Pathogenic variation in poplar rust <Emphasis Type="Italic">Melampsora larici-populina</Emphasis> from England

Using a leaf disc method, 19 isolates of the poplar rust, Melampsora larici-populina , and one isolate of M.populnea from England were inoculated on to 25 poplar clones belonging to Populus nigra and P.trichocarpa, and hybrids between P. deltoides and P. nigra, P. deltoidesand P. trichocarpa, P.tacamahaca and P.trichocarpa, and P. alba and P. tremula. Disease was scored based on the pustule area and inoculum density. In terms of whether sporulating uredinia formed, the 19 isolates showed seven different patterns to the tested poplar clones. The majority of the rust isolates infected P. nigra P3090 and Vereecken, P.nigra×P. deltoides Casale and Tasman, P. tacamahaca×trichocarpa 36 and Balsam Spire, and P.trichocarpa Blom. Populus trichocarpa×P. deltoides 69039/4 was infected by only three isolates collected from southern England. No visible symptoms appeared on P. alba ×P. tremulaTower and P.trichcarpa×P. deltoides×P. deltoides76028/5 in inoculations with M. larici-populina isolates. Populus alba×P.tremula Tower was infected only by M. populnea. When M. larici-populina isolates were tested using AFLP, no differences were found either between isolates from different geographical regions or between those having narrow spectrum of virulence and those showing wide spectrum of virulence on the tested clones. The results suggest that the UK rust populations possess virulences which were found in races E1, E2, E3 and E4 in continental Europe and that rust having virulence patterns similar to race E4 has occurred in UK poplar plantations since 1996.     

Identification of <Emphasis Type="Italic">Globodera rostochiensis</Emphasis> and <Emphasis Type="Italic">G. pallida</Emphasis> in the Ukraine by PCR

Multiplex polymerase chain reaction was used to identify the potato cyst nematodes in soil samples from the Ukraine. The results show the occurrence of Globodera pallida in the Uzhhorod region (Zakarpatska oblast), where only G. rostochiensis had been previously reported. In the mixed potato cyst nematode (PCN) populations, G. pallida was less prevalent (2–5%) than G. rostochiensis (95–98%). A phylogenetic analysis based on ribosomal DNA internal transcribed spacer sequences showed that the Ukrainian population of G. pallida had >99% sequence identity with other G. pallida pa2/3 isolates from Europe. This study has demonstrated that polymerase chain reaction-mediated amplification of specific regions of the potato cyst nematode genome is not only highly effective as a species diagnostic tool but is also a sensitive method which can be used for taxonomic purposes with cyst collections which vary in age.     

Relative susceptibilities of eleven potato cultivars and breeders' clones toGlobodera pallida pathotype Pa 3, with a discussion of the interpretation of data from pot experiments

Curves according to the equationPf=M (1-e ^–aPi/M ) fitted well to the relations according to Seinhorst's (1993) modelPf=y _e y _h M(1-e ^–aPi/M ) between initial egg densities of potato cyst nematodes (Globodera rostochiensis, G. pallida)Pi up to 5T _h (T _h =the tolerance limit of haulm weight) andPf at the end of the growing season (a=maximum rate of reproduction,M, M=different theoretical maximum egg densities). Variation of estimates ofM, due to variation of the parameters of the submodelsy _e andy _h for the effect of weight reduction of haulms (and, therefore, of roots) on cyst production and damage to root tissue on egg production, respectively, was small enough to be ignored relative to experimental error. Therefore, ratios of values ofM, determined in simultaneous pot experiments with different potato cultivars, are reliable measures of the relative host status of these cultivars at initial egg densitiesPi of these nematodes up to about 5T _h . Variation between potato cultivars of growth reduction and damage to root tissue by the nematodes reduces the reliability of ratios of rates of relative susceptibility of these cultivars.The ratios between the maximum rates of reproduction ofG. pallida, pathotype Pa 3, on 8 out of 9 cultivars and one breeder's clone of potatoes and this rate on the susceptible cvs Bintje and Irene (relative susceptibilitiesrs _a ) could be considered to be equal to the ratios of maximum population densitiesM on these cultivars (relative susceptibilitiesrs _M ) (relative susceptibilities independent of initial egg density). The latter ratios were 0.59 times the first (relative susceptibilities negatively correlated with initial egg density) in one cultivar and one breeder's clone. Relative susceptibilitiesrs _a andrs _M of the tested cultivars and breeders clones suggest the existence of continuous ranges of both relative susceptibilities between 0.50 and 0.15 with, in a great majority of cases,rs _a =rs _M .     

Some new races ofPuccinia striiformis

In the Netherlands in 1961 three new field races were found ofPuccinia striiformis, the causal fungus of yellow rust on wheat. Two of these, viz. the Falco race and the Opal race, together with the already known Etoile de Choisy race, constitute the so-called Nord-group of races, which is now clearly distinguished from the so-called Rubis-group. Most dangerous and interesting is the third of the new races. This so-called Cleo race proved to be the first to break both resistance to the Nord-group and resistance to the Rubis-group. On wheat seedlings in the greenhouse all three new races behaved like green-house race W(ageningen) 8=B(raunschweig) 55.Samenvatting Er zijn van de schimmelPuccinia striiformis Westend., de veroorzaker van de gele roest van tarwe, in Nederland in 1961 drie nieuwe veldfysio's gevonden. Dit zijn het Falco-fysio, het Opal-fysio en het Cleo-fysio. De eerste twee bleken met het reeds bekende Etoile de Choisy-fysio een groep te vormen, de z.g. Nord-groep, die zich duidelijk onderscheidt van de reeds beter bekende Rubis-groep van fysio's. Eerstgenoemde groep kenmerkt zich o.a. door de aantasting van het tarweras Nord, de laatstgenoemde door de aantasting van Rubis (zie tabel 1). Het belangrijkst en het gevaarlijkst is het Cleo-fysio. Dit bleek voor zover bekend het eerste en enige fysio te zijn dat zowel de resistentie tegen de Rubis-groep als die tegen de Nord-groep doorbreekt. Het heeft zodoende een zeer breed aantastingsspectrum. Van de tarwerassen die voorkomen in de 39ste Nederlandse Rassenlijst voor Landbouwgewassen (1964) zijn de volgende negen rassen vatbaar voor het Cleo-fysio met een aantastingsgraad van 6 tot 9 (Internationale Schaal): Cleo, Cappelle Desprez, Heine's VII, Hector, Stella, Sambo, Mado, Wodan en Jufy I. Bovendien bleek het Cleo-fysio in vergelijking met de andere veldfysio's goed te overwinteren en in 1964 reeds op vele plaatsen in Nederland aanwezig te zijn. Het Cleo-fysio maakt het des te noodzakelijker bij de selectie op resistentie tegen gele roest te werken op een brede genetische basis. Bij toetsing op tarwekiemplanten in de kas gedroegen de drie nieuwe fysio's zich alle als het kasfysio W(ageningen) 8 =B(raunschweig) 55 (zie tabel 2).     

Resistance of rose rootstocks to crown gall (Agrobacterium tumefaciens)

Various types of rose rootstocks were tested for their resistance to crown gall. The rootstock Iowa State University (ISU) 60–5 was the most resistant, followed by Brooks 48, Clarke 1957 and Welch. Rosa multiflora, R. noisettiana (Manetti) and Basye No. 3 were very susceptible. The inoculations were made with four isolates ofAgrobacterium tumefaciens (Smith et Townsend) Conn, respectively from aDahlia sp.,Rosa spp. andPrunus persica. It was found that the isolate fromDahlia was a different race to the isolates fromRosa andPrunus spp.Samenvatting Bij onderstammen van rozen, kunstmatig geïnoculeerd metAgrobacterium tumefaciens (Smith et Townsend) Conn., werden verschillen in resistentie tegen wortelknobbel gevonden. Het meest resistent was Iowa State University (ISU) 60–5, gevolgd door Brooks 48, Clarke 1957 en Welch, Zeer vatbaar warenRosa multiflora, R. noisettiana (Manetti) en Basye No. 3. De vier isolaten vanA. tumefaciens, gebruikt voor de inoculaties, waren respectievelijk afkomstig van eenDahlia sp.,Rosa spp. enPrunus persica. Het isolaat vanDahlia en de isolaten vanRosa enPrunus spp. behoorden tot twee verschillende fysiologische rassen. De vorming van tumoren was in sommige gevallen afhankelijk van de methode van inoculatie; inoculaties bij de stambasis waren meer succesvol dan in het midden van de stam.     

Bud necrosis,a storage disease of tulips. I. Symptoms and the influence of storage conditions

The symptoms of bud necrosis in tulip bulbs are described. This disorder is neither contagious nor hereditary. It is found more often and more severely in certain cultivars (viz Red Champion and White Sail) than in others (viz Rose Copland) in which it appears less frequently or not at all. The disease originates in the stamens of the flower-producing main bud during dry storage between lifting and planting of the bulbs. The occurrence of the disease in sensitive cultivars is promoted by storage of the bulbs under conditions of poor ventilation (viz packed in cardboard boxes) and at higher temperatures later in the storage period (after September 1).The experimental results suggest that there are several pathogenic factors and that the combined action of these factors can lead to bud necrosis; the primary factor is probably of physiological nature.Samenvatting Kernrot is een afwijking in tulpebollen die een bloeibare maat hebben. De ziekte ontstaat tijdens de droge bewaring van de bollen en begint met afwijkingen aan de meeldraden. Bij de beschrijving is onderscheid gemaakt tussen symptomen welke zich tijdens de droge bewaring voordoen aan de knoppen waaruit zich de bloeiende spruit zal ontwikkelen en symptomen welke zich voordoen aan de bollen tijdens de bloei te velde. Naar de mate waarin de knoppen en planten afwijken, wordt gesproken van meeldraadnecrose (Fig. 1A en 2), bloemnecrose (Fig. 1B en 4A), spruitnecrose (Fig. 1C en 4B), kernnecrose (Fig. 1D en 4C) en bolnecrose (Fig. 4D).Het is gebleken, dat de ziekte bij bepaalde cultivars (Red Champion en White Sail) vaak voorkomt, bij andere minder vaak (Rose Copland) of in het geheel niet. Er kon worden vastgesteld, dat de ziekte niet besmettelijk of erfelijk is.In de proeven werd het ontstaan van de afwijking bij Red Champion en White Sail bevorderd door de bollen te verpakken in kartonnen dozen (ventilatiebeperking) en door de bollen later in het seizoen (na 1 september) te bewaren bij hogere temperaturen dan daarvoor. Op grond van de proefresultaten wordt aangenomen dat de primaire oorzaak van de afwijking van fysiologische aard is. De aanwezigheid van schimmels, bacteriën en mijten moet als tweede oorzaak worden beschouwd.     

Effects of fusaric acid on cells from tomato cultivars resistant or susceptible toFusarium oxysporum f. sp.Lycopersici

Cell suspension cultures were set up from two tomato cultivars, one resistant, (Rio grande) and one susceptible (63.5) toFusarium oxysporum f. sp.lycopersici. Growth rates of the two cell cultures were comparable. Toxicity of fusaric acid, expressed as the fresh weight loss, was analyzed: It was significant in both cases after 10 h, but toxicity was twice as high for 63.5 suspension cells. In the same way, electrolyte leakage caused by fusaric acid was three times more important for 63.5 suspension cells. Moreover, fusaric acid treatment resulted in an acidification of the extracellular medium for 63.5 suspension cells (0.4 pH unit), whereas an alkalization was observed for Rio grande suspension cells (0.2 pH unit). Preliminary experiments suggest that fusaric acid was partially metabolized by Rio grande suspension cells, however, no detoxified forms of fusaric acid were detected either in cells or in culture filtrates. For these two tomato cultivars, the differences in sensitivity to fusaric acid of cultivated cells correspond to the differences in plant susceptibility toFusarium oxysporum f. sp.lycopersici.Abbreviations BAP 6-benzylaminopurine - conductivity - 2,4-D 2,4-dichlorophenoxyacetic acid - EtOAc ethyl acetate - FA fusaric acid - resistivity     

Alternaria alternata in cotton (Gossypium hirsutum) cv. Acala: Effects on gas exchange,yield components and yield accumulation

The effect ofAlternaria alternata on gas exchange proceses, on total yield and on yield components in cottonGossypium hirsutum cv. Acala SJ-2, was examined under field conditions. Variations in disease severity levels were achieved by using three fungicide treatments (Fentin Acetate). Disease developed more rapidly on the upper canopy layer (>661 cm height) than on lower ones. Infections were not accompanied by leaf shedding. The main effect of the disease was a reduction on bolls' number. A significant increase in seedcotton and lint yield, as compared to the untreated control, was achieved by applying fungicides. It was found that omitting early and late applications was not followed by significant yield reduction. Alternaria leaf spot reduced photosynthetic rate more and transpiration rate less, than could be explained by the extent of infected leaf area alone.Samenvatting Het effect vanAlternaria alternata op fotosynthese en transpiratie, opbrengstcomponenten en totale opbrengst bij katoen (Gossypium hirsutum L.) werd onderzocht onder veldomstandigheden. Verschillen in aantastingsniveaus werden verkregen door fungicide (fentin acetaat) behandelingen. De ziekte ontwikkelde zich het snelst boven in het gewas (> 60 cm). Infectie ging niet gepaard met bladval. Het belangrijkste effect van de ziekte vormde een afname van het aantal vruchtdozen (bolls). Een significante toename van zaadkatoen (seedcottton) en van vezels (lint) ten opzichte van de onbehandelde controle werd verkregen door toediening van fungiciden. Achterwege laten van vroege en late bestrijdingen had geen significante opbrenstverlaging tot gevolg.A. alternata verlaagde de fotosynthesesnelheid meer en de transpiratiesnelheid minder dan verklaard kon worden uit de geïnfecteerde oppervlakte van het blad.     

Influence of Inoculum Density of Races 0 and 5 of Fusarium oxysporum f. sp. ciceris on Development of Fusarium Wilt in Chickpea Cultivars

Artificial inoculation experiments were carried out at 25°C to determine the effects of inoculum density of Fusarium oxysporum f.sp. ciceris races 0 (Foc-0) and 5 (Foc-5) and susceptibility of chickpea cultivars P-2245 and PV-61 on development of Fusarium wilt. Foc-5 proved much more virulent than Foc-0. Increasing the inoculum density of F. oxysporum f.sp. ciceris caused an exponential reduction in disease incubation period and a monomolecular increase of disease incidence and the area under the disease intensity progress curve. The extent of these effects was highest in the most conducive P-2245/Foc-5 combination and decreased in the less susceptible PV-61 and for the less virulent Foc-0, in that order. For P-2245/Foc-5, the highest disease intensity was attained with 6 chlamydospores g^–1 of soil, the lowest inoculum density in the study. One thousand chlamydospores g^–1 of soil of the same race were needed to attain a comparable disease intensity in PV-61. Twenty thousand chlamydospores g^–1 of soil of Foc-0 were required for maximum disease intensity in P-2245.The disease intensity curves were adequately described by the Gompertz model. Using this model, a response surface for disease intensity was developed, in which the model parameters are expressed as a function of both time from inoculation and inoculum density. This response surface confirmed that the final amount of disease intensity increases in a monomolecular relationship with increasing inoculum density and showed that the relative rate of disease progress increases exponentially with increasing inoculum density of the pathogen.     

Effects of Potato-cyst Nematodes (Globodera pallida) and Soil pH on Root Growth, Nutrient Uptake and Crop Growth of Potato

Potato-cyst nematodes (Globodera pallida) cause severe yield losses in potato. Plants infected with potato-cyst nematodes generally have reduced concentrations of nitrogen, phosphorus and potassium in the foliage. This study investigated whether reduced growth of nematode-infected potato is caused by nutrient limitation.Experiments in the field and in containers showed that phosphorus concentration correlated best with total crop biomass at early stages of growth. The role of phosphorus in nematode damage was further investigated in the field and in the Wageningen Rhizolab. The experimental field was infested with potato-cyst nematodes and two levels of nematode density were established by fumigation with a nematicide. Prior applications of calcium carbonate resulted in pHKCl levels of 4.8 and 6.1. Two levels of phosphorus fertiliser were applied: either 0 or 225kgPha–1. In the Wageningen Rhizolab, soil of both pH levels from the field was used after treatment with 1MRad gamma irradiation to kill the nematodes. Subsequently, half of the soil was inoculated with cysts to give a nematode density of 30 viable juveniles per gram of soil.In the field, nine weeks after planting, the total crop biomass ranged from 107gm–2 for the treatment with nematodes at pHKCl 6.1 without phosphorus fertiliser to 289gm–2 for the fumigated treatment at pHKCl 4.8 with phosphorus fertiliser. The differences in total biomass for the various treatments were explained by differences in foliar phosphorus concentration. Nematodes induced or aggravated P deficiency and reduced total biomass. This was not the only damage mechanism as at high, non-limiting levels of foliar phosphorus concentration, nematodes still reduced total biomass.In the Wageningen Rhizolab, directly after planting, the number of roots visible against minirhizotrons was reduced by nematodes. However, the increase of root number in the nematode treatment continued longer than in the control, until root number was higher than that of the control. The compensary root growth of the nematode treatment was restricted to the top 30cm and nematodes reduced rooting depth.High soil pH reduced growth, mainly by reducing the availability of phosphate. Both nematodes and high soil pH reduced nutrient uptake per unit root length. Our results lead us to suggest an interaction between nematodes and soil pH, with nematode damage being higher at pHKCl 6.1 than at pHKCl 4.8.     

Yield of glasshouse tomatoes as affected by strains of tobacco mosaic virus

In the latter part of the summers of 1963, 1964 and 1965 the effect of an early inoculation with different strains of tobacco mosaic virus (TMV) upon yield was investigated. All experiments were carried out in the same glasshouse-compartment and with the tomato variety Moneymaker, the plants being grown in plastic pails containing ordinary potting soil. In 1963 a green strain, a distorting strain and a yellow (or aucuba) strain were used, all capable of causing local lesions onNicotiana tabacum White Burley, which reaction is characteristic for the tomato-type of TMV. Since unfavorable conditions affected control plants more than infected plants, the green strain caused no significant loss of yield. Plants infected by this strain yielded only 3% less weight of fruit than the control plants and even had a 12% greater number of fruits. Plants infected by either the distorting or the yellow strain yielded 43 and 65% respectively less weight of fruit and 32 and 48% respectively fewer fruits. In 1964 the same green strain of the tomato-type TMV was compared with a green strain of the tobacco-type TMV, which is characterized by a systemic reaction on White Burley. Plants infected by the tomato-type and tobacco-type green strains ylelded 16 and 21% respectively less weight of fruit than the control plants and 11 and 18% respectively fewer fruits. In 1965 the same green strain of the tomato-type TMV was used and compared with another green strain of the same virus, which in addition caused local lesions onN. glauca, and with a yellow ringspot strain, which showed the tomato-type reaction on White-Burley. Plants infected with these three strains yielded 14, 18 and 17% respectively less weight of fruit than the control plants, but only 2, 6 and 6% respectively fewer fruits. The relatively small reduction in the number of fruits and an appreciable delay of the harvest probably resulted from the fact that the plants in this experiment were inoculated in the seedling stage.Samenvatting In 1963,1964 en 1965 werd in de nazomer het effect van een vroege inoculatie met verschillende stammen van het tabaksmozalekvirus (TMV) op de opbrengst van tomaat nagegaan. Alle proeven werden uitgevoerd in dezelfde kas en met het tomateras Moneymaker; de planten werden opgekweekt in plastic emmers die met gewone potgrond gevuld waren.In 1963 werden een groene stam, een misvormende stam en een gele (of aucuba-)stam gebruikt, welke alle in staat zijn de voor het tomaattype TMV karakteristieke lokale lesies te veroorzaken opNicotiana tabacum White Burley. Aangezien ongunstige omstandigheden de controleplanten nadeliger beinvloedden dan de ge'infecteerde planten, veroorzaakte de groene stam geen duidelijk opbrengstverlies. Planten, die met deze stam geinfecteerd waren, brachten slechts 3% minder aan vruchtgewicht op dan de controleplanten en zelfs 12% meer vruchten. Planten, geïnfecteerd met de misvormende of met de gele stam, brachten respectievelijk 43 en 65% minder aan vruchtgewicht op en respectievelijk 32 en 48% minder vruchten.In 1964 werd dezelfde groene stam van het tomaattype vergeleken met een groene stam van het tabakstype TMV, welke gekarakteriseerd wordt door een systemische reactie op White Burley. Planten, geïnfecteerd met de groene stam van het tomaattype of tabakstype TMV, brachten respectievelijk 16 en 21% minder aan vruchtgewicht op dan de controleplanten en respectievelijk 11 en 18% minder vruchten.In 1965 werd opnieuw dezelfde groene stam van het tomaattype TMV gebruikt en vergeleken met een andere groene stam van hetzelfde virus, welke bovendien lokale necrotische lesies veroorzaakt opN. glauca, en met een gelekringvlekkenstam, welke eveneens de tomaattype-reactie vertoont op White Burley.Planten, welke met een van deze stammen waren geïnfecteerd, brachten respectievelijk 14, 18 en 17% minder aan vruchtgewicht op dan de controleplanten, maar slechts 2, 6 en 6% minder vruchten. De betrekkelijk geringe reductie in het aantal vruchten en een aanzienlijke oogstverlating waren waarschijnlijk het gevolg van het feit dat de planten in deze proef in het kiemplantstadium waren geïnoculeerd.Stationed at the Glasshouse Crops Research and Experiment Station, Naaldwijk.     

Characterisation of the saponin hydrolysing enzyme avenacoside-α-l-rhamnosidase from the fungal pathogen of cereals, Stagonospora avenae

The fungal pathogen Stagonospora avenae f. sp. avenaria infects oat leaves, which contain the saponins avenacoside A and B. The avenacosides are glycosylated steroidal saponins that occur within oat leaves in a non-fungitoxic form and are converted upon damage or pathogen invasion to their antifungal form by a plant enzyme. It has previously been shown that oat-attacking isolates of S. avenae are able to hydrolyse the sugar chain at the C3 position of the avenacosides. This carbohydrate moiety is a branched chain that consists of one -{scL}-rhamnose and two or three -{scD}-glucose residues in avenacosides A and B, respectively. Removal of the -{scL}-rhamnose residue is sufficient to detoxify the avenacosides. This work describes the purification of the avenacoside-degrading -{scL}-rhamnosidase–and determination of peptide sequence from the protein which represents the first -{scL}-rhamnosidase thus characterised in a fungal plant pathogen.     

Diapause in the nematodeGlobodera pallida

Over a period of 12 months new and old cysts ofGlobodera pallida were hatched in potato root diffusate according to a novel nematode-response hatching protocol. In this protocol, cysts were set to hatch at the beginning of autumn and then left to indicate when their hatching ability was exhausted (when less than 100 juveniles/replicate/week emerged) before another batch of cysts was set to hatch. At any time of the year for the 12 months this experiment was conducted there were cysts hatching. After 12 months of hatching, eight hatching curves were obtained. Based on the hatching curves of the new and old cysts, diapause was shown to be present in new cysts in autumn, winter and early spring. However, diapause was absent in late spring and summer.Infectivity assays to distinguish between juveniles obtained in the periods when cysts were in diapause and when cysts had overcome their diapause failed to show any significant difference in their infectivity. There was no significant difference in the number of eggs in new and old cysts. Based on this observation, it was suggested that high emergence in old cysts may not be a result of few eggs in the cyst but rather due to absence of diapause. Also the presence of large numbers of eggs in old cysts even after being stored for 12 months outdoors in the soil does not support the theories of spontaneous hatching, micro-organism induced hatching or persistence of hatching factors in the soil.     

Infection of Tomicus piniperda (Col., Scolytidae) with Canningia tomici sp. n. (Microsporidia, Unikaryonidae)

Canningia tomici sp. n. (Microsporidia, Unikaryonidae) infects the midgut epithelium, the gut muscules, Malpighian tubules, connective tissues, adipose tissues and the gonads of the pine shoot beetle, Tomicus piniperda (L.) (Coleoptera, Scolytidae). The infection is present in populations of Tomicus piniperda in Europe and in the United States. Uninucleate oval single spores occur in two sizes: 2.8±0.4× 1.4±0.4m and 3.8±0.3×2.0±0.2m. The polar filament of this microsporidium is fixed subapically in a flat anchoring disc. The thick posterior lamellae of the binary polaroplast are asymmetric due to the lateral fixation of the polar filament.     

Over de termen primair en secundair ziek in de fytopathologie

Summary It is proposed to use the terms primair and secundair ziek which were introduced byQuanjer (1916) only to indicate the primary and the secundary stage, respectively, of a continuous systemic infection, which occurs with seedborne diseases (e.g. virus diseases of potato, common bean mosaic,Peronospora destructor of onions). The primary stage (e.g. primary leaf-roll) develops after an infection during the season, the secondary from the infected seed and other types of propagating structures.For discontinuous disease cycles (e.g.Ascochyta pisi) the stage developing from the infected seed can better be described as: originating from the (infected) seed or in another way. The use of secundair ziek in this and other cases of interrupted disease cycles has to be avoided.     

Race-specific aspects of polygenic resistance of barley to leaf rust,Puccinia hordei

Partial resistance of barley to leaf rust,Puccinia hordei, is characterized by a reduced rate of epidemic development in spite of a susceptible infection type. Barley cultivars vary greatly for partial resistance and its components. In a test for interaction between host cultivars and pathogen isolates most variation was of a horizontal nature. However, in the combination between Julia and the rust isolate 18, a differential interaction (vertical effect) occured; Julia had lost a small part of its partial resistance. The same interaction was found for latent period (LP), the most important component of partial resistance. Julia showed a shortened LP for isolate 18. Genetic analyses revealed, that Julia carries a polygene for longer LP not present in the other cultivars. The effect of this polygene appears to be broken by isolate 18 indicative for a gene-for-isolate relation, and even for a gene-forgene relation.Samenvatting Partiële resistantie van gerst voor dwergroest,Puccinia hordei, is gekenmerkt door een vertraagde epidemie-opbouw ondanks een vatbaar infectietype. Gerstrassen variëren sterk in partiële resistentie en haar componenten. In een toets ter bestudering van de interactie tussen waardplant en pathogeenisolaat bleek de meeste variatie horizontaal van aard te zijn. In de combinatie van Julia met isolaat 18 werd echter een differentiële interactie waargenomen; Julia had een klein deel van zijn partiële resistentie verloren. Dezelfde interactie werd waargenomen voor de belangrijkste component van partiële resistentie, de latentieperiode (LP); Julia had een iets verkorte LP voor isolaat 18. Genetische analyses toonden aan, dat Julia een polygen voor langere LP bevat; dit gen is niet aanwezig in de andere bestudeerde rassen. Er wordt verondersteld dat het effect van dit polygen door isolaat 18 is doorbroken. Dit wijst op een gen-om-isolaat en zelfs op een gen-om-gen-relatie.     

The isolation of a further new race of Erysiphe graminis DC. f.sp. hordei Marchal and the genetical basis of the resistance of ‘Lyallpur 3645’

Besides the race Amsel-C₂, a new race of barley mildew (Erysiphe graminis DC. f.sp.hordei Marchal) has been isolated from a still resistant variety L 94. This race has provisionally been denoted Lyallpur 3645-C₁₇. It is able to overcome the resistance of Lyallpur 3645.It appears most likely that the barley variety Lyallpur 3645 has two closely linked genes for resistance, one for hypersensitivity and the other for an intermediate type of reaction. The first is present in the variety Amsel and both in Heine 4808.The reactions indicate that race Lyallpur 3645-C₁₇ has only one gene for virulence in addition to the ones present in race Amsel-C₂.A survey is given of the seedling reaction of some 60 previously very or moderately resistant varieties, after infection with the two new races.Samenvatting Twee fysio's van gerstemeeldauw,Erysiphe graminis DC. f.sp.hordei Marchal zijn gevonden, waarvan één niet eerder is beschreven. Dit laatste fysio wordt voorlopig aangeduid als Lyallpur 3645-C₁₇, het andere is het zgn. Amsel-C₂-fysio.Met behulp van beide fysio's is duidelijk geworden dat de lijn Lyallpur-3645 twee nauw gekoppelde resistentiegenen bezit; een gen voor een overgevoeligheidsreactie en een ander gen voor een intermediaire reactie.In het ras Amsel is alleen het overgevoeligheidsgen aanwezig, in Heine 4808, evenals Amsel afkomstig van kruisingen met Lyallpur 3645, zijn beide genen aanwezig.Het fysio Lyallpur-3645-C₁₇ lijkt daarom één gen voor virulentie meer te hebben dan Amsel-C₂.Tenslotte is een overzicht gegeven van de kiemplantreactie na infectie met beide fysio's van ongeveer 60 gerstrassen die eerder zeer goed of matig resistent waren.     

The Adoption of Integrated Pest Management (IPM) by Tropical Fruit Growers in Thailand as an Example of Change Management Theory and Practice

Integrated pest management (IPM) is a well-known innovation that accords with modern environmental management's (EMs) best practice. In this paper, it is examined in two ways. First, a recent IPM knowledge diffusion project in a region of Thailand, where durian is extensively grown, is described and analysed in relation to the adoption of both its philosophy and methods by growers. Particular use is made of a theory of innovation (Rogers) to depict the intensity, rate and scale of adoption by the durian growers. Second, attention is focused on IPM as an expression of theory and practice in change management. What is shown is that successful adoption of IPM depends upon a number of factors, notably durian growers perceptions of relative advantage and the way the approach is communicated and learned by them through practical application. In terms of change management theory, the intelligent way IPM knowledge was transferred, through the mediating role of agricultural extension workers (AEWs), reflected the current emphasis on collaborative partnerships and learning as an effective means of managing change in complex environments.     

Further observations on variation of lettuce mosaic virus in relation to lettuce (Lactuca sativa), and a discussion of resistance terminology

Two potyvirus isolates from endive, originating from southern France (Ls252) and from the Netherlands (Ls265), that were highly and poorly pathogenic on lettuce, respectively, were compared with a common isolate (Ls1) of lettuce mosaic virus (LMV) and with two highly deviant Greek isolates fromHelminthia (Picris) echioides (Gr4) and endive (Gr5), earlier recognized as LMV. The isolates could not be distinguished by particle morphology and serology, and were all identified as LMV. Leaf curling, plant stunting and necrosis were more characteristic of the virus than mosaic. The isolates studied varied considerably on differential host species and a range of lettce cultivars including pathotype differentials of Pink et al. [1992b]. Ls1 and Ls265 reacte largely as pathotype II, including the common strain of the virus, but Ls265 was least pathogenic on lettuce. Ls252 fitted pathotype IV and was very similar to LMV-E (the Spanish strain). The Greek isolates were very similar to each other in causing very severe symptoms on some non-lettuce hosts and a number of lettuce cultuvars. In lettuce variectal reaction Gr4 resembled pathotype I, but Gr5 severely affected Salinas 88, resistant to pathotypes I, II and III, and it appears to be a novel pathotype. Genetic interaction between lettuce and LMV is not following a simple yes-or-no pattern, and it is not a mere matter of resistance versus susceptibility. Adoption of a more realistic resistance terminology is proposed. None of the lettuce cultivars tested was resistant to the most pathogenic isolate Ls252, but resistance to it was found in 2 out of 12 wildLactuca species tested (Lactuca perennis andL. tatarica) while the symptomless plants ofL. perennis clearly reacted in ELISA.     

Strawberry latent ringspot virus in lilies

Strawberry latent ringspot virus was detected in the oriental lily hybrid Stargazer grown in Israel from bulbs imported from Europe. Virus infection was associated with asymmetrical opening of flowers. Virus identification was based on electron microscopy, host range and serology.