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).     

Integrated Biodiversity Management in Paddy Fields: Shift of Paradigm From IPM Toward IBM

The insect fauna in paddy fields is composed of resident, migratory and aquatic species each corresponding to the continuous cropping of rice in the same field, harvesting rice as an annual crop, and originating from still water habitats in wetlands. Although IPM is becoming popular in the control of rice pests, those minor insects and aquatic insects that have no direct economic impact on rice production have received little attention. Consequently, some of them are in danger of extinction requiring conservation. A new concept, Integrated biodiversity management (IBM), is proposed under which IPM and conservation are reconciled and made compatible with each other. As an operational concept in agroecology, premises for implementing IBM are suggested.     

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.     

Virulence inCeratocystis ulmi

Single-ascospore cultures ofCeratocystis ulmi from the progeny of a cross between a culture of compatibility type A from Belford, Massachusetts, U.S.A., and one of type B from Rotterdam, Netherlands, were morphologically heterogeneous and varied widely in virulence towards young elms inoculated in the greenhouse. Some cultures regularly caused typical wilt in callus cuttings ofUlmus hollandica Belgica (susceptible) but none in those ofU. carpinifolia Christine Buisman (resistant). Some cultures cuased wilt in trees of both clones; others in neither. All isolates produced abundant yeast-like cells in liquid culture. All isolated induced typical vascular discoloration except in the terminal inch of twigs. Similar progeny from a cross between cultures of types A and B, both from Naarden, Netherlands, however, were morphologically homogeneous and caused a fairly uniform level of symptoms in young trees ofU. hollandica Belgica (susceptible) and Commelin (moderately resistant), and in seedlings ofU. americana after inoculation in the nursery.Samenvatting Eén-ascosporecultures vanCeratocystis ulmi, verkregen uit de nakomelingen van een kruising tussen een cultuur van het compatibiliteitstype A uit Bedford, Massachusetts, V.S., en een type B uit Rotterdam, Nederland, waren morfologisch heterogeen en vertoonden grote verschillen in virulentie ten opzichte van jonge iepen, die in de kas waren geïnoculeerd. Enkele cultures veroorzaakten bij herhaling een typische verwelking bij callusstekken van de kloonUlmus hollandica Belgica (vatbaar), maar zij brachten geen verwelking teweeg bijUlmus carpinifolia Christine Buisman (resistent). Andere cultures veroorzaakten verwelking bij bomen van beide klonen, weer andere bij geen van beide. In alle gevallen ontstond na inoculatie de typische houtverkleuring, die echter uitbleef in de toppen van de twijgen (ca. 3cm). Alle cultures produceerden in een voedingsvloeistof een overvloed van gistachtige cellen.Eén-ascosporecultures, verkregen uit een kruising tussen de twee compatibiliteitstypen, beide afkomstig uit Naarden, Nederland, waren daarentegen morfologisch homogeen. Zij vertoonden bovendien vrijwel geen verschillen in virulentie na inoculatie in de kwekerij in jonge bomen vanU. hollandica Belgica en Commelin (respectivelijk vatbaar en matig resistent) en in zaailingen vanU. americana (vatbaar).A contribution of the Massachusetts Agricultural Experiment Station.     

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     

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.     

Metabolism of radioactively labeled quinic acid and shikimic acid in healthy and Fusarium-infected tomato plants

Radioactivity from quinic acid-U-¹⁴C was readily incorporated into chlorogenic acid and shikimic acid in healthy andFusarium-infected tomato plants of two varieties, Bonner Beste (susceptible) and Moneymaker (resistant); radioactively labeled shikimic acid, on the other hand, was converted neither to quinic acid nor to chlorogenic acid.Infection led to increased incorporation of¹⁴C inton-butanol extractives, and alcohol-soluble and insoluble esters, except in the resistant variety after feeding of shikimic acid-U-¹⁴C. After infection incorporation into the non-hydrolyzable fraction—which a.o. contains lignin—decreased in the susceptible variety, but it increased in the resistant variety, particularly after administration of shikimic acid-U-¹⁴C.Samenvatting Radioactief kinazuur werd in gezonde en metFusarium geïnfecteerde tomateplanten behorende tot twee variëteiten, Bonner Beste (vatbaar) en Moneymaker (resistent), in chlorogeenzuur en shikimizuur omgezet; laatstgenoemde verbinding werd echter noch in kinazuur, noch in chlorogeenzuur omgezet.Infectie leidde tot een toegenomen incorporatie van¹⁴C in metn-butanol extraheerbare verbindingen, en in alcohol-oplosbare en onoplosbare esters, behalve na toediening van radioactief shikimizuur in de resistente variëteit. Incorporatie in de zg. niethydrolyseerbare fractie, die o.a. lignine bevat, bleek in de vatbare variëteit na infectie geringer te zijn, maar in de resistente variëteit na infectie toe te nemen, vooral na toediening van shikimizuur.     

Rep-PCR Based Genomic Fingerprinting of Isolates of Leptosphaeria maculans from Poland

Leptosphaeria maculans, the ascomycete fungus which causes blackleg disease of oilseed rape, has been considered for a long time as a single species divided into aggressive and non-aggressive pathogenicity groups which differ in their economic importance. However, the development of accurate biochemical and molecular characterisation methods has demonstrated that the world-wide L. maculans population actually comprises at least two species. The aim of this research was to assess the ability of rep (repetitive element based)-PCR genomic fingerprinting methods, initially developed for bacterial identification, to characterise a collection of 90 isolates of L. maculans from Poland, in comparison with reference isolates from the IBCN (International Blackleg of Crucifers Network) collection. REP (repetitive extragenic palindromic)-, ERIC (enterobacterial repetitive intergenic consensus)-, and BOX primers for rep-PCR genomic fingerprinting, and primers derived from LMR1, a L. maculans specific repeated element, were tested. Rep-PCR and LMR1-based analyses were able to discriminate the different components of the species complex and to evaluate the genetic diversity within each member of the complex. These analyses suggested that Polish populations of L. maculans mainly belong to the non-aggressive species, rather than the aggressive species which is prevalent in Western Europe, Canada and Australia.     

A biochemical mechanism for the gene-for-gene resistance of tomato to Cladosporium fulvum

Model experiments were carried out with the tomato varieties Moneymaker (no resistance genes), Leaf Mould Resister No. 1 (resistance gene Cf 1), Vetomold (resistance gene Cf 2) and V 473 (resistance genes Cf 1 and Cf 2) and various physiological races ofCladosporium fulvum. Leaking of³²P from labelled leaf disks, was obtained on infiltration with high molecular weight excretion products from incompatible races ofC. fulvum but not with those from compatible races. These products were obtained by Sephadex G-25 gel filtration of culture filtrates.The observations are in line with our hypothesis that the gene-for-gene relation existing between tomato andC. fulvum is based on interaction of specific fungal excretion products with specific receptors in the host which may be located in the cell membrane. The presence of these fungal compounds is supposed to be controlled by four avirulence genes (A₁, A₂, A₃ and A₄) and that of the receptors by the four resistance genes (Cf 1, Cf 2, Cf 3 and Cf 4). Results obtained from experiments with tomatoes Cf 1, Cf 2 and Cf 1 Cf 2 suggest that leakage followed by the hypersensitivity reaction occurs whenC. fulvum races possessing a specific avirulence allele penetrate into a host carrying the corresponding resistance allele.It is not yet clear why growth ofC. fulvum is stopped when leakage of the host tissue resulting in the hypersensitive reaction takes place. No compound toxic toC. fulvum is present or is formed in homogenates of tomato leaves.Samenvatting Modelproeven werden uitgevoerd met de tomatenvariëteiten Moneymaker (geen resistentiegenen), Leaf Mould Resister No. 1 (resistentiegen Cf 1), Vetomold (resistentiegen Cf 2), V 473 (resistentiegen Cf 1 en Cf 2) en verschillende fysiologischen rassen vanCladosporium fulvum. Bladponsjes van radioactief gemerkte bladeren (gemerkt met³²P) werden geïnfiltreerd en geïncubeerd met cultuurfiltraatfracties (fractionering over Sephadex G-25) vanC.fluvum. Waargenomen werd dat ponsjes, behandeld met cultuurfiltraat van een niet compatibeleC.fulvum, een grotere uitlek van radioactief gemerkt materiaal te zien gaven dan in de, gevallen waarin een compatibele schimmel werd gebruikt.Deze waarnemingen stemden overeen met onze hypothese dat de gen-om-gen relatie die bestaat tussen tomaten enC.fulvum, gebaseerd, is op een interactie van specifieke schimmelprodukten met specifieke receptoren in de plantencellen, mogelijk in de membranen. De produktie van de specifieke stoffen door de schimmel zou worden bepaald door vier avirulentiegenen (A₁, A₂, A₃ en A₄), en de aanwezigheid van de specifieke receptoren in de plantencel door de vier resistentiegenen Cf 1, Cf 2, Cf 3 en Cf 4.De waarnemingen, verkregen uit de proeven met de tomaten Cf 1, Cf 2 en Cf 1 Cf 2, doen vermoeden dat de uitlek een gevolg is van een overgevoeligheidsreactie die optreedt, indien een fysio vanC.fulvum, dat een specifiek avirulentie allel bezit, een gastheer binnendringt die beschikt over een bijpassend resistentie allel.Het is tot nu toe niet duidelijk waarom de groei vanC.fulvum stopt indien, uitlek op gaat treden als gevolg van de overgevoeligheidsreactie. In homogenaten van tomatenbladeren werd geen stof gevonden die de groei vanC.fulvum remt.     

‘Economic Injury Level’ and preventive pest control

Abstract Although the Economic Injury Level (EIL) concept belongs to the keystones of IPM theory, its applicability to all pest problems is believed not to be universal. Current IPM theory claims that the EIL concept is of limited use (i) in situations where an injury-damage function cannot be established, (ii) if pest monitoring is impossible or EIL is very low, and (iii) with preventive measures of pests and pathogens. In this work, I argue that the two latter points may not be true. First, within IPM all types of chemical treatment, including preventive ones, should be economically justified via calculation of EIL, based on the comparison of the cost of preventive and responsive control measures and the cost of the forecasted/expected damage. The expected damage should be based on long-term (historical) damage records, manipulative experiments, risk assessment and biomathematical modelling of the evaluated pathosystem. Second, the absence of EIL in preventively controlled pests hampers completion of the consistent Stern–Pedigos classification of pest organisms according to mutual position of EIL and General Equilibrium Position (GEP) or Stationary Distribution of Population Densities (SDPD).     

Changes in susceptibility of bean leaves (Phaseolus vulgaris) to Sclerotinia sclerotiorum and Botrytis cinerea by pre-inoculative ozone exposures

The effects of ozone on the susceptibility of leaves ofPhaseolus vulgaris toSclerotinia sclerotiorum andBotrytis cinerea have been investigated. Seedlings of one ozone-sensitive (Pros) and five relatively ozone-insensitive cultivars (Gamin, Precores, Groffy, Narda, Berna) were exposed to different ozone concentrations (0, 120, 180 and 270 g m^–3) for 8 h. One day after the exposures, primary leaves were detached and immediately inoculated with spores of either pathogen suspended in water or in a 62.5 mM KH₂PO₄ (Pi) solution. Visible ozone injury differed between the cultivars and increased with increasing ozone concentration. On the leaves of non-exposed plants, spores of the pathogens suspended in water caused very few lesions, whereas fungal pathogenicity was stimulated by addition of Pi to the inoculum. Ozone-injured leaves of all cultivars exhibited lesions after inoculation of the leaves with the pathogens suspended in water, and the number of lesions was positively correlated with the level of ozone injury for either pathogen and cultivar. The increase in susceptibility of bean leaves in response to increasing ozone concentrations was greater forB. cinerea than forS. sclerotiorum when spores were suspended in water, but was similar when the spores were suspended in Pi.In general, the number of lesions following inoculation with spores in Pi increased with increasing ozone concentration. However, the number of lesions in the ozone-insensitive Groffy was reduced by an exposure to 120 g m^–3 but increased with higher concentrations. This pattern of susceptibility response to the pathogens was not found in the other ozone-insensitive cultivars and, thus, did not appear to be related to the inherent ozone-insensitivity in bean.     

Is a helper factor necessary for infection of cowpea protoplasts with blackeye cowpea mosaic virus?

Samenvatting Bij inoculatie van mesofyl protoplasten van cowpea met een zeer infectieuze, gezuiverde suspensie van blackeye cowpea mosaic virus (BlCMV), een potyvirus, kon een infectie niet worden geconstateerd. Wanneer echter de bovenstaande vloeistof, verkregen na centrifugering bij laag toerental van sap van BlCMV-geînfecteerde planten, als inoculum werd gebruikt, dan kon vermeerdering van het virus in de protoplasten worden vastgesteld door middel van infectiositeitsproeven, immunofluorescentie en elektronenmicroscopie. Werd deze bovenstaande vloeistof, ontdaan van zijn infectiositeit, hetzij door bestraling met UV-licht, hetzij door centrifugering bij hoog toerental, en vervolgens gemengd met gezuiverd virus, dan trad er geen infectie van de protoplasten op.     

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.     

A greenhouse test for screening sugar-beet (Beta vulgaris) for resistance to beet necrotic yellow vein virus (BNYVV)

Small differences in activity between batches of purified beet necrotic yellow vein virus (BNYVV) were observed in ELISA. A four-parameter modelled dose-response curve of purified BNYVV was used for the conversion of ELISA values to virus concentrations. Seedlings of the susceptible cultivar Regina and the partially resistant cultivars Nymphe and Rima were tested for resistance to BNYVV in a mixture of sand and infested soil. Plants were grown in a green-house with low nutrient supply and at temperatures below the optimum of both the vectorPolymyxa betae and BNYVV. Root systems were small and consisted mainly of lateral roots. Significant differences in average virus concentrations were found between cultivars, both when using the complete root systems and when using either the top or the bottom part of the root systems. Average virus concentrations in Regina were always significantly higher than in Rima and higher than in Nymphe on all occasions except one (P<0.05). Differences between Nymphe and Rima were less evident. Variation between plants was greatest within Rima. The test described in this paper can be used for the discrimination of different cultivars and for the identification of individual plants with resistance to BNYVV.     

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.     

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.     

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.     

Distribution of the FoToml gene encoding tomatinase in formae speciales of Fusarium oxysporum and identification of a novel tomatinase from F. oxysporum f. sp. radicis-lycopersici, the causal agent of Fusarium crown and root rot of tomato

The antifungal glycoalkaloid -tomatine accumulates in tomato plants and may protect plants from fungal infection. Fusarium oxysporum f. sp. lycopersici, the causal agent of vascular wilt of tomato, produces a tomatinase (FoToml) that degrades -tomatine to the nontoxic compounds tetrasaccharide lycotetraose and tomatidine. Induction of tomatinases and the distribution of FoToml homologs were examined among 30 strains belonging to 16 formae speciales of F. oxysporum. Tomatinase activity was found in 27 strains belonging to 15 formae speciales, but FoToml homologs (>98% sequence identity) were detected in only six strains belonging to four formae speciales. To identify tomatinases other than FoToml, -tomatine-inducible proteins of another tomato pathogen F. oxysporum f. sp. radicis-lycopersici were analyzed by two-dimensional gel electrophoresis. A protein with a molecular mass of 64kDa accumulated in the -tomatine-induced culture filtrates, and the protein had tomatinase activity, degrading -tomatine to lycotetraose and tomatidine.     

Scytalidium Wilt of Citrus

In July 1998, a sudden wilt of Star Ruby grapefruit (Citrus paradisi Macf.) occurred in Israel in a region with a warm Mediterranean climate. The wilt of the middle and upper canopy of main limbs was accompanied by gum oozing from the affected branches. The bark of these branches attained a dark colour and the epidermis sloughed off easily revealing a mass of black powder, resulting from copious sporulation of dark conidia. Both the bark and the xylem were intensively colonized with mycelium. The fungus was identified as Scytalidium lignicola Pesante, based on the characteristic mixture of some colourless and dark conidia, produced in branched chains by conversion of the vegetative hyphae. Artificial inoculations induced typical disease symptoms and the fungus was reisolated, thus confirming Koch's postulates. Similar symptoms appeared in Star Ruby interim segments which had been cut and top grafted with various citrus cultivars. This is the first detailed report of pathogenicity of S. lignicola to citrus trees. It is postulated that the disease developed after predisposition of Star Ruby grapefruit trees by comprehensive pruning followed by extremely hot conditions.     

Tylose formation in elms after inoculation with Ceratocystis ulmi,a possible resistance mechanism

After inoculation of elms withCeratocystis ulmi tylose formation in vessels of the susceptible clone Belgica appeared to be delayed in comparison with tylose formation in the resistant clone 390. It is suggested that tylose formation may be a resistance mechanism in elms to Dutch elm disease.Samenvatting Na inoculatie van iepen metCeratocystis ulmi bleek de thyllenvorming te zijn vertraagd in de vaten van de vatbare kloon Belgica in vergelijking met de thyllenvorming in de resistente kloon 390 (Fig. 1). Thyllenvorming zou een resistentie mechanisme kunnen zijn tegen de iepziekte.