Evidence for Lettuce big‐vein associated virus as the causal agent of a syndrome of necrotic rings and spots in lettuce

Lettuce big‐vein associated virus (LBVaV, genus Varicosavirus) was shown to be responsible for characteristic necrotic symptoms observed in combination with big‐vein symptoms in lettuce breeding lines when tested for their susceptibility to lettuce big‐vein disease (BVD) using viruliferous Olpidium virulentus spores in a nutrient film technique (NFT) system. Lettuce plants showing BVD are generally infected by two viruses: Mirafiori lettuce big‐vein virus (MiLBVV, genus Ophiovirus) and LBVaV. New mechanical inoculation methods were developed to separate the two viruses from each other and to transfer both viruses to indicator plants and lettuce. After mechanical inoculation onto lettuce plants MiLBVV induced vein‐band chlorosis, which is the characteristic symptom of BVD. LBVaV caused a syndrome of necrotic spots and rings which was also observed earlier in lettuce plants inoculated in the NFT system, resembling symptoms described for lettuce ring necrosis disease (RND). This observation is in contrast with the idea that LBVaV only causes latent infections in lettuce. De novo next‐generation sequencing demonstrated that LBVaV was the only pathogen present in a mechanically inoculated lettuce plant with symptoms, providing evidence that LBVaV was the causal agent of the observed necrotic syndrome and thus fulfilling Koch’s postulates for this virus. The necrotic syndrome caused by LBVaV in lettuce is referred to as LBVaV‐associated necrosis (LAN).     

Verschil in reactie van wilde sla ten opzichte van besmetting met het slamozaiekvirus

Samenvatting Hoewel wij in onze opzet nl. het vinden van resistenteLactuca serriola L. niet slaagden, vonden wij zeer grote verschillen in gevoeligheid voor hetLactuca virus 1 tussen de verschillende herkomsten.De herkomsten reageerden ieder voor zich zeer uniform op het virus. Uit de vaak zeer hevige reactie en het ontbreken van symptomen op de planten onder natuurlijke cundities, concluderen wij, dat het virus in Nederland niet op planten van wilde sla voorkomt.Summary Several botanicalLactuca spp. were tested for their susceptibility toLactuca vims 1 . All appeared to be susceptible. As the exact identifications are not certain, specific names are only mentioned ofL. virosa L. andL. serriola L. Of these we tested plants of respectively six and thirty-five different places of origin.The reaction ofL. virosa to lettuce mosaic was in all cases a necrosis. The seed originating from Cologne was distinctly less susceptible (see Table 1). The reaction ofL. serriola to lettuce mosaic was not the same for the different sources of this species. The most common symptom (see Table 2) was a veinclearing followed by mosaic and sometimes necrosis at the end of the growth period. ('s-Hertogenbosch, Arnhem, Blerick, Gulpen and Braunschweig). There were also plants from certain sources which first reacted with symptoms of necrosis (see Table 2). This was a total necrosis (Velp) and there were sources from which the plants grew on with mosaic symptoms and necrotic symptoms (Maurik and Voorst, Fig. 1). The plants from Stockholm and Lent were intermediate between the latter type and the former.L. serriola from Coimbra, received under the name ofL. scariola, reacted with reddish necrotic or glassy translucent spots along the veins (Fig. 2). This symptom disappeared afterwards and changed into faint mosaic symptoms.In September, there were great differences in height of growth between the sources. Therefore, we gave all plants a rating indicating their growth. The rating 0 was given for total necrosis or total stop of growth after infection and the rating 3 for normal growth, whereas ratings 1 and 2 indicated reactions in increasing severity between 0 and 3. When the growth of all 60 plants of one source was normal after infection a total of 60 × 3 = 180 was given.In Fig. 3 the resulting data were mapped. North of the river Waal strains ofL. serriola reacting with necrosis occur. Near Venlo and south of this town strains ofL. serriola were collected which showed the mosaic symptoms accompanied by bad growth, though some strains of plants were found which hardly showed a reduction of growth after infection with the virus.

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Instituut voor Plantenziektenkundig Onderzoek te Wageningen; gedetacheerd bij het Proefstation voor de Groenteteelt in de volle grond te Alkmaar     

SOME EFFECTS OF DROPLET SIZE AND POSITION ON THE RESPONSE OF LETTUCE TO SUB-LETHAL DOSES OF MCPA

Summary. Senescent, expanded or expanding leaves of lettuce plants were treated individually with MCPA. The subsequent development of auxin-induced symptoms throughout the plants was recorded. The number of leaves affected and the severity of the symptoms increased with application to the less mature leaves. In addition more leaves were affected when the application was made to larger plants than to smaller ones. These results were associated with absorption/translocation differences in the leaves of different ages, and in their spatial relationship to tissues at particularly susceptible stages of development. In further experiments, applications of similar weights of MCPA to lettuce plants in droplet sizes of approximately 100 μ and 500 μ were compared. There were consistent trends for the small droplets to produce greater effects.
Quetques effets de la taille des gouttelettes et de leur position sur la reaction de la laitue à des doses sub-létales de MCPA     

Effects of inoculum density,leaf wetness duration and nitrate concentration on the occurrence of lettuce leaf spot

In Ehime Prefecture, Japan, lettuce leaf spot (Septoria lactucae) caused huge losses in marketable lettuce yields. To explore potential measures to control disease outbreaks, the effects of inoculum density, leaf wetness duration and nitrate concentration on the development of leaf spot on lettuce (Lactuca sativa) were evaluated. Conidia were collected from diseased plants in an infested field by single-spore isolation and were used to inoculate potted lettuce plants with different conidial concentrations. Lesions developed on inoculated lettuce plants at inoculum concentrations from 10⁰ to 10⁶ conidia/ml. The disease was more severe when the inoculum exceeded 10² conidia/ml, and severity increased with increasing concentrations. Assessment of the relationship between disease development and the duration of postinoculation leaf wetness revealed that symptoms appeared when the inoculated plants remained wet for 12 h or longer. The number of lesions and total nitrogen content in the lettuce leaves both increased when nitrate was applied.     

Molecular and biological characterization of two potyviruses infecting lettuce in southeastern France

Several potyviruses affect lettuce (Lactuca sativa) and chicory (Cichorium spp.) crops worldwide and are important constraints for production because of the direct losses that they induce and/or because of their seed transmission. Here, the molecular and biological properties are described of two potyviruses that were recently isolated from lettuce plants showing mosaic or strong necrotic symptoms in an experimental field in southeastern France. The first potyvirus belongs to the species Endive necrotic mosaic virus and is present in a large number of wild plant species, especially Tragopogon pratensis. It is unable to infect lettuce cultivars with a resistance to Turnip mosaic virus that is present in many European cultivars and probably conferred by the Tu gene. The second potyvirus belongs to the tentative species lettuce Italian necrotic virus and was not observed in wild plants. It infected all tested lettuce cultivars. Wild accessions of Lactuca serriola, Lactuca saligna, Lactuca virosa and Lactuca perennis were identified as resistant to one or the other potyvirus and could be used for resistance breeding in lettuce. No resistance against these two potyviruses was observed in the tested Cichorium endivia cultivars. In contrast, all tested Cichorium intybus cultivars or accessions were resistant.     

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.     

Lettuce ring necrosis,caused by a chytrid-borne agent distinct from lettuce big-vein ‘virus’

Ring necrosis is a serious disease of lettuce (Lactuca sativa) with often coalescing necrotic rings and ring-like patterns on middle leaves of plants or groups of plants in glasshouses during winter. Affected leaves may decay and plants rapidly become unmarketable. The disease was shown to be soil-borne and transmitted by the zoospores ofOlpidium brassicae. Symptoms in lettuce do not appear before seven weeks after inoculation via the soil. Additives to the inoculum and chilling of source leaves, inoculum buffer and utensils enabled mechanical transmission of a pathogenic agent toChenopodium quinoa, C. amaranticolor, Nicotiana benthamiana, N. clevelandii, N. hesperis, andN. occidentalis but not to lettuce. TheChenopodium spp. reacted with local lesions, infection was symptomless inN. clevelandii and mostly so inN. benthamiana, butN. hesperis andN. occidentalis reacted with leaf spotting and plant stunting. With zoospores of an originally pathogen-free fungus culture further cultivated on the roots of cuttings from sap-inoculated plants ofN. clevelandii andN. occidentalis, the agent could be transferred back to lettuce and the symptoms of ring necrosis be reproduced. The agent biologically resembles those of lettuce big-vein (LBV) and freesia leaf necrosis and the tobacco stunt virus. In lettuce it often occurs together with LBV virus but differs in longer incubation period, type of symptoms and symptom appearance only during winter. It could be separated from a mixture with LBV virus by serial transfer always selecting plants without LBV symptoms. So far cultural hygiene, including soil disinfection addressing the vector, is the main means of control.     

Identification of Rhizoctonia solani AG 1-IB in Lettuce, AG 4 HG-I in Tomato and Melon, and AG 4 HG-III in Broccoli and Spinach, in Brazil

Fungi isolated in Brazil, from lettuce, broccoli, spinach, melon and tomato, were identified as Rhizoctonia solani. All lettuce isolates anastomosed with both AG 1-IA and IB subgroups and all isolates from broccoli, spinach, melon and tomato anastomosed with AG 4 subgroup HG-I, as well as with subgroups HG-II and HG-III. DNA sequence analyses of ribosomal internal transcribed spacers showed that isolates from lettuce were AG 1-IB, isolates from tomato and melon were AG 4 HG-I, and isolates from broccoli and spinach were AG 4 HG-III. The tomato isolates caused stem rot symptoms, the spinach, broccoli and melon isolates caused hypocotyl and root rot symptoms on the respective host plants and the lettuce isolates caused bottom rot. This is the first report on the occurrence in Brazil of R. solani AG 4 HG-I in tomato and melon, of AG 4 HG-III in broccoli and spinach and of AG 1-IB in lettuce.     

Pythium wilt of lettuce caused by <Emphasis Type="Italic">Pythium uncinulatum</Emphasis> in Japan

Pythium wilt of lettuce was found in two fields in Hyogo Prefecture, Japan in January 2007. On the basis of morphological characteristics and the rDNA-ITS sequences, the causal organism was identified as Pythium uncinulatum, unknown in Japan. A pot inoculation test demonstrated that P. uncinulatum was pathogenic to lettuce and garland chrysanthemum (both in Compositae), but no symptoms developed on nine species of vegetable plants from Amaranthaceae, Apiaceae, Brassicaceae, Cucurbitaceae, Liliaceae, or Solanaceae.     

Effects of rust infection of Senecio vulgaris on competition with lettuce

The effects of rust (Puccinia lagenophorae) on competition between groundsel (Senecio vulgaris L.) and lettuce were investigated. In small tubs in a glasshouse environment, rust infection reduced the dry weight yield of groundsel both in monocultures and mixtures with lettuce at total densities from 35 to c. 1100 plants m^-2. While rust did not affect the relative mixture response of groundsel it resulted in a significant decrease in the relative mixture response of lettuce. Indeed, the yield-density relationship of lettuce in mixture with rusted groundsel was not significantly different from that of lettuce in monoculture. At 1120 groundsel plants m^-2, lettuce yield in mixture with uninoculated groundsel was inhibited by c. 50% when the ratio of lettuce: groundsel (L:G) was 1:3. By contrast, in mixture with rusted groundsel, lettuce yield per tub was not changed by varying L:G, eight lettuce with 24 groundsel yielding no less than 32 in monoculture. Although the severity of rust did not appear to differ, infection inhibited groundsel yield and modified competitive interactions only when L:G was low. In a field experiment the fresh weight yield of lettuce was significantly inhibited by interference from uninoculated groundsel at all sowing densities from 250 to 65 000 seed m^-2. Conversely, lettuce yield was not significantly reduced by rust-infected groundsel until weed density exceeded c. 4000 m^-2. Even at greater sowing densities, lettuce yield in plots with rusted groundsel was two to three limes greater than in plots with uninoculated groundsel. Inoculation with rust reduced the impact of groundsel upon lettuce yield without causing any significant increase in groundsel mortality. The effects of rust were consistent in both glasshouse and field and were attributed to the decreased competitive ability of infected groundsel plants, especially at high density. The implications of the results for the use of rusts in biocontrol are discussed. Effets d'une infection par la rouille de Senecio vulgaris L. sur la competition en culture de laitue Les effets de la rouille (Puccinia lagenophorae) sur la compétition entre le séneçon vulgaire (Senecio vulgaris L.) et la lailue ont étéétudiés. Dans des petits containers sous serre, une infestation de rouille a réduit le poids sec du seneçon tant seul qu'en mélange avec des laitues, à des densités de 35 à 1100 plantes m^-2). Bien que la rouille n'ait pas affecté le comportement relatif du seneçon en mélange, on a abouti à une réduction significative de la réponse de la laitue en mélange. En effet, la relation poids-densité de la laitue en mélange avec des seneçons ‘rouilles’, n'était pas significativement différente de celle de la laitue seule. A 1120 pieds de seneçons m^-2 le poids de lailue en mélange avec des pieds sains était réduit de 50% pour un ratio laitue/seneçon de 1/3. Par opposition, en mélange avec le seneçon ‘rouillé’, le poids de laitue par container n'a pas changé même en faisant varier le rapport L/S. 8 laitues avec 24 seneçons ne pèsent pas moins que 32 laitues seules. Bien que la sévérité de l'attaque de rouille ne semble pas entraîner de différence, l'infection a contrarié le poids des seneçons et modifié la compétition inter espèces seulement quand le rapport L/S était bas. Au champ, le poids frais de laitue a été significativement réduit par l'interférence de seneçons sains à toutes les densités de semis (de 250 à 65000 graines m^-2). Au contraire, le poids de laitue n'a pas été signifieativement réduit par des seneçons attaqués par lu rouille jusqu’à des densités supérieures à 4000 m^-2. Même à des densités de semis plus importantes, le poids des laitues dans les parcelles avec des seneçons attaqués était 2 à 3 fois plus grand que dans les parcelles sans inoculation de seneçon. L'inoculation avec la rouille a réduit l'impact du seneçon sur le poids de la laitue sans engendrer d'augmentation sensible de la mortalité du seneçon. Les effets de la rouille ont été conséquents à la fois en serre et en plein champs et on leur a attribué la baisse de compétitivité des plantes de seneçon infestées, spécialement aux fortes densités. Les implications de ces résultats sur l'utilisation possible des rouilles en lutte biologique fait l'objet d'une discussion. Die Beeinflussung der Konkurrenzkraft von Senecio vulgaris auf Salat durch Rostinfektionen In der vorliegendcn Arbeit wurde der Einfluss von Rost (Puccinia lagenophorae) auf die Konkurrenzkraft des Gemeinen Kreuzkrauts (Senecio vulgaris L.) gegenüber Salat (Lactuca sativa cv. ‘Avon Defiance’) untersucht. Unter Gcwächshausbedingungen reduzierten Rostinfektionen das Trockengewicht von S. vulgaris, sowohl in Monokultur als auch in Mischung mit L. sativa, bei einer totalen Pflanzendichte von 35 bis 1100 Pflanzen m^-2. Während die Rostinfektionen die relative Wachstumsreaktion von S. vulgaris, wenn in Mischung mit L. sativa wachsend, nicht beinflusste, hatte die Infektion eine signifikante Reduktion der Konkurrenzwirkung auf Salat zur Folge. Tatsächlich war die Beziehung zwischen Ernie und Pflanzendichte von in Mischkultur mit rostinfiziertem Kreuzkraut wachsendem Salat nicht signifikant verschieden von dem in Monokultur gewachsenem Salat. In Gegenwart von 1120 nicht infizierten Kreuzkrautpfianzen m^-2 war die Salaternte um 50% verringert, wenn das Zahlenverhältnis Salat/Kreuzkraut (S:K) 1:3 war. Demgegenüber wurde in Mischung mit infizieriem Kreuzkraut die Salaternte pro Kulturbehälter durch die Veränderung des Verhältnisses S:K nicht beeinflusst; 8 Salatpflanzen, mit 24 Kreuzkrautexemplaren zusammen kultiviert, ergaben nicht weniger Ernteertrag als 32 in Monokultur gewachsene Salatpflanzen. Obwohl die Schwere der Infektionen nicht verschieden zu sein schien, reduzierten sie die Krautgewichte der Kreuzkrautpflanzen und ihre Konkurrenzkraft nur, wenn das Verhältnis S: K klein war. In einem Feldversuch wurden die Erntefrischgewichte des Salats durch die Konkurrenz nicht infizierter Kreuzkrautpfianzcn, bei Saatdichten von 250–65 000 Samen m^-2, signifikant gehemmt, Im Gegensatz dazu vermochte rostinfizierles Kreuzkraut, bis zu einer Unkrautdichte von 4000 Pflanzen m^-2, die Erntegewichte des Salats nicht signifikant herabzusetzen. Sogar bei höheren Unkrautdichten war die Salaternte in Parzellen mit infiziertem Kreuzkraul 2–3 mal höher als in Gegenwart von nicht befallenem Unkraut. Die Infektion von S. vulgaris durch Rost verringerte die Beeintächtigung der Salaternte durch das Unkraut, ohne dass eine signifikante Erhöhung der Mortalität des letzteren feststellbar war. Die durch Rostinfektioen bedingten Effekte waren in Gewächshaus und Freiland übereinstimmend und wurden der verminderten Konkurrenzkraft befallener Kreuzkrautpflanzen zugeschrieben; dies war speziell manifest in Gegenwart hoher Unkrautdichten. Die Ergebnisse dieser Untersuchung werden im Lichte der Möglichkeit diskutiert, Rostinfektionen als biologischcs Bekämpfungsmittel einzusetzen.     

Characterisation of Rhizoctonia solani Anastomosis Groups Causing Bottom Rot in Field-Grown Lettuce in Germany

Bottom rot caused by Rhizoctonia solani is an increasing problem in field-grown lettuce in Germany. During the growing seasons of 1999 and 2000, 95 isolates of R. solani from lettuce plants with bottom rot symptoms were collected from eight locations. The isolates were characterised using hyphal anastomosis, pectic zymograms and morphological characteristics. Ninety-three isolates were identified as anastomosis group (AG) 1-IB, one as AG 1-IC and one as AG 2-1. Optimum hyphal growth was measured over a temperature range of 20–30 °C with an optimum at 25 °C. Aggressiveness of the AG 1-IB isolates varied from weak to strong when tested on detached lettuce leaves. The pathogenic potential of six AG 1-IB isolates was determined on 14 plant species in comparison with lettuce under conditions favourable for the fungus. Radish, broccoli, kohlrabi, spinach and millet seedlings were as severely infected as lettuce seedlings. The same isolates caused little symptoms on maize, tomato and onion. Knowledge about the host range of AGs of R. solani are important for planning an effective crop rotation as part of a control management system.     

A new race of Fusarium oxysporum f. sp. lactucae of lettuce

Fusarium oxysporum f. sp. lactucae, the causal agent of fusarium wilt of lettuce (Lactuca sativa), occurs in most countries in which lettuce is grown and causes serious economic losses. Three races (1, 2 and 3) of the pathogen have previously been identified on the basis of their ability to cause disease on differential lettuce cultivars, as well as by means of molecular tools developed to characterize different races of this pathogen. Only race 1 has been detected in Europe so far. In this study, two isolates of F. oxysporum, obtained from lettuce plants grown in the Netherlands showing symptoms of wilt, have been characterized by combining the study of pathogenicity with differential cultivars of lettuce and molecular assays to determine whether the isolates are different from the known races of F. oxysporum f. sp. lactucae. This study reports the presence of F. oxysporum f. sp. lactucae for the first time in the Netherlands. The causal pathogen has been identified, using the IRAP‐SCAR technique, as a new race of F. oxysporum f. sp. lactucae. Specific primers have been designed to identify this new race.     

First report of Rhizomonas sp. causing corky root of lettuce in Europe

Typical symptoms of corky root were observed on iceberg lettuce (Lactuca sativa L.) in the Netherlands and England, on prickly lettuce (Lactuca serriola L.) in Spain, and on sowthistle (Sonchus oleraceus L.) in Greece. Slow-growing bacteria with similar colonies as strains ofRhizomonas suberifaciens or otherRhizomonas species were isolated from soil surrounding plants with corky root symptoms using lettuce seedlings as bait. Crude lysate from all strains was tested for DNA homology with DNA fromR. suberifaciens strain CA1 (R. sub. CA1) andRhizomonas sp. strain WI4 (R. sp. WI4). Strains that had homology values higher than that ofR. sp. WI4 orR. sub. CA1 were tested for pathogenicity on 1-wk-old lettuce seedlings, cv. Salinas, Two strains from the Netherlands induced typical symptoms of corky root on lettuce. These strains tested negative with monoclonal antibody MAb-Rs1 specific forRhizomonas suberifaciens in an enzyme-linked immunosorbent assay (ELISA). They had low DNA homology withR. sub. CA1 (4–9%) and low to moderate DNA homology withR. sp. WI4 (4–17%). Some nonpathogenic strains had moderate to high levels of DNA homology withR. sub. CA1 orR. sp. WI4 (19–84%). All strains had fatty acid profiles similar to those ofRhizomonas species. This is the first report ofRhizomonas sp. causing corky root of lettuce in Europe.     

Identification and Partial Characterisation of <Emphasis Type="Italic">Lettuce big-vein associated virus</Emphasis> and <Emphasis Type="Italic">Mirafiori lettuce big-vein virus</Emphasis> in Common Weeds Found Amongst Spanish Lettuce Crops and their Role in Lettuce Big-vein Disease Transmission

The potential role of 10 frequently occurring weed species found amongst Spanish lettuce crops as host plants for the two viruses associated with the lettuce big-vein disease, Lettuce big-vein associated virus (LBVaV) and Mirafiori lettuce big-vein virus (MLBVV), was studied. The results showed that both viruses can infect naturally growing Sonchus oleraceus (common sowthistle) plants, the unique susceptible species detected among the analysed weeds. The sequences of the coat protein (CP) genes of the LBVaV and MLBVV isolates recovered from S. oleraceus plants were determined. Phylogenetic studies revealed a very close relationship between the CP sequences from these weed isolates and those from Spanish lettuce. Moreover, we showed that S. oleraceus can act as a source of lettuce infection by means of Olpidium brassicae, the vector fungus of both viruses.     

Life tables of Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae) (Lepidoptera: Noctuidae) on different cultivated plants

Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae) is a cosmopolitan species that feeds on numerous cultivated plants and herbaceaus plants. Agrotis ipsilon causes significant economic losses in various agricultural products, especially in indisturial plants and vegetables in Turkey and worldwide. In this study, the effects on the biology and reproductive potential of lettuce (Lactuca sativa L., Asteraceae), maize (Zea mays L., Poaceae), sugarbeet (Beta vulgaris var. saccharifera Alef.,Amaranthaceae) and potato (Solanum tuberosum L., Solanaceae) which are essential host plants of A. ipsilon were investigated under climatized conditions of 28?±?1 °C, 60%?±?10 r.h., and 16 h daily artificial light. The data obtained were analyzed by using an age-stage two-sex life table. Agrotis ipsilon had five larval stages fed on lettuce, whereas seven larval stages were fed on other host plants. Agrotis ipsilon showed the best development and reproduction on lettuce, followed by sugar beet. When A. ipsilon is reared on lettuce the intrinsic rate of increase (r?=?0.1237 d^?1), finite rate of increase (λ?=?1.1317 d^?1) and net reproductive rate (R₀?=?403.06 offspring/individual) were found higher and the mean generation times (T?=?48.51 d) is shorter than those in other host plants. According to these results, A. ipsilon can cause the most damage to lettuce among studied plants.

     

Pythium uncinulatum sp. nov. and P. tracheiphilum pathogenic to lettuce

From mature lettuce heads with bottom rot symptoms severalPythium species were isolated, two of which were studied further. Pathogenicity tests with both fungi have been carried out only with seedlings. P. uncinulatum sp. nov. was isolated frequently. Its relation to otherPythium spp. with spiny oogonia is discussed. It proved to be very pathogenic to seedlings of lettuce and somewhat less to those of cucumber and tomato. P. tracheiphilum, which was isolated far less frequently, proved to be highly pathogenic to seedlings of lettuce, cucumber and cauliflower, and somewhat less to tomato and pea.Samenvatting Van slaplanten met smetsymptomen werden behalveBotrytis cinerea, Rhizoctonia sp. enSclerotinia spp. dikwijlsPythium-soorten geïsoleerd. Twee hiervan zijn nader onderzocht. P. uncinulatum (Fig. 1, 3a, 3b), die dikwijls werd geïsoleerd, wordt beschreven als nieuwe soort en de relatie met anderePythium-soorten met gestekelde oögoniën wordt besproken. In proeven bleekP. uncinulatum zeer pathogeen te zijn voor zaailingen van sla en iets minder voor zaailingen van komkommer en tomaat (Fig. 4a, Tabel 2). Het planten van slazaailingen in metP. uncinulatum geïnoculeerde grond had meer effect dan het dopen van de wortels in een myceliumsuspensie van de desbetreffende schimmel vóór het planten. Het effect uitte zich in groeivertraging van de plant en beschadiging (necrose) van het gehele wortelstelsel. P. tracheiphilum (Fig. 2, 3c), die slechts een paar maal werd geïsoleerd, was zeer pathogeen voor zaailingen van sla, komkommer en bloemkool (Fig. 4a) en iets minder voor zaailingen van tomaat en erwt (Tabel 2). Groeiremming van de planten was sterker wanneer de wortels van de zaailingen vóór het planten in een myceliumsuspensie vanP. tracheiphilum gedoopt werden (Fig. 4b), dan wanneer de zaailingen in geïnoculeerde grond geplant werden. De hoofdwortels van de zaailingen werden ernstig beschadigd, maar zich later vormende zijwortels werden weinig door de schimmel beïnvloed.Tot nu toe zijn de pathogeniteitsproeven slechts met zaailingen uitgevoerd. Proeven met volwassen planten moeten volgen om te onderzoeken of er een verband kan bestaan tussen de aanwezigheid van de betreffendePythium-soorten in de grond en het optreden van smet in sla.     

Colonization of lettuce cultivars and rotation crops by Fusarium oxysporum f. sp. lactucae,the cause of fusarium wilt of lettuce

The severity of fusarium wilt is affected by inoculum density in soil, which is expected to decline during intervals when a non‐susceptible crop is grown. However, the anticipated benefits of crop rotation may not be realized if the pathogen can colonize and produce inoculum on a resistant cultivar or rotation crop. The present study documented colonization of roots of broccoli, cauliflower and spinach by Fusarium oxysporum f. sp. lactucae, the cause of fusarium wilt of lettuce. The frequency of infection was significantly lower on all three rotation crops than on a susceptible lettuce cultivar, and the pathogen was restricted to the cortex of roots of broccoli. However, F. oxysporum f. sp. lactucae was isolated from the root vascular stele of 7·4% of cauliflower plants and 50% of spinach plants that were sampled, indicating a greater potential for colonization and production of inoculum on these crops. The pathogen was also recovered from the root vascular stele of five fusarium wilt‐resistant lettuce cultivars. Thus, disease‐resistant plants may support growth of the pathogen and thereby contribute to an increase in soil inoculum density. Cultivars that were indistinguishable based on above‐ground symptoms, differed significantly in the extent to which they were colonized by F. oxysporum f. sp. lactucae. Less extensively colonized cultivars may prove to be superior sources of resistance to fusarium wilt for use in breeding programmes.     

Zwartrot in sla,veroorzaakt doorRhizoctonia solani

Summary In the autumn of 1962, a so-called blackrot of lettuce occurred on a relatively large scale. The leaves of older and younger plants which lie on the soil, become black and rotten from the point of contact with the soil. The disease spreads over the whole leaf, leaving the remains as a black print on the soil surface. From diseased leavesRhizoctonia solani Kühn (stat. mycel. ofThanatephorus cucumeris (Frank) Donk) andBotrytis cinerea Pers. ex Fr. were isolated. Since the latter was expected to be a secondary invader, it was not used in the subsequent inoculation experiments. After inoculation with theR. solani isolate, the typical blackrot symptoms appeared within eight days. This isolate was not pathogenic to tomatoplants. Possibly the disease may be controlled with PCNB, an effective fungicide againstRhizoctonia. As far as the authors are aware, this is the first record of blackrot of lettuce, caused byR. solani.     

Development of Myzus persicae on a partially resistant and on a susceptible genotype of lettuce (Lactuca sativa) in relation to plant age

The relationship between lettuce (Lactuca sativa) andMyzus persicae is influenced by internal and external factors. For the improvement of screening methods and the evaluation of the resistances found, a better knowledge of these factors is wanted.In five experiments the influence of plant age on resistance level was investigated for a partially resistant and a susceptible cultivar. Criteria for resistance were: remaining percentage of aphids (RPA), aphid developmental rate, insect biomass, and larvae production.It appeared that the aphids developed faster and grew better on older plants compared with younger plants, resulting in a decrease of overall level of resistance. The absolute differences between the susceptible and the resistant genotype for parameters such as biomass increased if plants were older and aphids were allowed to utilize all parts of the plant. It is concluded that with older plants (plant age e.g. 30–40 days) a better discriminative selection can be carried out.Samenvatting De relatie tussen sla enMyzus persicae wordt door zowel interne als externe factoren beïnvloed. Voor de verbetering van toetsmethoden en voor de evaluatie van gevonden resistenties, is meer kennis omtrent deze factoren noodzakelijk.In vijf proeven (I–V) werd de invloed van leeftijd van de plant op het resistentieniveau onderzocht bij een partiëel resistent en een vatbaar ras. Criteria voor resistentie waren: overblijvend percentage luizen 7 dagen na inoculatie, ontwikkelingssnelheid van de luizen, de insekt-biomassa en de larvenproduktie.Het bleek dat de groei en de ontwikkeling van de luizen op oudere planten beter was dan op jongere. Dit resulteerde in een afname van het resistentieniveau bij zowel het partiëel resistente als het vatbare genotype. De absolute verschillen tussen het vatbare en het resistente genotype voor bepaalde eigenschappen zoals biomassa namen echter toe naarmate planten ouder waren in de experimenten IV en V waarbij de luizen toegestaan was zich op alle delen (bladeren en stengel) van de planten te vestigen. Op basis van deze toegenomen absolute verschillen tussen resistente en vatbare planten is het dus beter om bij oudere planten (bijv. 30–40 dagen oud) op resistentie te selecteren.