reduced survival of FORL propagules significantly. The effectiveness of pathogen control was improved by combing solarization with manure, or extending the solarization treatment to 27 days. In a closed greenhouse, solarization and biofumigation with bovine manure proved effective in reducing the viability of FORL chlamydospores, disease incidence and in increasing commercial yield[37].

Under field conditions, the combination of T. harzianum with soil solarization or with a reduced dose of methyl bromide resulted in significant disease control of FCRR of tomato induced by FORL[54]. Combination of the biocontrol agent Pseudomonas fluorescens with the mineral element zinc significantly reduced disease severity of FCRR of tomato, however, P. fluorescens strain CHA0 alone was only moderately effective[64].

Diseases caused by Fusarium spp. are important limiting factors in the production of tomato. Several types of diseases associated with these pathogenic fungi have been identified, including Fusarium wilt, foot rot and crown and root rot disease. One of the most damaging soilborne pathogens of tomato is Fusarium oxysporum f. sp. radicis-lycopersici, which causes Fusarium crown and root rot. Several control procedures have been attempted for managing FCRR in the greenhouse and field. Possibilities to manage fusarium wilt, rot by using fungicides or resistant cultivars, are limited. The use of Fusarium-resistant tomato cultivars can provide some degree of control of this disease, but the occurrence and development of new pathogenic races is a continuing problem and currently there are no commercially acceptable cultivars with adequate resistance to F. oxysporum f. sp. radicis-lycopersici. Although fungicides such as benomyl or captafol have been demonstrated to be effective, captafol is no longer labeled for usage and there is an imminent possibility of fungicide resistance.

Fumigation with methyl bromide (MBr) + chloropicrin formulations have been the most commonly used pre-plant practice for control of FCRR. Application of MBr + chloropicrin significantly reduces the incidence and severity of the disease. However, disease incidence is still very high. Even with the use of MBr as a pre-plant fumigant, epidemics of fusarium crown and root have occurred in commercial production fields.

Research has demonstrated that biological control of FCRR has been successful in some instances under greenhouse and field conditions. Biocontrol of FCRR with antagonistic microbes such as the fungus Trichoderma or non-pathogenic strains of F. oxysporum are areas of research. There are also some bacteria, especially Pseudomonas spp., that have been shown to be effective in controlling FCRR. More research needs to be done on biological and integrated management of FCRR.

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