Analysing spatial patterns of spread of Lettuce necrotic yellows virus and lettuce big-vein disease in lettuce field plantings

Spatial patterns of spread of lettuce big‐vein disease (LBVD) and Lettuce necrotic yellows virus (LNYV) were examined in two plantings each consisting of two blocks of lettuce. LBVD came from planting land infested with viruliferous Olpidium brassicae resting spores, while LNYV was introduced by aphid vectors from external sources consisting of LNYV‐infected sowthistle (Sonchus oleraceus) weeds. Clustering of LBVD was obvious in an area where the soil was heavily infested with only sporadic occurrence elsewhere. There was a steep decline in LNYV incidence over distance from a concentrated external weed source, with clustering of LNYV‐infected plants at the crop edge closest to it. There was no evidence of secondary spread with LBVD or LNYV.     

Transgenic resistance to Mirafiori lettuce virus in lettuce carrying inverted repeats of the viral coat protein gene

Mirafiori lettuce virus (MiLV), a plant RNA virus belonging to the genus Ophiovirus, is considered to be a causal agent of lettuce big-vein disease. In this study, inverted repeats of a fragment of the coat protein (CP) gene of MiLV in a binary vector pBI121 were transferred via Agrobacterium tumefaciens-mediated transformation into lettuce (Lactuca sativa L.) in order to generate MiLV-resistant lettuce. Forty T₁ lines were analyzed for resistance to MiLV by detecting MiLV in leaves, and two lines (lines 408 and 495) were selected as resistant to MiLV. Both lines were susceptible to Lettuce big-vein associated virus (LBVaV), and line 495 showed higher resistance to MiLV than line 408. Further analysis indicated that line 495 showed resistance to big-vein symptoms expression. Small interfering RNA (siRNA) molecules derived from the transgene were detected in plants of line 495. MiLV was detected in roots but not in leaves of line 495 plants after MiLV inoculation, suggesting that resistance to MiLV is less effective in roots than in leaves.     

Effects of fungicides and surfactants on the zoospores of Olpidium brassicae

Lettuce big-vein disease is transmitted from diseased to healthy plants by zoospores of the lettuce root-infecting fungus Olpidium brassicae. A laboratory technique based on microscopical examination of Olpidium Zoospores is described for assaying the toxicity of chemicals to zoospores. Chemcials found to kill zoospores in <1 h included copper (4 μ/ml), zinc (10μ/ml), diluted preparations of carbendazim (methyl-2-yl-benzimidazole carbamate) as Bavistinand a formulation of Bavisitin containing no carbendazim. Bavistn controlled the disease when introduced at a concentration of 0.6 g/litre into a lettuce crop grown in a re-circulated film of nutrient. Various surfactants inlcuding Agral, Cetrimide, Deciquam, Ethylan CPX, Hyamine 1622, Manoxol/OT and sodium lauryl sulphate were toxic to zoospores at concentrations of 1–10 μ/ml.     

Lettuce big-vein virus: mechanical transmission and relationships to tobacco stunt virus

Big-vein diseased lettuce plants contained an agent that could consistently be transmitted mechanically to Chenopodium quinoa, in which it caused characteristic local lesions. Mechanical transmission was also possible to five other plant species including Nicotiana benthamiana, N. clevelandii and N. occidentalis, but not to lettuce. Symptoms in N. occidentalis were reminiscent of those of tobacco stunt disease. With zoospores of originally virus-free Olpidium brassicae, subcultured on the roots of N. occidentalis-P1, sap-inoculated either from lettuce or via C. quinoa, the agent could be transferred back to lettuce in which characteristic symptoms of big-vein were reproduced.
Infectivity in sap at room temperature was reduced by half after 2 h, and was practically lost after one day. Thermal inactivation was considerable at 45°C and complete at 50°C. Most infectivity was lost at dilution 1:5, and the dilution end-point was 1:10. The agent survived well in leaf material stored at -80°C, or in sap from leaves ground in buffer with DIECA and activated charcoal and freeze-dried. Mechanical transmission required low dilution (1:2) in the buffer with charcoal, and chilling of materials and utensils.
In lettuce, N. occidentalis-P1 and C. quinoa, with all isolates tested but one, infection was always associated with the presence of rod-shaped particles which in the literature have been associated with lettuce big-vein, and are similar to those described for tobacco stunt. Results obtained corroborate the assumption that these particles are the virions of lettuce big-vein virus. The virus also resembles tobacco stunt virus in mechanical transmissibility, instability in sap and symptoms on N. occidentalis.     

The incidence and distribution of viruses infecting lettuce, cultivated Brassica and associated natural vegetation in Spain

A virus survey was conducted during the spring and autumn of 2001 and 2002 to determine the presence, prevalence and distribution in Spain of the viruses that are most commonly found infecting lettuce and Brassica worldwide. Crop plants showing virus symptoms from the principal lettuce and Brassica-growing regions of Spain, and some samples of the annual and perennial flora nearby, were tested by enzyme-linked immunosorbent assays using specific commercial antibodies against the following viruses: Alfalfa mosaic virus (AMV), Broad bean wilt virus 1 (BBWV-1), Beet western yellows virus (BWYV), Cauliflower mosaic virus (CaMV), Cucumber mosaic virus (CMV), Lettuce mosaic virus (LMV), Pea seed-borne mosaic virus (PSbMV), Turnip mosaic virus (TuMV) and Tomato spotted wilt virus (TSWV). Samples were also tested with a Potyvirus genus antibody. Virus incidence was much lower in spring than in autumn, especially in 2001. In spring 2002, CMV and LMV were the most prevalent viruses in lettuce, while CaMV was the most important virus present in Brassica crops grown in Navarra, followed by CMV and BWYV. In the autumn, the spectrum of viruses was different; potyviruses were widespread in lettuce grown in Madrid, but TSWV and BWYV were predominant in the Murcia region. The prevalent Potyvirus detected in lettuce fields was LMV, but none of the samples collected were positive for PSbMV or TuMV. In Brassica crops, TSWV was the most abundant in autumn-sown crops, especially in the Navarra region. All of the viruses present in lettuce and Brassica were also frequently detected in their associated natural vegetation at the same time, suggesting that they probably play an important role as virus reservoirs. Sonchus spp. were particularly common and were frequently infected with CMV, LMV and BWYV. Another common species, Chenopodium album, was often infected with TSWV and BWYV. Multiple infections were common, especially in non-crop plants, and the most common combination was BWYV and TSWV. The role of weeds in the epidemiology of viruses that infect lettuce and Brassica crops in Spain is discussed.     

Development of marker-free transgenic lettuce resistant to <Emphasis Type="Italic">Mirafiori lettuce big</Emphasis>-<Emphasis Type="Italic">vein virus</Emphasis>

Lettuce big-vein disease caused by Mirafiori lettuce big-vein virus (MLBVV) is found in major lettuce production areas worldwide, but highly resistant cultivars have not yet been developed. To produce MLBVV-resistant marker-free transgenic lettuce that would have a transgene with a promoter and terminator of lettuce origin, we constructed a two T-DNA binary vector, in which the first T-DNA contained the selectable marker gene neomycin phosphotransferase II, and the second T-DNA contained the lettuce ubiquitin gene promoter and terminator and inverted repeats of the coat protein (CP) gene of MLBVV. This vector was introduced into lettuce cultivars ‘Watson’ and ‘Fuyuhikari’ by Agrobacterium tumefaciens-mediated transformation. Regenerated plants (T₀ generation) that were CP gene-positive by PCR analysis were self-pollinated, and 312 T₁ lines were analyzed for resistance to MLBVV. Virus-negative plants were checked for the CP gene and the marker gene, and nine lines were obtained which were marker-free and resistant to MLBVV. Southern blot analysis showed that three of the nine lines had two copies of the CP gene, whereas six lines had a single copy and were used for further analysis. Small interfering RNAs, which are indicative of RNA silencing, were detected in all six lines. MLBVV infection was inhibited in all six lines in resistance tests performed in a growth chamber and a greenhouse, resulting in a high degree of resistance to lettuce big-vein disease. Transgenic lettuce lines produced in this study could be used as resistant cultivars or parental lines for breeding.     

Detailed characterization of <Emphasis Type="Italic">Mirafiori lettuce virus</Emphasis>-resistant transgenic lettuce

Lettuce big-vein disease is caused by Mirafiori lettuce virus (MiLV), which is vectored by the soil-borne fungus Olpidium brassicae. A MiLV-resistant transgenic lettuce line was developed through introducing inverted repeats of the MiLV coat protein (CP) gene. Here, a detailed characterization study of this lettuce line was conducted by comparing it with the parental, non-transformed ‘Kaiser’ cultivar. There were no significant differences between transgenic and non-transgenic lettuce in terms of pollen fertility, pollen dispersal, seed production, seed dispersal, dormancy, germination, growth of seedlings under low or high temperature, chromatographic patterns of leaf extracts, or effects of lettuce on the growth of broccoli or soil microflora. A significant difference in pollen size was noted, but the difference was small. The length of the cotyledons of the transgenic lettuce was shorter than that of ‘Kaiser,’ but there were no differences in other morphological characteristics. Agrobacterium tumefaciens used for the production of transgenic lettuce was not detected in transgenic seeds. The transgenic T₃, T₄, and T₅ generations showed higher resistance to MiLV and big-vein symptoms expression than the resistant ‘Pacific’ cultivar, indicating that high resistance to lettuce big-vein disease is stably inherited. PCR analysis showed that segregation of the CP gene was nearly 3:1 in the T₁ and T₂ generations, and that the transgenic T₃ generation was homozygous for the CP gene. Segregation of the neomycin phosphotransferase II (npt II) gene was about 3:1 in the T₁ generation, but the full length npt II gene was not detected in the T₂ or T₃ generation. The segregation pattern of the CP and npt II genes in the T₁ generation showed the expected 9:3:3:1 ratio. These results suggest that the fragment including the CP gene and that including the npt II gene have been integrated into two unlinked loci, and that the T₁ plant selected in our study did not have the npt II gene. DNA sequences flanking T-DNA insertions in the T₂ generation were determined using inverse PCR, and showed that the right side of the T-DNA including the npt II gene had been truncated in the transgenic lettuce.     

Endophytic fungi occurring in fennel, lettuce, chicory, and celery — commercial crops in southern Italy

The occurrence of endophytic fungi in fennel, lettuce, chicory, and celery crops was investigated in southern Italy. A total of 186 symptomless plants was randomly collected and sampled at the stage of commercial ripeness. Fungal species of Acremonium, Alternaria, Fusarium, and Plectosporium were detected in all four crops; Plectosporium tabacinum was the most common in all crop species and surveyed sites. The effect of eight endophytic isolates (five belonging to Plectosporium tabacinum and three to three species of Acremonium) inoculated on lettuce plants grown in gnotobiosis was assessed by recording plant height, root length and dry weight, collar diameter, root necrosis, and leaf yellowing. P. tabacinum and three species of Acremonium, inoculated on gnotobiotically grown lettuce plants, showed pathogenic activity that varied with the fungal isolate. Lettuce plants inoculated with the isolates Ak of Acremonium kiliense, Ac of Acremonium cucurbitacearum, and P35 of P. tabacinum showed an increased root growth, compared to the non-inoculated control. The high frequency of P. tabacinum isolation recorded in lettuce plants collected in Bari and Metaponto, and in fennel plants from Foggia agricultural districts, suggests a relationship not only between a crop species and P. tabacinum, but also between the occurrence of the endophyte and the crop rotation history of the soil.     

Finding of lettuce big vein virus in Czechoslovakia

Big vein disease occurring on lettuce plants cv. ‘Pra?an’ was identified and the susceptibility to this disease was tested on 9 lettuce cultivars of foreign origin. Lettuce big vein virus was confirmed to be the cause of the disease. The transmission of the LBVV by the fungusOlpidium brassicae (Wor.)Dang., the sporangia of which were found by microscopy in the cells of surface tissues of lettuce roots, was experimentally proved. Rod-like particles, the average length of which was estimated at 244 nm, were found in the roots of diseased plants. LBW transmission by mechanical inoculation and by aphids could not be proved.     

Transmission of plant viruses by fungi

The evidence for the mechanisms involved in virus transmission by fungi is reviewed in relation to the non-persistent and persistent categories usually recognised. Non-persistent transmission by Olpidium spp. has been little studied in the last 20 years, but appears to depend on adsorption of virus to the outside of the fungal zoospores. This seems to be under the genetic control of both the virus (via its coat protein) and the vector. Such viruses are not transmitted in the fungal resting spores. The route by which the virus is transferred from the vector to the host may involve uptake into the zoospore but this deserves further study. Persistent transmission by Olpidium, Polymyxa and Spongospora spp. is less well characterised. Some of the evidence used for its support is inconclusive. The viruses are probably always carried inside zoospores, and they also persist in the fungal resting spores. Transmission depends on the genome of the vector and the virus, but not exclusively on the virus coat protein.     

Temporal and spatial spread of Lettuce mosaic virus in lettuce crops in central Spain: factors involved in Lettuce mosaic virus epidemics

Lettuce mosaic virus (LMV) is transmitted by aphid vectors in a nonpersistent manner as well as by seeds. The virus causes severe disease outbreaks in commercial lettuce crops in several regions of Spain. The temporal and spatial patterns of spread of LMV were studied in autumn 2002 in the central region of Spain. Symptomatic lettuce (var. Cazorla) plant samples were collected weekly, first at the seedling stage from the greenhouse nursery and later outdoors after transplantation. The exact position of symptomatic plants sampled in the field was recorded and then material was tested by enzyme‐linked immunosorbent assay to assess virus infection. Cumulative spatial data for infected plants at different growth stages were analysed using spatial analysis by distance indices. For temporal analysis, the monomolecular, Gompertz, logistic and exponential models were evaluated for goodness of fit to the entire set of disease progress data obtained. The results indicated that the disease progress curve of LMV epidemics in the selected area is best described by a Gompertz model and that the epidemic follows a polycyclic disease progression. Our data suggest that secondary cycle of spread occurs when noncolonising aphid species land on the primary infected plants (probably coming from infected seed) and move to adjacent plants before leaving the crop. The role of weeds growing close to lettuce fields as potential inoculum sources of virus and the aphid species most likely involved in the transmission of LMV were also identified.     

Soil fumigation with methyl bromide: bromide accumulation by lettuce plants

Lettuce plants grown in beds of soil previously fumigated with methyl bromide accumulated water-extractable bromide, the amount present in the tissues depending on the concentration of inorganic bromide produced in the soil by the breakdown of the fumigant. Samples of lettuce plants from commercial nursery soils fumigated with methyl bromide at rates of 1–2 lb/ 100 ft² (49–98 g/m²) gave rise to soil bromide levels of n-6i/μg/g. The corresponding bromide concentrations in the plants ranged from i-6 to io-1 mg/g of dry tissue. The bromide concentrations in whole lettuce plants grown in pots of soil supplemented with 0–5 mg/g inorganic bromide, as potassium bromide, ranged up to 100 mg/g of dry tissue. Bromide taken up from the soil by lettuce plants was located mainly in the outer leaves. Lettuce was relatively insensitive to the presence of bromide in the soil; no phytotoxic symptoms were observed in plants growing in soils containing 5 mg/g inorganic bromide. Implications in relation to possible tolerance limits for the bromide content of lettuce plants are discussed.     

Effect of long-term organic and mineral fertilization strategies on rhizosphere microbiota assemblage and performance of lettuce

Long-term agricultural fertilization strategies gradually change soil properties including the associated microbial communities. Cultivated crops recruit beneficial microbes from the surrounding soil environment via root exudates. In this study, we aimed to investigate the effects of long-term fertilization strategies across field sites on the rhizosphere prokaryotic (Bacteria and Archaea) community composition and plant performance. We conducted growth chamber experiments with lettuce (Lactuca sativa L.) cultivated in soils from two long-term field experiments, each of which compared organic versus mineral fertilization strategies. 16S rRNA gene amplicon sequencing revealed the assemblage of a rhizosphere core microbiota shared in all lettuce plants across soils, going beyond differences in community composition depending on field site and fertilization strategies. The enhanced expression of several plant genes with roles in oxidative and biotic stress signalling pathways in lettuce grown in soils with organic indicates an induced physiological status in plants. Lettuce plants grown in soils with different fertilization histories were visibly free of stress symptoms and achieved comparable biomass. This suggests a positive aboveground plant response to belowground plant–microbe interactions in the rhizosphere. Besides effects of fertilization strategy and field site, our results demonstrate the crucial role of the plant in driving rhizosphere microbiota assemblage.     

Occurrence and distribution of lettuce mosaic disease in Tehran province from Iran

Lettuce mosaic virus (LMV) Cucumber mosaic virus (CMV) and Tomato spotted wilt virus (TSWV) were identified in fields of Tehran province. In this study 452 leaf samples were collected from the fields throughout the Tehran province during 2002 and 2003. Distribution of Lettuce mosaic virus (LMV), Cucumber mosaic virus (CMV), Tomato spotted wilt virus (TSWV)and Arabis mosaic virus (ArMV) was determined with DAS-ELISA. Percentage of single Infection to LMV. CMV or TSWV was 20.58, 15.93 and 9.96% respectively. Also 15.28% of samples were co- infected with LMV+CMV, 8.19% with LMV+TSMV and 7.74% with CMV+TSWV. 4.65% of samples were Infected to all of these three viruses. LMV was found in 48.69%, CMV in 43.59% and TSWV in 30.54% of samples totally. Therefore LMV is major dominant agent of lettuce mosaic disease in Tehran province. This is the first report of occurrence of TSWV on lettuce in Iran and first report of CMV and LMV in Tehran province.     

Association mapping and marker-assisted selection of the lettuce dieback resistance gene Tvr1

Background  

Lettuce (Lactuca saliva L.) is susceptible to dieback, a soilborne disease caused by two viruses from the family Tombusviridae. Susceptibility to dieback is widespread in romaine and leaf-type lettuce, while modern iceberg cultivars are resistant to this disease. Resistance in iceberg cultivars is conferred by Tvr1 - a single, dominant gene that provides durable resistance. This study describes fine mapping of the resistance gene, analysis of nucleotide polymorphism and linkage disequilibrium in the Tvr1 region, and development of molecular markers for marker-assisted selection.     

Growth promotion of maize and lettuce by phosphate-solubilizing Rhizobium leguminosarum biovar. phaseoli

Rhizobium leguminosarum bv. phaseoli strains P31 and R1, Serratia sp. strain 22b, Pseudomonas sp. strain 24 and Rhizopus sp. strain 68 were examined for their plant growth-promoting potential on lettuce and forage maize. All these phosphate solubilizing microorganisms (PSM) were isolated from Québec soils. The plants were grown in field conditions in three sites having high to low amounts of available P. In site 1 (very fertile soil), strains R1 and 22b tended to increase the dry matter yield of lettuce shoots (p≤0.10). Lettuce inoculated with rhizobia R1 had a 6% higher P concentration (p≤0.10) than the uninoculated control. In site 2 (poorly fertile soil), the dry matter of lettuce shoots was significantly increased (p≤0.05) by inoculation with strain P31 and 24 plus 35 kg ha^-1 P-superphosphate, or with strain 68 plus 70 kg ha^-1 P-superphosphate. In site 3 (moderately fertile soil), the dry matter of maize shoots was significantly increased (p≤0.05) by inoculation with strain 24 plus 17.5 kg ha^-1 P-superphosphate, or with strain P31 plus 35 kg ha^-1 P-superphosphate. Inoculation with PSM did not affect lettuce P uptake in the less fertile soil in site 2. In site 3 with the moderately fertile soil, maize plants inoculated with strain R1 had 8% higher P concentration than the uninoculated control (p≤0.01), and 6% with strains P31 and 68 (p≤0.05). The results clearly demonstrate that rhizobia specifically selected for P solubilization function as plant growth promoting rhizobacteria with the nonlegumes lettuce and maize. The P solubilization effect seems to be the most important mechanism of plant growth promotion in moderately fertile and very fertile soils when P uptake was increased with rhizobia and other PSM.     

Screening of bacteria for the suppression of Botrytis cinerea and Rhizoctonia solani on lettuce (Lactuca sativa) using leaf disc bioassays

Two leaf disc bioassays were developed for screening bacteria as putative biological control agents of Botrytis cinerea and Rhizoctonia solani on lettuce. Aerobic spore and non‐spore forming bacteria were isolated from the phylloplane, rhizoplane and rhizosphere of symptom‐free lettuce plants grown in the presence and absence of chitin or composted bark soil amendments. Bacteria, previously isolated from other plants, were also included in the primary screen initially against B. cinerea. One hundred and twenty‐seven of 700 isolates reduced botrytis rotting of lettuce leaves by more than 50% in the primary screen. Following a secondary screen against B. cinerea, the lead 50 isolates were also tested for suppression of R. solani infection. Four isolates significantly reduced both botrytis and rhizoctonia leaf rotting. Eleven and five isolates gave control of botrytis and rhizoctonia, respectively, equal to that given by the standard fungicides Rovral WP (iprodione) and Basilex (tolclofos methyl). The two most effective isolates against B. cinerea and R. solani were both identified as Bacillus subtilis. Use of soil amendments did not increase the proportion of efficacious isolates recovered. Effective isolates were originally recovered from roots of oilseed rape and lettuce leaves. In general, it was found that bacteria which controlled one disease effectively did not control the second disease nearly as well. The bioassay protocols developed in this study were used successfully in screening a large number of bacterial isolates in a short time.     

The bioavailability of cadmium to lettuce and cabbage in soils previously treated with sewage sludges

The application of sewage sludges to soils may lead to increased soil-Cd levels. The bioavailability of Cd is determined by the interaction of a number of soil physico-chemical and plant variables, of which pH is the most important. Duplicate samples of sludge-treated soils were transferred to tubs in the field, one of each pair being limed to pH 7±0.5. Lettuce and cabbage were grown to maturity and analysed for Cd. Liming always reduced Cd uptake by the plants. Three soil extractants, 1 M NH₄NO₃, 0.05 M EDTA-(Na)₂ and 0.05 M CaCl₂ were used as indices of Cd bioavailability. CaCl₂ proved to be the most effective for both lettuce and cabbage. Multiple linear regression equations were derived to describe the uptake and accumulation of Cd by both crops. The relative influence of soil variables differed between the two species. Unlike those of a number of pot experiments conducted in glasshouses, the data from this experiment are comparable with those of crop samples taken from the field.     

Genetic analysis and mapping of resistance to lettuce dieback: a soilborne disease caused by tombusviruses

A diverse collection of modern, heirloom and specialty cultivars, plant introduction (PI) accessions, and breeding lines of lettuce were screened for susceptibility to lettuce dieback, which is a disease caused by soilborne viruses of the family Tombusviridae. Susceptibility was evaluated by visual symptom assessment in fields that had been previously shown to be infested with Lettuce necrotic stunt virus. Of the 241 genotypes tested in multiple field experiments, 76 remained symptom-free in infested fields and were therefore classified as resistant to dieback. Overall, resistant genotypes were as prevalent among modern cultivars as in heirloom cultivars or primitive germplasm. Within modern germplasm, however, all crisphead (iceberg) cultivars were resistant, while all romaine cultivars were susceptible. Using enzyme-linked immunosorbent assay, tombusviruses were detected in leaves of some plants of resistant genotypes that were grown in infested fields, suggesting that symptom-free plants are not immune to viral infection. The inheritance of resistance was studied for Salinas, a modern iceberg cultivar, and PI 491224, the progenitor of recently released romaine germplasm with resistance to lettuce dieback. Resistance was conferred by a dominant allele at a single locus in both genotypes. The tombusvirus resistance locus from Salinas, Tvr1, was mapped in an intraspecific Lactuca sativa population to a location that corresponds to linkage group 2 on the consensus map of Lactuca. The largest cluster of resistance genes in lettuce, the Dm1/Dm3 cluster, is found on this linkage group; however, the precise position of Tvr1 relative to this cluster has not yet been determined. To our knowledge, Tvr1 is the first tombusvirus resistance gene identified for any plant host.Electronic Supplementary Material Supplementary material is available for this article at     

Nursery treatments with resistant inducers,soil amendments and biocontrol agents for the management of the Fusarium wilt of lettuce under glasshouse and field conditions

Lettuce Fusarium wilt, caused by Fusarium oxysporum f. sp. lactucae, represents a major problem in most lettuce production areas worldwide. In the present study, a number of resistance inducers, organic amendments and biocontrol agents were applied in a preventative way, in experimental and commercial situations, to soils artificially or naturally infested with race 1 of the pathogen, and to moderately susceptible lettuce cultivars. Potassium phosphite, acibenzolar‐S‐methyl, green composts, and Bacillus subtilis Qst713, Trichoderma asperellum + Trichoderma gamsii, and Pseudomonas strains achieved the most consistent disease control under the experimental conditions. Moreover, potassium phosphite, green compost, B. subtilis Qst713 and T. asperellum + T. gamsii, also showed a positive effect on plant development. In general, the results of the different treatments in naturally infested soil were similar to those observed in glasshouse trials under artificial inoculation. Potassium phosphite provided a consistent disease reduction (48%–62% in artificially infested soil and 60%–75% in naturally infested soil). The effects of adding 10% compost to a peat growing medium in the nursery, followed by a soil mixing application when lettuce was transplanted, significantly reduced the severity of Fusarium wilt (50%–59% efficacy) and increased fresh biomass production. Compost enrichment with Trichoderma TW2 generally further increased its efficacy. When tested under field conditions, the commercially available Trichoderma spp. and B. subtilis, together with experimental strains of Pseudomonas and Trichoderma spp., applied at the nursery level, provided a disease reduction of 30%–78%. Early application of the different control measures under nursery conditions and at lettuce transplant is noteworthy because it was carried out at a more localized level, with reduced amounts of products. Their use in practice should be integrated with other control strategies.