Healthy Lettuce Research Articles

Response Surface Methodology for Development of Nutrient Solution Formula for Hydroponic Lettuce Based on the Micro-Elements Fertilizer Requirements at Different Growth Stages

In order to precisely obtain the impact of nutritional elements on lettuce yield and quality, in the present study, a nutrient solution formula suitable for lettuce hydroponic production was development using response surface methodology based on the determination for micro-elements in three growth stages and taking the interaction between elements into account. Then, the formula was optimized and validated, aiming for the goal of improving lettuce yield and quality. The results showed that 200 healthy lettuce leaves contained similar amounts of macro-elements, and there was no significant difference in the unit content of micro-elements among the seedling, rosette, and harvest stages. Quadratic regression models between shoot fresh weight, SPAD value, soluble sugar content, Vc content, and nutrient content were established (R2 = 0.91, 0.95, 0.98, and 0.81, respectively). The optimal concentrations of P, K, Ca, and Mg obtained by multi-objective optimization of the quadratic regression models for fresh weight, SPAD value, soluble sugar content, and Vc content were 2.71 mmol·L−1, 6.42 mmol·L−1, 5.58 mmol·L−1, and 7.11 mmol·L−1, respectively. The nutrient solution formula (T1) was found to be the optimal nutrient solution formula for improving lettuce growth and quality. Overall, we developed a specific and targeted nutrient solution formulation for lettuce; this formulation not only meets lettuce’s demand for nutrients, but also improves lettuce yield and quality, providing more choices for lettuce production in a region with high salts and high pH in the irrigation water.

Growth of Romaine Lettuce in Eggshell Powder Mixed Alginate Hydrogel in an Aeroponic System for Water Conservation and Vitamin C Biofortification.

Vitamin C is crucial for physical well-being, and its deficiency can lead to severe health consequences. Biofortification has been used to address this deficiency by enhancing vitamin C in plants. Additionally, soilless agriculture has been used to conserve and optimize water use in comparison to conventional agriculture. While hydrogels have been shown to improve water conservation and are used for biofortification in crops, their application has only been explored in soil-based and hydroponic farming. The aeroponics system is a plant-growing method that has shown potential for increasing yields and biomass while conserving water and nutrients. In this paper, we have developed an aeroponic-compatible medium to grow romaine lettuce (Lactuca sativa L.) with eggshell powder (ESP) mixed with calcium-alginate hydrogel as a substrate and nutrient source aiming to conserve water and incorporate vitamin C through biofortification. Herein, lower water spray time and higher intervals, with varied gel types and ESP concentrations, resulted in healthy lettuce growth. Plants treated with 0.5% ascorbic acid-absorbed ESP-mixed alginate hydrogel for biofortification showed higher levels of vitamin C compared to the traditional method. This study suggests using an alginate hydrogel-ESP-based substrate in aeroponics to reduce water usage and enhance plant biofortification of vitamin C.

First Report of Golovinomyces bolayi Causing Powdery Mildew on Lettuce in Mexico.

In August 2022, powdery mildew symptoms were detected on lettuce (Lactuca sativa) in a commercial field located in Quecholac, Puebla, Mexico. Signs appeared as whitish powdery masses on leaves. Disease incidence was about 100% and signs covered up to 40% of leaf surface. Mycelium was amphigenous forming white patches. Hyphal appressoria were indistinct or nipple-shaped and solitary. Conidiophores (n= 30) were hyaline, erect, arising from the upper surface of hyphal mother cells or lateral, and of 90 to 201 μm long. Foot cells were cylindrical, of 49 to 92 × 10-15 μm, followed by 1-3 shorter cells, and forming conidia in chains. Conidia (n= 100) were hyaline, ellipsoid-ovoid, doliiform-subcylindrical, 27 to 40 × 14 to 20 μm. Conidial germination belonging to the Euoidium type. Chasmothecia were not observed. The morphological characters were consistent with those of Golovinomyces bolayi (Braun et al. 2019). A voucher specimen was deposited in the Herbarium of the Department of Agricultural Parasitology at the Chapingo Autonomous University under accession number UACH451. To confirm the identification of the fungus, genomic DNA was extracted from conidia and mycelium following the CTAB method (Doyle and Doyle 1990), and the internal transcribed spacer (ITS) region was amplified by PCR using the primers ITS5/ITS4 (White et al. 1990) and sequenced. The resulting 506 bp sequence had 100% identity to those of G. bolayi (LC417109 and LC417106). Phylogenetic analyses using the Maximum Likelihood and Maximum Parsimony methods were performed and confirmed the results obtained in the morphological analysis. The isolate UACH451 grouped in a clade with isolates of G. bolayi. The ITS sequence was deposited in GenBank under accession number OR467546. Pathogenicity was confirmed by gently dusting conidia onto ten leaves of healthy lettuce plants. Five non-inoculated leaves served as controls. The plants were maintained in a greenhouse at 25 to 30 ºC, and relative humidity of 70%. All inoculated leaves developed similar symptoms to the original observation after 10 days, whereas control leaves remained disease free. Microscopic examination of the fungus on inoculated leaves showed that it was morphologically identical to that originally observed. Based on morphological data and phylogenetic analysis, the fungus was identified as G. bolayi. This pathogen has been previously reported causing powdery mildew on lettuce in Argentina, Canada, Chile, Ecuador, Peru, USA and Venezuela (Braun et al. 2019; Mieslerová et al. 2020). To our knowledge, this is the first report of G. bolayi causing powdery mildew on lettuce in Mexico.

Evidence for the natural infection of cucurbit chlorotic yellows virus (CCYV) in lettuce plants from India.

In the years 2021 and 2022, lettuce plants showing blistering, chlorosis, mosaic, rosetting/ excess proliferation, and stunting symptoms were subjected to leaf-dip transmission electron microscopy, RT-PCR followed by sequence analysis and bio-assay to unfold the identity ofassociated virus(es). The association of longfilamentous virions (~ 850nm in length) as seen through leaf-dip transmission electron microscopy suggested the possible infection by a potyvirus or crinivirus, either singly or in combination. RT-PCR assays using generic primers targeting the RdRp region of criniviruses and the NIb region of potyviruses revealed the association of both a crinivirus as well as a potyvirus. The gel-purified RT-PCR products derived from the RdRp region of criniviruses upon cloning, sequencing, and NCBI BLAST analysis indicated the associated crinivirus as cucurbit chlorotic yellows virus (CCYV). Further, RT-PCR assays using specific primers targeting CP and CP minor genes of CCYV followed by cloning and sequencing confirmed its association with the diseased lettuce plants. Besides, the bioassay based on whitefly-mediated virus transmission followed by RT-PCR confirmed the infectivity of CCYV from diseased to healthy lettuce plants. The results of this study confirmed the natural infection of CCYV in lettuce host for the first time in the world indicating its distribution across the crop families.

Potential use of Terasi, an Indonesian traditional fermented seafood paste, to provide healthy lettuce (Lactuca sativa L.) seedling

Terasi paste consists of plant nutrients and microorganisms that potentially improve the health of plant seedlings. We evaluated the effects of terasi paste on seed germination, seedling growth, and root rot disease severity caused by Fusarium oxysporum f.sp. lactucae (FOL) in lettuce. Terasi paste was tested at four different weights (i.e., 0.00 g [T0], 1.00 g [T1], 2.50 g [T2], and 5.00 g [T3]) in the 100 g nursery soil. The results showed that lettuce seeds from T1 and T2 treatments were fully germinated, as observed for the T0 treatment. However, 40.0% of lettuce seeds from T3 treatment failed to germinate. Thus, 1.00 g and 2.50 g terasi applications in the 100 g soil did not affect lettuce seed germination. Moreover, T1 and T2 treatments tended to improve the shoot and root biomass of lettuce seedlings over T0 treatment, while T3 treatment showed the opposite effects. Following FOL infection, the lettuce seedlings from T2 and T3 treatments had 31.1% significantly lower disease severity than those from the T0 treatment. Therefore, T2 treatment (i.e., 2.50 g terasi application in the 100 g soil) imparts maximum health-promoting benefits to lettuce seedlings.

Identification and characterization of the phytoplasma associated with lettuce chlorotic leaf rot disease together with its natural reservoirs and leafhopper vectors in China

Lettuce (Lactuca sativa L.) is an important vegetable crop that is produced commercially in many countries. Lettuce chlorotic leaf rot disease (LCLRD) was first reported in China in 2005, and in 2008, a severe outbreak in Yongan county occurred with an incidence of approximately 25%, causing significant economic losses. However, the causal agent, insect vector, and alternative hosts of this disease are still unknown. The results from the phylogenetic tree and virtual RFLP patterns of the 16S F2nR2 region showed that the phytoplasma associated with LCLRD was more related to the 16SrI-B subgroup with 99% nucleotide identity and a similarity coefficient of over 0.97. Only leafhopper Macrosteles striifrons could transmit LCLRD phytoplasma to the healthy lettuce plants under experimental conditions. Six out of 52 plant hosts, which were collected from some of the fields, could harbor phytoplasma, but only the chinaberry tree (Melia azedarach) might serve as an alternative host with a similarity coefficient of 1.0 to LCLRD phytoplasma. The other hosts of the head lettuce, pepper, and sweet potato could also harbor the phytoplasma, which had the same KpnI digestion profiles from leafhopper of Deltocephalinar spp.. Interestingly, the phytoplasmas from wild and longleaf lettuce belonged to the subgroup of 16SXII-A, which was not detected in leafhoppers in this study. Identification and characterization of the LCLRD agent, its insect vector, and alternative host plants are useful for developing strategies to control this phytoplasma-induced disease in lettuce.

Lead and copper-induced hormetic effect and toxicity mechanisms in lettuce (Lactuca sativa L.) grown in a contaminated soil

Lead (Pb) and copper (Cu) contamination seriously threatens agricultural production and food safety. This study aims to investigate Pb and Cu induced hormetic effect and toxicity mechanisms in lettuce (Lactuca sativa L.) and establish reliable empirical models of potentially toxic elements (PTEs) transfer in the soil–plant system. The content and distribution of Pb and Cu at subcellular levels in lettuce plants were examined using inductively coupled plasma-mass spectrometry, differential centrifugation and micro-X-ray fluorescence spectroscopy. The PTE-loaded capacity of Pb that ensures food safety was lower than that of Cu in the studied soil, but the PTE-loaded capacity of Pb that limits yield was higher than that of Cu. Lead in lettuce roots mainly accumulated in the cell wall (41%), while Cu mainly accumulated in the vacuoles (46%). The Pb and Cu were primarily distributed in the radicle of lettuce seeds under severe PTE stress, resulting in no seed development. Iron plaque formed on the root surface of lettuce seedlings and sequestered Pb and Cu via chelation. At the same concentration, lettuce was less tolerant to Cu in contaminated soil than Pb due to the higher activity of Cu ions in the soil. Lead was more phytotoxic to lettuce than Cu, however, since the radicle emerged from the seed under severe Cu levels, while it did not protrude under severe Pb levels. The potentially damaging effect of Pb in the visually healthy lettuce appeared to be higher than that of Cu under the same soil contamination level.

Assessment of the Risk of Microbial Contamination of an Urban Crop in the City of Daloa (Côte d'Ivoire): Case of Lettuce (Lactuca sativa L.)

The growing population of the city of Daloa, together with its growing urbanization, has a diversified need for food resources. Urban and periurban production of lettuce is a food resource for its people. This study was designed to assess the risk of microbial contamination. For the conduct of the study, first a survey was carried out on different sites of urban production to know the technical process of production. Then, mature lettuce, ready for sale and then consumed from a site, was subjected to microbiological characterization according to the standards in force. Microbiological analyzes revealed a high contamination. For mesophilic aerobic germs, the charges in CFU/g ranged between 1.7x107 and 6.7x107. The loads in CFU/g for yeasts and molds ranged from 3.4x105 to 9.3x105. As for fecal coliforms and enterobacteria, their loads ranged between 1.2x105 to 2.8x105 CFU/g for the first and 7.2x105 to 106 CFU/g for the second. These samples were contaminated with both Escherichia coli, Staphylococcus aureus c + and Salmonella sp. The loads in E. coli and S. aureus c+ ranged between 9.4x104 and 1.8x105 for the first and 4x103 to 1.1x104 CFU/g for the second. Salmonella sp. was found with loads ranging from 6.1x104 to 8.2x104 CFU/g. Empirical production process would increase the risk of microbial contamination. It is necessary to produce healthy lettuce for Daloa’s consumers.

Postharvest Survival of Porcine Sapovirus, a Human Norovirus Surrogate, on Phytopathogen-Infected Leafy Greens

Leafy greens are increasingly being recognized as an important vehicle for human noroviruses (HuNoV), which cause recurring gastroenteritis outbreaks. Leafy greens often become infected by phytopathogens in the field, which may cause symptoms on the edible parts. Whether plant pathogen infections enhance the survival of HuNoV on leafy greens is unknown. Lettuce and spinach plants were infected with a bacterium, Xanthomonas campestris pv. vitians strain 701a, and with Cucumber mosaic virus strain Fny, respectively. The survival rate of porcine sapovirus (SaV), a HuNoV surrogate, on infected and noninfected postharvest leaves was then assessed. In addition, acibenzolar-S-methyl, a commercial chemical elicitor of plant systemic defense, was used to assess whether stimulating the plant host defense affects the postharvest survival of SaV. Leaves harvested from control and treated plants were inoculated with SaV and incubated for 7 days at 4°C. The infectivity (tissue culture infectious dose affecting 50% of the culture [TCID50]/ml) and RNA (genomic equivalent/ml) titers of SaV were assayed using immunohistochemistry staining and SaV-specific TaqMan real-time reverse transcription PCR. Our results showed that cucumber mosaic virus Fny induced mild, nonnecrotic symptoms on spinach leaves and had no effect on SaV survival. In contrast, X. campestris pv. vitians 701a induced small localized necrotic lesions and significantly enhanced SaV survival on lettuce leaves. Treatment with acibenzolar-S-methyl was effective in reducing X. campestris pv. vitians 701a-induced lesions on infected lettuce plants but had no direct effect on SaV survival when used on healthy lettuce plants. These findings indicate that phytopathogen-induced necrotic lesions may enhance the postharvest survival of HuNoV on lettuce leaves. Therefore, preventive measures aiming to maintain healthy plants and minimize preharvest biological damage are expected to improve the safety of leafy greens.

Identification of Mirafiori lettuce big‐vein virus and Lettuce big‐vein associated virus infecting Lactuca sativa with symptoms of lettuce big‐vein disease in Argentina

Lettuce big-vein disease (BVD) affects all major lettuce-producing areas of the world. The causal agent is Mirafiori lettuce big-vein virus (MLBVV), an ophiovirus transmitted by the soil-borne fungus Olpidium brassicae (Lot et al., 2002). MLBVV has been detected in many different areas of the world but never in Argentina. La Plata has about 700 ha of lettuce with a production of about 13 000 tonnes, and with about 70% of the total production from Buenos Aires Province. BVD has been detected in different areas in the north and west of the La Plata horticultural green belt. Many of the plants with BVD symptoms had leaf distortions of moderate severity, which affected their commercial value. Over the winters of 2007, 2008 and 2009, forty samples of lettuce with BVD symptoms, representing most of the commercial varieties, were taken from fields near La Plata. Samples tested positive by RT-PCR with universal primers for the genus Ophiovirus (Vaira et al., 2003) and/or by DAS-ELISA using a polyclonal antiserum against the viral coat protein of MLBVV (kindly provided by Dr Y. Kawazu, National Institute of Vegetable and Tea Science, Japan). The identity of MLBVV was verified by cloning and sequencing amplicons (GenBank Accession Nos. FJ555204-05), which shared 97% identity with both Italian (AY2046741) and Brazilian (DQ8548131) MLBVV isolates. Specific primers, Cps1 (5′-CTCATGACAAAAGAAGAGAAGC) and Cpas1 (5′-CACATCAAATATGAAGTTGTGCTC) were designed from MLBVV-RNA 3, to allow specific MLBVV detection in RNA extracts from collected samples. The primers were tested using samples with symptoms that were positive by RT-PCR and DAS-ELISA. An amplicon of the expected 450 bp was amplified from all infected samples. Sequence analysis (GU295451) demonstrated high identity (98%) with sequences deposited in the public databases. A block sampling method was employed to collect random field samples and estimate the incidence of the disease. These were scored for symptoms and tested for infection by RT-PCR (primers Cps1/Cpas1). In the three lots tested, the incidence of the disease reached 60% of plants inoculated. Additionally, soil transmission experiments were conducted using healthy lettuce seedlings planted into contaminated soil or by watering with rinse water from the roots of diseased field plants. After 5 weeks in a greenhouse up to 70% of the 15 seedlings inoculated with five field isolates showed big-vein symptoms and tested positive by DAS-ELISA and RT-PCR. Three of the plants used for transmission were also positive for Lettuce big-vein associated virus (LBVaV), detected by RT-PCR using specific primers (VP248 and VP249; Navarro et al., 2004). This is the first report confirming the presence of MLBVV and LBVaV infecting lettuce plants in Argentina. The authors would like to thank Dr. L.W. Timmer for helpful discussion and manuscript revision.

Survival and Dissemination of Escherichia coli O157:H7 on Physically and Biologically Damaged Lettuce Plants

The ecology of the vegetable leaf surface is important to the survival of enteric pathogens. Understanding changes in ecological parameters during the preharvest stages of production can lead to development of approaches to minimize the hazard of contamination of fresh fruits and vegetables with foodborne pathogens. In this study, survival levels of Escherichia coli O157 over a 10-day period were compared among traumatically injured, phytopathogen-damaged, and healthy lettuce plants. Leaves from lettuce plants cracked along the central vein, plants infected with Xanthomonas campestris pv. vitians, and healthy plants were inoculated with E. coli O157:H7. The presence of E. coli O157:H7 populations on inoculated leaves and non-inoculated leaves of these same plants was determined for 10 days. The density of E. coli O157:H7 decreased over time on the inoculated leaves regardless of the treatment. The population of E. coli O157:H7 remained higher on traumatically injured leaves than on healthy plants (P < 0.001). E. coli O157:H7 was detected on leaves other than the direct inoculation site of the enteric pathogen in all three treatments groups. Preharvest damage, especially that caused by traumatic injury, impacted the survivability of E. coli O157:H7. Maintaining healthy plants and minimizing physical damage around the time of harvest might improve the safety of fresh produce.

Suppressing spread of Tomato spotted wilt virus by drenching infected source or healthy recipient plants with neonicotinoid insecticides to control thrips vectors

Summary Field experiments were done to determine whether drenching plants with two systemically active neonicotinoid insecticides, thiamethoxam and imidacloprid, suppresses spread of Tomato spotted wilt virus (TSWV) by thrips vectors. Separate treatments to TSWV ‘infector’ tomato (source) and healthy lettuce (recipient) plants provided information on the relative importance of targeting control at virus acquisition by nymphs versus virus transmission to healthy plants by adults. Drenches were applied either to seedlings just before transplanting or to soil around plants. The thrips vectors recorded were Frankliniella occidentalis, F. schultzei and Thrips tabaci, but F. schultzei and T. tabaci predominated. Overall ratios of external to internal TSWV spread into and within plots without insecticide ranged from 1 : 2.3 to 1 : 2.8 between field experiments. Applying thiamethoxam as a soil drench to both young source plants and recipient seedling transplants suppressed TSWV incidence by 86%, while such application to either young source or recipient seedlings diminished incidence by 67–70%. When thiamethoxam was applied either as a soil drench to old source plants and concurrently as a seedling drench to recipient plants or as a seedling drench to recipient plants alone, incidence was suppressed by 65–73% and 54–73%, respectively. Thiamethoxam applied as a soil drench to old source plants diminished incidence by only 33% or not significantly. Imidacloprid applied either as a soil drench to old source plants and concurently as a seedling drench or as a seedling drench alone, suppressed TSWV incidence by 90–92% and 80% respectively. Although adult vector thrips and nymph numbers were low, fewer adults and/or nymphs were sometimes recorded due to insecticide application. Drenching healthy seedlings with neonicotinoid insecticides just before transplanting can be an effective chemical control measure to include in integrated disease management strategies to suppress TSWV epidemics in short-duration crops like lettuce.

First Report of Lettuce big-vein virus and Mirafiori lettuce virus in Chile

Lettuce big-vein disease (BVD) is a serious virus disease of lettuce. Recent evidence has brought into question the role of Lettuce big-vein virus (LBVV) in the etiology of BVD, and suggested that Mirafiori lettuce virus (MiLV) and not LBVV is the causal agent of BVD (1,2). Lettuce plants (Lactuca sativa L.) with symptoms similar to those of BVD were observed during the winter of 2003 in open-field and hydroponic-grown lettuce plants located in the Chacabuco Province of central Chile. Symptomatic plants exhibited leaves with chlorotic vein banding that became ruffled and distorted. Symptoms were usually accompanied by reduced plant size and absence of head formation. Roots from symptomatic plants were analyzed by light microscopy-acid fuchsin staining. Zoosporangia and resting spores of Olpidium brassicae were identified on the basis of their morphology and structure. Additionally, soil transmission experiments were performed with 50 healthy lettuce seedlings replanted into contaminated soil collected from lettuce fields having symptomatic crops. After 3 weeks, one-half of the seedlings showed differing degrees of big-vein symptoms, and the presence of spores of O. brassicae was confirmed in the roots by light microscopy. Seedlings raised in sterilized soil showed no symptoms after the same period of time. On the basis of nucleotide sequences of LBVV and MiLV from the GenBank database, primers specific to the coat protein genes of each virus were designed as follows: MiLVV-CP1: 5'-CAAATCTGTCCACAATTCC-3'; MiLVV-CP2: 5'-TCTCACTTGAAAACCTTCC-3'; MiLVV-CP3: 5'-TTGCAACGTGATGAAACC-3'; MiLVV-CP4: 5'-AAAGAAGAGAAGCCTGTTCC-3'; LBVV-CP1: 5'-AAGCTTTCCGTACTGTCC-3'; LBVV-CP2: 5'-CCTTGATACAGTTTTTGACC-3'; LBVV-CP3: 5'-GTATGCTGATTTCTGTAGACC-3'; LBVV-CP4: 5'-TAGATGTTCTTCGCCACC-3'. The primers were used in reverse transcription-polymerase chain reaction (RT-PCR) assays with dsRNA as a template. Amplicons of the expected size were obtained in each of the five symptomatic plants analyzed, using each of the designated primer sets: MiLVV-CP1/MiLVV-CP2: 562 bp; MiLVV-CP3/MiLVV-CP4: 743 bp; LBVV-CP1/LBVV-CP2: 485 bp; and LBVV-CP3/ LBVV-CP4: 570 bp. No amplicons were obtained from healthy lettuce plants. The identity of both viruses was verified by cloning and sequencing of the amplicons. Nucleotide sequences were compared with those in the GenBank database. Sequences derived from the Chilean isolates resulted in identities of 87 to 97% for MiLV and 97 to 99% for LBVV. All samples analyzed were from the Chacabuco Province where 43% of the lettuce crops in Chile are grown. Thus, the impact that BVD may have on lettuce availability for local consumption may be significant. To our knowledge, this is the first report of Lettuce big-vein disease and Mirafiori lettuce virus infecting lettuce and the first report of BVD in Chile.

Isolamento, seleção de bactérias e efeito de Bacillus spp. na produção de mudas orgânicas de alface

Isolados bacterianos epifíticos e endofíticos, obtidos de plantas sadias de alface, foram avaliados para promoção de crescimento de mudas e plantas, respectivamente em estufa e campo de cultivo orgânico (Chã Grande-PE). Nos experimentos em estufa, foi utilizada a cultivar Verônica e em campo, as cultivares Verdinha e Verônica. Os isolados foram aplicados por bacterização simultânea nas sementes e substrato. Em campo, foram utilizados os isolados mais eficientes, C25 (Bacillus thuringiensis subvar. kenyae) e C116 (Bacillus pumilus), separadamente e em mistura, após teste de compatibilidade. Em estufa, foram avaliadas a matéria fresca de raízes (MFR), da parte aérea (MFPA) e total (MFT), 21 dias após a bacterização. Em campo, foi determinado o peso da matéria fresca total de plantas comercializáveis 21 e 28 dias após o transplante, respectivamente para as cultivares Verdinha e Verônica. Os mecanismos de ação de BPCP analisados foram produção de ácido indol acético, ácido cianídrico, solubilização de fosfatos e alterações dos teores foliares dos macronutrientes, N, P, K, Ca e Mg. Em estufa, as mudas apresentaram aumento significativo em relação à testemunha para MFR, MFPA e MFT quando foi utilizado o isolado C116 e para MFR e MFT utilizando-se o C25. No campo, não houve promoção significativa no crescimento nas plantas das cultivares Verdinha e Verônica, tratadas com C25 e C116 separadamente ou em mistura. Dos mecanismos de ação analisados verificou-se apenas elevação significativa (P=0,05) do teor foliar de N pelo isolado C25.

Characterization of Distinct Tombusviruses that Cause Diseases of Lettuce and Tomato in the Western United States

A soilborne disease of lettuce, associated with necrosis and dieback, has been found with increasing frequency in California and Arizona over the last 10 years. An isometric virus, serologically related to Tomato bushy stunt virus (TBSV), was consistently isolated from lettuce plants with these disease symptoms. Back-inoculation to healthy lettuce plants and subsequent reisolation of the virus from symptomatic lettuce leaves suggested that this virus was the causal agent of this disease. A tombusvirus was also associated with a necrosis disease of greenhouse-grown tomatoes in Colorado and New Mexico. Complementary DNA representing the 3' end of viral genomic RNAs recovered from diseased lettuce and tomato plants had identical nucleotide sequences. However, these sequences were divergent (12.2 to 17.1%) from sequences of the previously described strains of TBSV, Petunia asteroid mosaic virus (PAMV), Artichoke mottled crinkle virus, and Carnation Italian ringspot virus. Additional tombusvirus isolates were recovered from diseased lettuce and tomato plants and these were most closely related to the TBSV-cherry strain (synonymous with PAMV) and to Cucumber necrosis virus based on comparison of 3'-end sequences (0.1 to 0.6% and 4.8 to 5.1% divergence, respectively). Western blot analysis revealed that the new tombusvirus isolated from diseased lettuce and tomato plants in the western United States is serologically distinct from previously described tombusvirus species and strains. Based on genomic and serological properties, we propose to classify this virus as a new tombusvirus species and name it Lettuce necrotic stunt virus.

Influence of Aster Yellows Phytoplasma on the Fitness of Aster Leafhopper (Homoptera: Cicadellidae)

This study revealed that feral aster leafhoppers, Macrosteles quadrilineatus Forbes, exposed to aster yellows phytoplasma live longer and may lay more eggs than nonexposed leafhoppers. Aster leafhoppers were reared on asters infected with either of 2 strains of aster yellows phytoplasma or uninfected asters. After eclosion, adults were placed on uninfected healthy lettuce or oat plants and transferred periodically. The life span of test leafhoppers and the number of offspring they produced were compared. Females reared on noninfected aster plants lived for an average of 19 d, those reared on ‘severe’ and ‘bolt’ strain aster yellows phytoplasma-infected plants lived 26 and 28 d, respectively. The mean number of offspring produced by females reared on the bolt strain of aster yellows phytoplasma-infected asters was almost twice the number produced by nonexposed leafhoppers. The life span of feral leafhoppers or the number of eggs laid did not differ for leafhoppers maintained on either oats or lettuce after exposure to aster yellows phytoplasma-infected asters. Female leafhoppers lived twice as long as males. Our results suggest that the aster leafhopper may have had a long association with aster yellows phytoplasma. The longer life and higher fecundity of phytoplasma-infected leafhoppers may influence disease dynamics of aster yellows in lettuce.

Growth and survival of the mycoparasiteConiothyrium minitans on lettuce leaves in contact with soil in the presece or absence ofSclerotinia sclerotiorum

Lettuce leaves co-inoculated withSclerotinia sclerotiorum andConiothyrium minitans and controls were placed on, or buried in, soil for a period of two weeks to study development and survival ofC. minitans. OnS. sclerotiorum-infected leaves on the soil surface, the number of colonies ofC. minitans recovered was about 40% of the number of pycnidiospores applied. When buried in the soil there was a reduction to about 2% of the spores applied. WhenC. minitans was applied on healthy lettuce leaves, which were subsequently placed on or in soil, the recovery was about 4%. It is argued that these figures indicate multiplication ofC. minitans onS. sclerotiorum-infected lettuce leaves on the soil, and good survival in all other cases.

Studies on the lettuce big‐vein virus and its vector Olpidium brassicae (Wor.) Dang

SUMMARYIt was demonstrated by serial graft transmission experiments that the big‐vein disease of lettuce was caused by lettuce big‐vein virus (LBVV).An Olpidium sp., always found in the roots of naturally affected plants, induced the disease when its zoospores were transferred to healthy plants. The fungus was identified as O. brassicae (Wor.) Dang.The same fungus was found in the roots of healthy lettuce, but zoospores from this source would not induce big‐vein when transferred to healthy plants.When a non‐viruliferous isolate of O. brassicae was grown in the roots of lettuce affected by big‐vein the fungus acquired LBVV and was able to transmit the disease. Conversely, the viruliferous Olpidium was converted to a non‐viruliferous form by culture in the roots of Plantago major or Veronica persica. These results were not caused by the selection of non‐vector strains of the fungus during its culture in these species, as the fungus reacquired and transmitted LBVV when again cultured in big‐vein affected lettuce roots. The results suggested that LBVV did not multiply in P. major or V. persica, or in the fungus during its growth in them.Attempts to transmit LBVV with Cuscuta campestris, C. gronovii, C. reflexa and C. subinclusa and by sap inoculation were unsuccessful.No causal relationship between tobacco necrosis virus (TNV) and big‐vein was found: the reasons for the frequent association of TNV with the disease are discussed.

Bobbelblad bij sla

In The Netherlands symptoms of big vein were found in lettuce and endive (Cichorium endivia L.). In the roots of these plants sporangia ofOlpidium occurred. In the glasshouse healthy lettuce plants showed symptoms when planted in infected soil.