Splash dispersal of conidia of Mycocentrospora acerina in the field

The dispersal of conidia of Mycocentrospora acerina was studied in caraway field trials. A Burkard spore trap, rotorods, inverted Petri dishes containing sucrose agar and rain gauges were used to trap conidia of M. acerina . Sporulation was stimulated by rainfall (2 mm) and moderate temperatures (around 15°C). Solar radiation had a negative effect on sporulation. Hardly any conidia were found in the spore traps on rainless days. Short distance (9 m) spread of M. acerina is mainly caused by splash dispersal of its conidia. Trap plants at 0, 0.1, 1 and 4 m from the inoculum source were readily infected under moist conditions. Beyond 9 m from an inoculum source no infection of caraway trap plants was found. Trap plants at 9 m from an inoculum source were infected in one out of three seasons only. Long distance (>9 m) spread could not be demonstrated by the techniques used in this study. The results suggest that, usually, a caraway field is infected by inoculum sources within that field.     

Production of cell-wall polysaccharide degrading enzymes in carrot root tissues infected by Mycocentrospora acerina.

Mycocentrospora acerina , the causal agent of liquorice rot of carrot roots, produces several cell-wall polysaccharide degrading enzymes in vitro . To assess the involvement of these enzymes in tissue invasion, the production and localization of pectin methylesterases, polygalacturonases, pectate lyases and endoglucanases were measured in root tissue infected by M. acerina . Isoelectrofocusing studies demonstrated the production of three isoforms of pectin methylesterase in healthy tissue. In infected tissue, two isoforms already observed in the culture filtrate and two novel isoforms were detected. Tissue maceration was associated with in vivo production of pectin methylesterase and polygalacturonase, which were first detected on the second day whereas pectate lyase and endoglucanase activities were detected only from the fourth day after infection. Maceration was always detected ahead of the mycelium, indicating diffusion of these enzymes.     

Effect of root injury on lesion development of caraway roots infected by Mycocentrospora acerina

The effect of injury on disease incidence, incubation period and lesion development rate on caraway roots by Mycocentrospora acerina was studied in three laboratory experiments. After inoculation with M. acerina, disease incidence of injured roots was significantly (P< 0.001) higher than of non-injured roots. The incubation period of M. acerina was significantly (P < 0.001) shorter on injured roots than on non-injured roots. The incubation period shortened with increasing root injury level. Younger injured roots tended to be more resistant to M. acerina infection than older injured roots, expressed by longer incubation periods. The lesion development rate was, on average, higher on heavily injured roots than on non- or slightly injured roots. The lesion development rate remained fairly constant after the first emergence of the symptoms on the caraway root, until the whole root was colonized. Caraway roots carefully dug up in autumn frequently showed injuries enabling M. acerina to penetrate the roots. However, the correlation between root injury and root rot after cold storage was weak. Injury of roots had a stimulating effect on infection and development of M. acerina, but roots without wounds could be infected too. Some relevant field observations are discussed.     

Mycocentrospora cladosporioides on olive in Greece1

Mycocentrospora cladosporioides has been recorded as present and widely distributed on olive fruits and leaves in Greece since 1972. The fungus frequently appears on the lower surface of the older leaves, growing like a dark olivaceous mould. The disease, known as cercosporiosis, sometimes causes severe defoliation. Green and mature olives are occasionally attacked. The severity of fruit infection seems to be related to mild humid weather in the 3 months prior to harvest. Diseased fruits show superficial slightly sunken irregular dry spots often covered by the characteristic fructifications of the fungus. Infection of artificially inoculated detached olives starts with the appearance of slow-growing brown lesions surrounding the lenticels. Cultivars producing large fruits for table use (e.g. Conservolia) are more susceptible. The fungus grows very slowly in culture and forms only a few conidia. Epidemiology and control have been little studied. Copper-based fungicides used for olive scab control are not always effective against cercosporiosis.     

Lettuce mosaic virus

Lettuce mosaic virus (LMV) is an economically significant virus of lettuce and endive. The virus has spread world-wide due to the exchange of seed of lettuce varieties. A very high proportion of infected plants may result from a low level of infected seed, because of very efficient transmission by a number of aphid species. The symptoms are characteristic but the diagnosis can be difficult, particularly on lettuce, because numerous viruses may coinfect this species. A very reliable and sensitive method by ELISA has been established for diagnosis and detection, which gives a good estimation ofthe contamination level in a seed batch. The use of virus-free seed, preventive cultural practices and the use of tolerant varieties were shown to be good methods for control if rigorously applied. Up to now, strains able to overcome the genes g and mo, considered to be identical, were shown to be non-seed-transmissible. Studies carried out with several virulent isolates have shown that genes mo and g are different and probably allelic, and that one strain infects seed at a very high level on susceptible and tolerant genotypes. These features have necessitated the production of virus-free seed, including systematic checks on all cultivars, and have stimulated research on new sources of resistance. Recent molecular studies have provided clones for detection and strain differentiation. Assays to introduce different LMV genes into lettuce seem promising.     

Occurrence of Mirafiori Lettuce Virus and Lettuce Big-vein Virus in Relation to Development of Big-vein Symptoms in Lettuce Crops

Big-vein disease (BV) of lettuce has been attributed to infection by Lettuce big-vein virus (LBVV), vectored by the soil fungus Olpidium brassicae. The finding of a second soil-borne virus in lettuce, Mirafiori lettuce virus (MiLV), led to a re-investigation of the role of LBVV in big-vein disease, with evidence emerging that both MiLV and LBVV are vectored by O. brassicae, and that MiLV, not LBVV, is the cause of BV (Lot et al. (2002), Phytopathology 92: 288–293). The two viruses have coat proteins of similar size but have different morphologies and are serologically unrelated. We tested individual lettuce plants in BV-prone fields and protected crops in France and Italy for the presence of the two viruses, using DAS-ELISA and antisera specific for each virus. Both MiLV and LBVV were found at high incidence, often together but sometimes separately. Symptoms were frequently found to be associated with MiLV alone or both viruses, but rarely LBVV alone. However, no absolute correlation emerged, because sometimes MiLV was present in the absence of symptoms, and vice versa. To clarify the situation, individual lettuce plants were examined over a period of time in two further surveys. In surveys of protected crops in France, plants with big-vein were always ELISA-positive for MiLV, or else symptomless plants positive for MiLV were later seen to develop big-vein symptoms. Presence or absence of LBVV appeared to have no effect on symptom development. In surveys of open fields in Italy, all combinations were found: presence of both viruses, apparent absence of both viruses, or presence of each one alone, in plants that developed BV. At the end of the observation period, nearly all plants had BV and contained both viruses.     

莴笋、油麦菜中烯酰吗啉的GC-ECD分析

建立了气相色谱法检测莴笋、油麦菜中烯酰吗啉残留的分析方法。样品采用乙腈提取,Canbon-NH2柱净化,DM-1色谱柱分析。莴笋、油麦菜中烯酰吗啉在0.064、0.128、0.320mg/kg 3个添加水平的平均加标回收率在范围86.4%~97.4%内,RSD(n=3)均10,方法理论最小检出限为0.01mg/kg。该方法满足莴笋、油麦菜中烯酰吗啉农药残留的检测要求。     

Pathogenicity Groups in Fusarium oxysporum f. sp. lactucae on Horticultural Types of Lettuce Cultivars

Isolates of Fusarium oxysporum f. sp. lactucae obtained from six localities in Japan were divided into three patho-genicity groups. Group 1 was highly pathogenic to lettuce cultivars of crisphead and red leaf types and was less pathogenic to butterhead and green leaf type cultivars. Group 2 was highly pathogenic to butterhead type and less pathogenic to crisphead and leaf types. Group 3 was less pathogenic to all lettuce types than groups 1 and 2. These results indicated pathogenic differentiation in F. oxysporum f. sp. lactucae, roughly relating to horticultural types of host lettuce cultivars. Received 21 February 2001/ Accepted in revised form 28 May 2001     

Diversity Among Strains of Xanthomonas campestris pv. vitians from Lettuce

ABSTRACT Diversity in host range, pathogenicity, phenotypic characteristics, repetitive extragenic palindromic polymerase chain reaction (rep-PCR) profiles, and sequence of the 16S-23S rDNA spacer region was examined among 44 Xanthomonas strains isolated from lettuce. Forty-two of the strains were divided into two groups, designated A and B. Seventy percent were Group A, and most of the remaining strains including a reference strain (LMG 938) were Group B. Group A strains induced both local and systemic symptoms, whereas Group B strains caused only distinct necrotic spots. Two strains, including the X. campestris pv. vitians type strain, were distinct from the Group A and B strains and were not pathogenic on lettuce. Analysis of fatty acid profiles, serotype, carbon substrate utilization patterns, and protein fingerprints confirmed this grouping. The Group A and B strains also formed two unique clusters (I and II) by rep-PCR profiling that corresponded to the two groups. Direct sequencing of a PCR-amplified DNA fragment (680 bp) from the 16S-23S rDNA spacer region of four representative strains, however, did not differentiate these groups. Serology and rep-PCR fingerprinting can be used to diagnose and identify X. campestris pv. vitians strains, while the other analyses evaluated are useful for strain characterization.     

The Use of Green Fluorescent Protein-Tagged Recombinant Viruses to Test Lettuce mosaic virus Resistance in Lettuce

ABSTRACT Seed certification and the use of cultivars containing one of two, probably allelic, recessive genes, mo1(1) and mo1(2), are the principal control methods for Lettuce mosaic virus (LMV) in lettuce. Although for a few LMV isolates, mo1(2) confers resistance with most isolates, the genes mo1(1) or mo1(2) confer a tolerance, and virus accumulation is readily detected in mo1-carrying plants. This phenotype complicates evaluation of the resistance status, in particular for mo1(1), for which there are no viral strains against which a true resistance is expressed. Two green fluorescent protein (GFP)-tagged viruses were constructed, derived from a non-resistance breaking isolate (LMV-0) and from a resistance-breaking isolate (LMV-E). An evaluation of 101 cultivars of known status was carried out with these recombinant viruses. Using the LMV-0-derived recombinant, identification of mo1-carrying cultivars was simple because, contrary to its wild-type parent, systemic movement of LMV-0-GFP was abolished in resistant plants. This assay detected four cases of misidentification of resistance status. In all these cases, further tests confirmed that the prior resistance status information was incorrect, so that a 100% correlation was observed between LMV-0-GFP behavior and the mo1 resistance status. Similarly, the LMV-E-derived recombinant allowed the identification of mo1(2) lettuce lines because its systemic movement was restricted in mo1(2) lines but not in susceptible or in mo1(1) lines. The tagged viruses were able to systemically invade another host, pea, irrespective of its resistance status against another member of the genus Potyvirus, Pea seed-borne mosaic virus. The use of these recombinant viruses could therefore greatly facilitate LMV resistance evaluation and speed up lettuce breeding programs.     

Sources of Verticillium dahliae Affecting Lettuce

ABSTRACT Since 1995, lettuce in coastal California, where more than half of the crop in North America is grown, has consistently suffered from severe outbreaks of Verticillium wilt. The disease is confined to this region, although the pathogen (Verticillium dahliae) and the host are present in other crop production regions in California. Migration of the pathogen with infested spinach seed was previously documented, but the geographic sources of the pathogen, as well as the impact of lettuce seed sparsely infested with V. dahliae produced outside coastal California on the pathogen population in coastal California remain unclear. Population analyses of V. dahliae were completed using 16 microsatellite markers on isolates from lettuce plants in coastal California, infested lettuce seed produced in the neighboring Santa Clara Valley of California, and spinach seed produced in four major spinach seed production regions: Chile, Denmark, the Netherlands, and the United States (Washington State). California produces 80% of spinach in the United States and all seed planted with the majority infested by V. dahliae comes from the above four sources. Three globally distributed genetic populations were identified, indicating sustained migration among these distinct geographic regions with multiple spinach crops produced each year and repeated every year in coastal California. The population structure of V. dahliae from coastal California lettuce plants was heavily influenced by migration from spinach seed imported from Denmark and Washington. Conversely, the sparsely infested lettuce seed had limited or no contribution to the Verticillium wilt epidemic in coastal California. The global trade in plant and seed material is likely contributing to sustained shifts in the population structure of V. dahliae, affecting the equilibrium of native populations, and likely affecting disease epidemiology.