Lettuce Fields Research Articles

Temporal Distribution of Aster Leafhopper Sex Ratios and Spatial Pattern of Aster Yellows Phytoplasma Disease in Lettuce

The aster leafhopper, Macrosteles quadrilineatus Forbes, is an important pest of fresh market vegetable crops as the primary vector of the aster yellows (AY) phytoplasma. Two sampling methods, sticky trapping and inverted cage trapping, were used to monitor male and female leafhopper populations over three growing seasons in leaf lettuce fields in Ohio. Captures by one sampling method could not be used to estimate captures by the other method, because sticky traps captured significantly more male leafhoppers and cage traps captured significantly more females. The proportion of females collected in cage traps decreased significantly in individual plantings as lettuce matured and also over the course of the season in one of 2 yr of sampling by using both techniques. Subsamples of captured leafhoppers were tested for AY phytoplasma infection by using a polymerase chain reaction (PCR) assay. In one year of our study, more male than female leafhoppers were infected with AY phytoplasma. Because the field distribution of insect-vectored diseases can reveal information about vector movement, as well as vector identity, spatial analysis was performed. Analyses indicated the distribution of symptomatic AY phytoplasma-infected lettuce plants was significantly clustered and followed a beta-binomial distribution. Our data suggest that females may be responsible for at least the early season inoculation of clustered symptomatic lettuce plants.

Comparative Analyses of Lettuce Drop Epidemics Caused by Sclerotinia minor and S. sclerotiorum.

Temporal and spatial patterns of lettuce (Lactuca sativa) drop caused by Sclerotinia minor and S. sclerotiorum were determined in lettuce fields in the Salinas, Santa Maria, and San Joaquin Valleys in California during 1995 to 1998. Of the 25 commercial fields assessed, 14 had predominantly S. minor, 9 had predominantly S. sclerotiorum, and 2 had varying levels of both species. Sclerotinia infections were classified based on symptoms: those caused directly by eruptive germination of sclerotia (type I) and those caused by the airborne ascospores (type II). The precise location of diseased and healthy plants was mapped and lettuce drop progress was determined at different crop growth stages. Spatial patterns of disease incidence were analyzed using 1-by-1-, 2-by-2-, 3-by-3-, and 4-by-4-m quadrat sizes. Regardless of the analytical method employed, disease incidence with type I infection showed an aggregated pattern in a majority of the fields evaluated and random patterns in fields where incidence was low. In all fields with type I infection, disease progress followed the monomolecular model, typical of soilborne diseases. For fields with aggregated distribution, spatial dependence was observed up to 10 m and was either isotropic or random in direction, suggesting the potential influence of tillage operations on inoculum distribution and disease incidence. Lettuce drop incidence in fields with type II infection was erratic in time and peaked within a very short time. However, disease incidence showed an aggregated pattern in all fields evaluated. Spatial dependence of quadrats generally was detected in two adjacent directions, suggesting a directional gradient perhaps caused by wind direction during ascospore dissemination. Increasing quadrat sizes usually increased the degree of aggregation of lettuce drop, but not the distribution pattern itself. These results demonstrate that the source of inoculum and the type of infections they cause are most likely to determine spatial patterns of lettuce drop in the field.

Effect of Planting Date, Cultivar, and Stage of Plant Development on Incidence of Fusarium Wilt of Lettuce in Desert Production Fields

Fusarium wilt of lettuce, first recognized in Japan in 1955, has since been discovered in the United States (California in 1990, Arizona in 2001), Iran (1995), Taiwan (1998), and Italy (2001). In Arizona, the causal agent, Fusarium oxysporum f. sp. lactucae, has been recovered from lettuce plants in 27 different lettuce fields during the 2001 to 2003 production seasons. Studies were initiated to examine the impact of planting date, cultivar, and stage of plant development on the incidence of disease in the field. In 2002 and 2003, tested lettuce cultivars were sown in at least one of the following planting windows; early-season (September), mid-season (October), and late-season (December). Within each planting window, significant differences in disease incidence among lettuce cultivars were noted at plant maturity. The mean incidence of Fusarium wilt on cultivars sown in September, October, and December was 92.3, 15.1, and 2.0%, respectively, in 2002 and 74.2, 5.1, and 0.7%, respectively, in 2003. The mean soil temperatures at the10-cm depth during the September, October, and December plantings for both years were 26, 14, and 14°C, respectively. Initial symptoms of Fusarium wilt were apparent as early as 14 days after seeding, with increasing incidence of disease noted as the crop developed and reached maturity. Among all lettuce cultivars planted in September, only one and two cultivars of romaine in 2002 and 2003, respectively, reached maturity with ≤5% incidence of Fusarium wilt, whereas the lowest incidence of disease among crisphead, green leaf, red leaf, or butterhead cultivars was 73.7, 27.0, 20.2, and 65.7%, respectively, in 2002 and 62.1, 29.0, 100, and 100%, respectively, in 2003. For October plantings, all romaine cultivars had ≤5% incidence of Fusarium wilt at maturity, whereas disease incidence among tested cultivars of crisphead lettuce in 2002 and 2003 ranged from 0.8 to 66.8% and 0.3 to 43.3%, respectively. When planted in December, 82 and 88% of tested cultivars, including all romaine entries, reached maturity with ≤1% incidence of Fusarium wilt. Selection of appropriate lettuce cultivars and planting times should allow successful production of lettuce in the southwestern Arizona production region with minimal or no incidence of disease in fields infested with F. oxysporum f. sp. lactucae. On the other hand, successful production of lettuce in infested fields when temperatures favor disease development will not be possible until lettuce cultivars are developed that possess high tolerance or resistance to the pathogen.

Solarization and fungicides for the control of drop, bottom rot and weeds in lettuce

Soil solarization was applied for 60 days during the summer in lettuce fields in Mogi das Cruzes (latitude 22°31′S, longitude 48°11′W), southeastern Brazil, from December to January, 1998/99 and 1999/2000, after which two consecutive lettuce crops were grown in each cropping season. Soil solarization was tested in association with the fungicides pencycuron and procymidone, with the purpose of controlling bottom rot and lettuce drop, caused by Rhizoctonia solani AG1-I and Sclerotinia minor, respectively. In the first crop of both years the solarized treatments presented very low incidences of the diseases and shortened the crop cycle by 10 days as compared to the non-solarized plots. In both years the incidence of lettuce drop in the second (winter) crops was very low in the solarized plots, but a 25–40% loss of lettuce heads to the disease was observed in the control plots (no solarization and no fungicides). Although the incidence of diseases in the non-solarized plots was reduced by the fungicides, they did not offer additional benefits in the solarized areas in both crops. Reductions in biomass C and in the number of fungi and bacteria were observed in the solarized treatments, but the amount of fluorescent pseudomonas was not affected by the soil treatments. The concentrations of soil nutrients NH 4–N and DTPA-extractable-Mn increased sharply due to solarization. Changes in soil nutrient availability and probably in the microbial community, among other non-detected benefits, may be related to the reduction of the crop cycle in the present work.

Sclerotinia sclerotiorum에 의한 결구상추 균핵병(Sclerotinia rot)의 발생과 병원성

2003년 1윌부터 5월까지 경상남도 부림면 신반리의 결구상추 재배 플라스틱 하우스에서 자연발병된 균핵병의 발병율를 조사한 결과, 5.7%<TEX>${\sim}$</TEX>39.0%로서 평균 21.9%였다. 병든 밑둥부위에서 분리하여 병원성이 가장 강한 것으로 확인된 YR-1 균주를 형태 및 배양적 특성 등을 조사하여 기존의 보고와 비교한 결과 Sclerotinia sclerotiorum로 동정되었으며, S. sclerotiorum YR-1으로 명명하였다. YR-1 균주의 병원성 검정을 위하여 균사조각 부유액을 접종원으로하여 접종원의 최적 처리량과 최적 농도를 선발한 결과, 50%에 가까운 발병도를 보인 <TEX>$A_{550}$</TEX>=0.4와 0.6농도의 균사조각 부유액 40 ml로 선발하였다. 균사조각 부유액으로 병원성 검정 결과, 결구상추 재배 농가에서 자연발병된 병징과 동일한 병징이 나타났다. This studies were investigated the occurrence of sclerotinia rot at the crisphead lettuce field in Uiryeong-Gun, Gyeongsangnam-Do from January to May in 2003. Average incidence rates of sclerotinia rot on crisphead lettuce was up to 21.9% at the five plastic houses. A total of 140 isolates of Sclerotinia sp. were obtained from diseased leaves of crisphead lettuce. Among them, the fungi YR-1 was isolated, which showed highly virulent on the whole plant. the YR-1 was identified as Sclerotinia sclerotiorum based on the formation, color, shape and size of sclerotium and apothecium. For the pathogenicity test, the most suitable inoculum quantity of YR-1 strain was selected as the triturated mycelial suspension of <TEX>$A_{550}$</TEX>=0.8, 40 ml showing disease incidence of 94%, and the symptom showed as same as at the fields, the leaves and stem had rotten and developed white downy mycelial at the diseased lesion on the leaves and stems, and produced black and irregular sclerotinia. This is the first report on the pathogenicity test using by triturated mycelial suspension-inoculum of the pathogen for the sclerotinia rot of crisphead lettuce.

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

SummarySpatial 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.

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.

Steinbeck Goes to High School: A U.K. Educational Resource Project on Of Mice and Men

John Steinbeck has long been a popular choice for teachers in England to read with their high-school literature students. Educational editions of The Pearl, The Moon Is Down, and The Red Pony are regularly reprinted for use in Key Stage 3 of the U.K. system (ages 11-14), while both The Grapes of Wrath and East of Eden have been featured in advanced level studies for 16-18 year olds. But by far the most popular book has been Steinbeck's enduring classic Of Mice and Men. Every year it is the examination text for hundreds of thousands of 16 year olds in England—and the only book that is offered by every one of the five examination boards who administer testing for the General Certificate of Secondary Education (GCSE). DCS is a U.K. educational advisory service that exists to provide innovative support for teachers of all subjects as they strive to improve their pedagogic skills. As part of that remit, their digital Media Education Centre(dMEC)in Exeter, Devon, produces printed and digital resources for classroom use. Their most successful series has been the products tailored for use by English teachers. As well as a wide range of material on poetry from different cultures and traditions, two of the most popular titles to date have been American texts: To Kill a Mockingbird and Arthur Miller's play A View from the Bridge. Martin Phillips, the director of dMEC, explains why they chose Of Mice and Men as the next text in their series: Quite simply, we selected Of Mice and Men because it is by far and away the most popular novel [End Page 163] being studied for examination at 16. Part of the assessment criteria for gcse Literature require our students to show an understanding of the social, historical, and cultural context of the works they study. On the principle of "show not tell," our productions aim to bring alive the culture of exam texts. We visited Monroeville, Alabama, (the real-life equivalent of Maycomb) to make our Mockingbird resource. We interviewed local people, both black and white, about their memories of the deep South in the 1930s. We also filmed extracts of the annual performance in the Monroeville museum and interviewed those playing the parts about their characters. It was very well received by teachers, so Of Mice and Men will follow a similar format. Martin and his colleague Tim Arnold spend most of their time working as educational advisers, running courses or working with teachers in their classrooms—but they double up as a camera crew when necessary! In July they brought their equipment to California to do the location work for the DCS project. The main focus of the visit was an in-depth interview with Dr. Susan Shillinglaw of the Center for Steinbeck Studies at San Jose State University. As well as offering a perspective on the contextual issues, such as the Great Depression, the Dustbowl, and notions of the American Dream, the interview explored different aspects of the language, themes, and characterization in the novel. Having worked with Susan, the pair then filmed a range of location material in the area: modern-day migrant workers in the lettuce fields around Soledad, exhibits at the National Steinbeck Center, Steinbeck's houses and other locations in Salinas and Monterey, and an interview with Bill Ramsey, a Salinas resident whose family arrived from Texas as "Okies" in the 1930s. All the California material will now be edited together with work completed in England using actors to explore sections of the text. This footage was devised by two local Devon teachers, Jon Seal and Connor McGee. They have been instrumental in bringing the resource to production. It is a feature of DCS's work that they draw on the expertise of cutting-edge working practitioners to inform their training programs. Jon wrote the...

Rhizoctonia soiani에 의한 결구상추 밑둥썩음병(Bottom rot)의 발생과 병원성

2003년 11월 경상남도 부림면 신반리의 결구상추 재배 플라스틱 하우스내에서 늦가을부터 밑둥썩음병이 발생하여 수확기인 11월에 평균 5.3%의 발병율이 조사되었다. 이는 비교적 낮은 수치이나, 2차적으로 균핵병과 잿빛곰팡이병이 유발되어 큰 경제적 피해를 야기하였다. 이때의 병징은 아랫잎의 밑둥 부위에 수침상의 원형 내지 타원형의 담갈색 또는 갈색점무늬가 나타나고 진전되면 윗잎쪽으로 병반이 부정형으로 크게 확대되어 결국 잎이 무르게 썩고 누렇게 변하며 종이처럼 말라죽는 증상이 관찰되었다. 이에, 밑둥썩음병이 발생한 지역의 병든 식물체와 토양으로부터 병원균을 분리하였으며 이 중 병원성이 가장 강한 PY-1 균주가 Rhizoctonia solani AG1 (IB)로 동정되었다. 균사생육 최적온도는 25∼30<TEX>$^{\circ}C</TEX>였으며, 병원성 검정을 위해 가장 효과적인 접종원으로 검정 방법이 용이하면서도 효율적인 균사조각 부유액 접종원을 생육상내 폿트 검정용으로 선발하였고, 토양내 부생적 착생을 위한 플라스틱 하우스내 토경 재배 검정용으로는 WRSP배지 접종원을 선발하였다. 또한, 선발된 각 접종원의 최적 처리농도 및 처리량에서는 각각 균사조각 부유액 접종원의 경우, 평균 주당 51.1%의 발병도를 보인 <TEX>$A_{550}$</TEX>=1.0농도를 가장 적합한 처리 농도로 선발하였으며, WRSP 배지 접종원의 처리량 비교에서는 평균 주당 61.6%의 발병도를 보인 40 <TEX>$m\ell$</TEX>를 최적 처리량으로 선발하였다. This study was investigate the occurrence of bottom rot caused by Rhizoctonia solani at the crisphead lettuce fields in Uiryeong-gun, Gyeongsangnam-do from November to December in 2003. Incidence of bottom rot on crisphead lettuce was up to 5.3% at the six plastic houses. A total of 30 isolates of R. solani were obtained from diseased leaves of plants and were tested by artificial inoculation to the host. Among them, PY-1 isolate was selected showing highly virulent on the whole plant and was identified as R. solani AG1 (IB) based on the anastomosis test, morphological and cultural characteristics. Symptoms of bottom rot by PY-1 isolate produced small dark brown, depressed and elliptical spots on the lower part of leaves in the early stage as same as at the fields, were enlarged onto the upper part of leaves later, and the infected plant wilted and ultimately died in the end. For the pathogenicity test, triturated mycelia-inoculum (A<TEX>$_{550}$</TEX>=1.0) of PY-1 isolate was selected the most effective inoculum showing disease incidence of 51.1% for the mycelial inoculation at pot assay. Otherwise, WSRP media-inoculum (wheat brane ： sawdust ： rice brane ： PDB media=30 g ： 10 g ： 10 g ： 100 ml, w/w/w/v) of PY-1 isolate was effectual inoculum showing disease incidence of 61.6% for soil inoculation at the plastic house. Also, in selection of density and amount of inoculum, most suitable density of triturated mycelia-inoculum and amount of WSRP media- inoculum were determined as <TEX>$A_{550}$</TEX>=1.0 and 40 ml, respectively. This is the first report on the pathogenicity test using by WSRP media-inoculum of R. solani PY-1 isolate for the bottom rot of crisphead lettuce.

Relationship Between Plant Morphological Traits and Resistance to Sclerotinia minor in Lettuce

A low to moderate incidence of lettuce drop caused by Sclerotinia minor is commonly observed in commercial lettuce fields of all types of lettuce (e.g. crisphead, romaine, leaf, butter) and although partial resistance has been reported, no sources of immunity have been described. We sought to determine whether there was variability between different types of lettuce and among cultivars within types. Replicated experiments were conducted in an infested field using established inoculation procedures. Significant variation in susceptibility to S. minor was detected among cultivars within as well as between major lettuce types. Correlations between lettuce drop susceptibility and plant canopy size, seedling vigor, and additional morphological traits were determined. Variability between different field experiments was also evaluated for several traits and a subset of cultivars. Our results suggest that cultivated germplasm may provide genes that are as useful or more useful than those found in genotypes with more primitive growth habits in developing cultivars with tolerance to lettuce drop.

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

SummaryA 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.

GENETIC ANALYSIS OF RESISTANCE TO LETTUCE DROP CAUSED BY SCLEROTINIA MINOR

GENETIC ANALYSIS OF RESISTANCE TO LETTUCE DROP CAUSED BY SCLEROTINIA MINOR

Insensitivity to the Fungicide Fosetyl-Aluminum in California Isolates of the Lettuce Downy Mildew Pathogen, Bremia lactucae.

Lettuce downy mildew, caused by Bremia lactucae, is the most important foliar disease of lettuce in California. In recent years, there were apparent failures of fungicides containing fosetyl-aluminum (Aliette) to control downy mildew in commercial lettuce fields in California. Consequently, we characterized 134 isolates collected over 2 years from throughout the coastal growing areas of California for insensitivity to the fungicides fosetyl-aluminum and maneb, pathotype, and mating type. Tests using seedlings in controlled growth room conditions demonstrated the widespread occurrence of insensitivity to fosetyl-aluminum in California populations of B. lactucae. Fifty percent of the isolates assayed sporulated profusely in the presence of fosetyl-aluminum applied at rates twice the normal field dosage, and an additional 40% showed moderate sporulation at this rate. Fosetyl-aluminum-insensitive isolates were detected from all regions sampled. Insensitivity was also observed in multiple pathotypes. Insensitivity was not complete, however, because quantitative analysis of the number of lesions on older plants revealed that applications of fosetyl-aluminum could reduce the levels of disease by 50%. Therefore, while fosetyl-aluminum may have utility under low disease pressure in the field, other control measures are required to provide control under conditions favorable to the disease.

Occurrence of Yersinia pseudotuberculosis in iceberg lettuce and environment.

The virF-positive Y. pseudotuberculosis serotype O:2 strain was isolated from food in Finland for the first time. Y. pseudotuberculosis is considered to be a food-borne pathogen, but it has rarely been isolated from foods. Iceberg lettuce are grown on open fields and irrigated with water from lakes, rivers, and ponds on Ahvenanmaa Island. Y. pseudotuberculosis is frequently isolated from many domestic and wild animals (Fukushima and Gomyoda, 1991; Sanford et al., 1995; Niskanen et al., 2001). There is a large population of deers and hares on Ahvenanmaa Island. These animals can be asymptomatic carriers and reservoirs of Y. pseudotuberculosis (Griffin et al., 1992). Wildlife had access to lettuce fields and the ponds used to collect irrigation water. It is possible, that wild animals contaminate water, soil, and lettuce on the fields with their feces containing Y. pseudotuberculosis (Inoue et al., 1991; Fukushima et al., 1995). Contamination of iceberg lettuce was likely resulted from use of irrigation water contaminated with animal feces. Also nontypable Y. pseudotuberculosis strains without the virulence plasmid were recovered from environmental samples and a lettuce sample. Non-pathogenic Y. pseudotuberculosis serotypes O:7-O:14 have been previously recovered in environment (Nagano et al., 1996). Lettuce produced on open fields irrigated with water from natural water systems may contain pathogenic microbes, and should be concern as a possible source of Y. pseudotuberculosis infections.

Identification of Lettuce (Lactuca sativa L.) Germplasm with Genetic Resistance to Drop Caused by Sclerotinia minor

Incidence of the disease lettuce drop caused by Sclerotinia minor is often high in California lettuce fields despite the use of cultural and chemical controls. Development of resistant lettuce cultivars has been hindered by the difficulty of evaluating resistance in field tests and the lack of a screening procedure that reliably predicts field performance. Several lettuce genotypes of diverse geographic origin and plant architecture including modern and heirloom cultivars, plant introduction accessions, and breeding lines, were evaluated for resistance to S. minor using several methods. Resistance was evaluated in fields that contained naturally occurring S. minor, in a field that contained both naturally occurring and manually incorporated S. minor inoculum, and in the greenhouse using two types of inocula. Many genotypes exhibited partial resistance to S. minor, with significantly reduced disease incidence relative to susceptible controls. The similarity of disease ratings observed in replicated field tests supports the conclusion that partial resistance is under genetic control. Ratings obtained in some greenhouse tests were highly correlated with field ratings, but this was not true for all tests. Therefore, although greenhouse evaluation with adequate replication and repetition can be used as a selection tool, field testing remains an essential component of S. minor resistance breeding programs.

First Report of Fusarium Wilt of Lettuce Caused by Fusarium oxysporum f. sp. lactucae in Arizona.

A new wilt and root rot disease was observed in 6 and 11 commercial fields of lettuce (Lactuca sativa) in western Arizona during the fall of 2001 and 2002, respectively. Distance between infested sites ranged from approximately 0.5 to 39 km. Five head lettuce cultivars as well as a red leaf lettuce cultivar were affected. Disease symptoms included yellowing and wilting of leaves, as well as stunting and plant death. The cortex of the crown and upper root of infected plants usually was decayed and reddish brown. Disease symptoms first appeared at the time of plant thinning and continued to develop up to plant maturity. Fusarium oxysporum was consistently isolated from symptomatic plant roots. Seeds of cv. Lighthouse were planted in nonsterile vermiculite within 3.0-cm-square × 7.0-cm-deep cells in a transplant tray and thinned to a single plant per cell. When the first true leaves were emerging, 10 individual seedlings were inoculated with a single-spore isolate of F. oxysporum recovered from diseased lettuce root cortex tissue. Inoculum was prepared by growing the fungus on potato dextrose agar in 100-mm-diameter × 15-mm-deep plastic petri dishes at 28°C with a 12-h photoperiod under fluorescent light. Once the fungus completely covered the agar surface, 50 ml of sterile distilled water was added to the dish, and the mycelia and conidia on the surface were scraped off the agar and suspended in the water. This fungal suspension was decanted, and a 2-ml aliquot containing 1.8 × 105 CFU was pipetted into the vermiculite near the stem of each lettuce seedling. Ten plants grown in noninfested vermiculite served as uninoculated controls. After inoculation, plants were maintained in a growth chamber at 28°C with a 12-h photoperiod under fluorescent light for 3 weeks. Symptoms of yellowing, wilt, vascular decay, and often plant death developed during the incubation period on all inoculated plants but not on control plants. Fusarium oxysporum was consistently reisolated from inoculated plants but not from uninoculated plants. The experiment was repeated and yielded the same results. A wilt and root rot disease of lettuce attributed to F. oxysporum f. sp. lactucae was first reported in Japan in 1967 (3) and subsequently in the United States (San Joaquin Valley of California) in 1993 (2), and Italy in 2002 (1). The researchers of the U.S. report did not cite the earlier work from Japan and described the pathogen as F. oxysporum f. sp. lactucum. The Arizona isolate used to demonstrate pathogenicity was of the same vegetative compatibility group as an isolate of the pathogen from lettuce in California reported in 1993. Several companies grow and harvest lettuce in Arizona and California. At the end of production and harvest in the fall, tractors, implements, and harvesting equipment are transported from the San Joaquin Valley in California to western Arizona. The similarity between the isolate of F. oxysporum f. sp. lactucae from western Arizona and the San Joaquin Valley of California suggest a possible introduction of the pathogen into Arizona from California, perhaps on soil adhering to farm equipment. To our knowledge, this is the first report of F. oxysporum f. sp. lactucae infecting lettuce in Arizona.

Erigeron bonariensis: hospedeira alternativa do Lettuce mosaic virus no Brasil

O gênero Erigeron (Asteraceae), de plantas da vegetação espontânea, encontra-se amplamente disseminado nas regiões Sul e Sudeste do Brasil, sendo freqüentemente encontrado em lavouras perenes e anuais. Plantas de E. bonariensis com sintoma de mosaico, típico do induzido por vírus, foram coletadas no município de São Paulo e submetidas a análises ao microscópio eletrônico de transmissão, testes biológicos, sorológicos e moleculares. Em cortes ultrafinos do tecido foliar original, observaram-se inclusões tubulares e cata-ventos dispersos no citoplasma. Através de inoculação mecânica, somente Chenopodium amaranticolor, C. quinoa, Nicotiana benthamiana e N. clevelandii foram infetadas. Os resultados obtidos em ELISA foram negativos quando se utilizaram antissoros contra o Turnip mosaic vírus (TuMV) e diferentes estirpes do Potato virus Y (PVY), constatando-se relacionamento sorológico com o Lettuce mosaic virus (LMV). Com a utilização de oligonucleotídeos específicos para LMV amplificaram-se fragmentos esperados de aproximadamente 280 pb, que seqüênciados confirmaram a identidade do vírus. A ocorrência do LMV em E. bonariensis, gênero da mesma família botânica da alface (Lactuca sativa), é de grande importância, pois talvez possa atuar como reservatório para infecção de campos de produção de alface. Este é o primeiro relato, no Brasil, de vírus infetando Erigeron sp., o qual só havia sido reportado como hospedeira natural do Bidens mottle virus (BiMoV) e do Tomato spotted wilt virus (TSWV) nos Estados Unidos.

Attractiveness of three weed species to polyphagous predators and their influence on aphid populations in adjacent lettuce cultivations

AbstractThe utilization of olfactory responses of predators and ­parasitoids to the allelochemicals emitted by phytophagous arthropods and their host plants is becoming more important in biological pest control. The effects of three weed ­species, i. e. wormwood Artemisia vulgaris L., tansy Tanacetum vulgare L. and stinging nettle Urtica dioica L., which were planted as accompanying vegetations into a lettuce field, were examined for the predatory species Coccinella septempunctata L., Adalia bipunctata (L.), Propylea quatuordecimpunctata (L.) (Coleoptera, Coccinellidae) and Chrysoperla carnea (Steph.) (Neuroptera, Chrysopidae), as well as for aphids during the summer of 2000. The presence of weeds significantly increased the density of adults and larvae of the predators on the lettuce plants in relation to the control (lettuce field without weeds). However, the differences remained smaller for eggs and pupae. C. septempunctata tended to be the most abundant species, followed by P. quatuordecimpunctata. Remarkable differences among the attractiveness levels of the weeds in the 3 treatments were not observed. The increased populations of predators were accompanied by significantly reduced infestation rates with aphids in the treatments in relation to the control. Myzus persicae Sulzer was in general the most abundant aphid species followed by Nasonovia ribisnigri Mosley and Macrosiphum euphorbiae Thomas. The results of the present study are discussed with respect to the management of agroecosystems and the use of weeds in biological control.

Occurrence of Sclerotinia Rot on Composite Vegetable Crops and the Causal Sclerotinia spp.

Composite vegetable crops grown in greenhouses and open fields in Korea were surveyed from 1995 to 1999. Occurrence of Sclerotinia rot was observed in 123 of 277 lettuce fields, in 11 of 18 head lettuce fields, in 12 of 14 endive fields, and in 4 of 38 garland chrysanthemum fields surveyed during the growing seasons. The disease most severely occurred up to 80% on lettuce. Incidence of the disease was as high as 20% at its maximum on endive and garland chrysanthemum but relatively low on head lettuce. Symptoms of the disease developed on leaves of all the composites, crowns of lettuce and head lettuce, and stems of garland chrysanthemum. Out of 240 isolates of Sclerotinia species obtained from the diseased composites, 169 isolates were identified as Sclerotinia sclerotiorum, and the others as S. minor based on their morphological and cultural characteristics. S. sclerotiorum was isolated from all the composites, and S. minor only from lettuce and endive. Eight isolates of S. sclerotiorum and four isolates of S. minor were tested for their pathogenicity to the composites by artificial inoculation. All the isolates of the two Sclerotinia spp. induced rot symptoms on the plants of the composites by artificial inoculation, which were similar to those observed in the fields. The pathogenicity tests revealed that there is no significant difference in virulence of the isolates to the composites and in susceptibility of the composites to the isolates.

Validation of weather and leaf wetness forecasts for a lettuce downy mildew warning system

Leaf wetness and weather forecasts were important inputs in a warning system developed previously for lettuce downy mildew in coastal California. To improve the predictive ability of this warning system, reliability of weather and leaf wetness forecasts were evaluated. Weather forecasts from two commercial companies were compared with weather data measured in or near commercial lettuce fields during 1994-1998, at several different locations in coastal California. Measured leaf wetness data were compared with forecasts by company A. and by a dew simulation model using weather forecasts of company B or measured weather data (except cloud cover) as inputs of the model. When forecast and observed weather data pooled over 24 h were compared, the forecasts appeared to be accurate, but when data were analyzed hourly, forecasts had large errors during early morning hours, which coincided with the critical period for infection by the pathogen. Correspondingly, both company A and the dew simulation model frequently gave incorrect leaf wetness forecasts during morning and evening hours although incorrect forecasts were rare from Pacific standard time 10:00 to 17:00 and 00:00 to 06:00. Weather and leaf wetness forecasts were generally better for the Salinas Valley than for the Santa Maria Valley. The errors in weather forecasts and the innate weaknesses in the dew simulation model might have resulted in the inaccuracy in leaf wetness forecasts. This, in turn, led to errors in warnings of downy mildew infection by the previous system.