Schurft op Pyrancantha

Summary Spilocaea pyracanthae (Otth) v. Arx comb. nov. (Syn.:Fusicladium pyracanthae (Otth) Viennot-Bourg.), the scab fungus ofPyracantha coccinea, overwinters in its conidial stage on the living leaves and on twigs. No perithecial stage could be found the last three years on overwintered fallen leaves.     

De waardplantenreeks van phomopsis dauci

Zusammenfassung Infektionsversuche haben ergeben, dassPhomopsis dauci von Arx scharf aufDaucus carota L. spezialisiert ist und andere Umbelliferen nicht antasten kann. Der Pilz kommt sowohl auf der Wildform wie auf den gezüchteten Rassen vonDaucus vor und geht leicht von der einen Form auf die andere über.     

Enkele waarnemingen over het opreden van de verwelkingsiekte bij luzerne in Zeeland in 1957

Summary Some observations have been made on lucerne wilt disease during the growing season in 1957, and in experimental inoculation tests in a glasshouse in 1958. In the province of Zeeland the disease does harm to the crop and prevents many farmers from keeping their crops longer than two years.In a survey data have been collected on soil type, preceding crops, green manuring and manuring with stable dung, pH, structure and weeds. No correlations between these factors and the degree of attack by the disease could be proved, except that on heavy soils the disease was less severe than on lighter soils (as a mean of all observations).Development of the disease is facilitated by damage caused by mechanical equipment to the crop. Damage to the crown of the plants should be avoided as much as possible. The disease is distributed extensively after frequent mowing. On attacked leaves left behind on the mowed stalks, and on decayed leaves dropped to the soil, masses of spores ofVerticillium albo-atrum are produced, which form a potential source of infection in a weakened crop.It seems that the disease is more air-borne than soil-borne, since one year old crops are seldom attacked.From attacked lucerne plants in the field the following fungi have been isolated:Verticillium albo-atrum Reinke & Berth.,Gliocladium roseum Bain andFusarium avenaceum (Fr.) Sacc. (table 1).Isolations have been made from 50 different species of weeds, out of which the following fungi were obtained:Verticillium albo-atrum Reinke & Berth. fromCapsella bursa-pastoris Med. andPlantago major L.,V. dahliae Kleb. fromPrunella vulgaris L.,Capsella bursa-pastoris Med. andThlaspi arvense L., andV. lecanii (Zimm.) Viegas fromPlantago major L.In inoculation experiments with all these isolates, in roots, stalks and on cut plants of lucerne, onlyV. albo-atrum, both from lucerne and weeds, andV. lecanii proved to be able to cause wilting of leaves and stalks. WithGliocladium, Fusarium andVerticillium dahliae no such symptoms could be obtained. The results were the same whatever method of inoculation was used.     

Het parasitaire karakter van Septotinia populiperda Waterman & Cash

Summary Mature apothecia ofSeptotinia populiperda Waterman andCash are formed in spring and early summer on leaves shed the previous autumn. The ascospores ofS. populiperda produce an infection in the form of leafblotches only when an injury of the leaf is present.P. candicans Ait. is more susceptible to the parasite thanP. marilandica Bosc. andP. brabantica Houtz. Leafblotches are produced in equal numbers when the ascospores are sprayed with an atomizer upon the upper or the lower leafsurface. The germination of the ascospores is inhibited in vitro at a concentration of 7 to 8 mg copper per liter, when copper is used as Bordeaux mixture, or as coppersulfate (CuSO₄5H₂O).     

De Rijpingsvreterij van den Ooftboom-Spintkever,Scolytus Mali Bechst. (Col.: Scolyti Dae)

Summary 1. The maturation feeding ofScolytus mali Bechst. is described for the first time. 2. This feeding takes place at the base of the spurs (short-shoots) where a short hole is bored by the young beetle. 3. The damaged spurs whither and die, finally dropping to the ground. 4. The described infestation originated from a dead apple, pear and plum tree in the neighbourhood, the bark of which was heavily perforated by the emerging beetles. The breeding galleries are shortly described; attention is further drawn to the occurrence of numerous horizontal galleries the significance of which is not yet clear. 5. The parasites reared from the infested wood are enumerated. The most numerous is a Chalcid, subfam. Eulophinae, viz.Elachistus leucogramma Rtzb. but it occurred in a small percentage, perhaps 2–5%, only. 6. Removing and destroying all dead or dying fruit trees, which are infested by these Scolytids, is strougly advised.      

Het optreden van plaatselijke rassen bij Helopeltis antonii Sign. op Java

Summary Helopeltis antonii was bred at different constant temperatures at the experimental station at Semarang in 1939 and 1940. Some of the results concerning the duration of development from hatching of the egg untill the imaginal stage are recorded in the table and the graph. By comparing these data with those published byLeefmans (1916) (Helopeltis bred on tea in West Java) andZehntner (1901) (Helopeltis bred on cocoa in Central Java) we find that the time of development mentioned by these authors is shorter. In accordance with the experiments ofBonnemaison (1946) this probably can be attributed to the breeding ofHelopeltis at alternating temperatures (day and night). The duration of development at 19°C, mentioned byLeefmans (19 days), however, differs so much from the duration found in this investigation (32–34 days) that this explanation seems improbable. Other experiments at constant temperature, that will be published elsewhere, show that the population used for breeding was well adapted to the average temperature at the estate, Djati Rungo, from where this population came. This extreme adaptation suggests that this may also occur at other localities. Therefore it is probable that the populations ofH. antonii from different localities do not have the same genetical composition. The differences mentioned above can probably be attributed therefore to this fact. Data are given to strengthen this supposition. The existence of genetically different populations has already been suggested by ProfessorRoepke in 1909. He called them biological races. The occurrence of the varietybradyi ofH. antonii is probably caused by such a difference too and thus could better be considered as a subspecies.Helopeltis theobromae Mill. is without doubt only a subspecies ofH. theivora.Thus we suggest:Helopeltis antonii subspec.bradyi Wat. new comb. Helopeltis theivora subspec.theobromae Mill. new comb.

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Deventer     

Is verrijking van de mycoflora mogelijk?

Samenvatting Van verrijking der mycoflora is slechts dan sprake, als nieuwe elementen in een gebied verschijnen door invoer van elders, door nieuwe kruisingen of door nieuwe mutaties van zwammen. Het ontdekken van een vroeger onbekende zwam is nog geen bewijs voor een verrijking der mycoflora. Het kan zijn dat de onbekende vroeger alleen aan de aandacht is ontsnapt.Voorbeelden van verrijking der flora door het overbrengen van parasitaire zwammen van het eene werelddeel naar een ander zijn gemakkelijk te geven. Uit de verspreiding van de populierenroesten over de aarde blijkt, dat het gevaar van overbrengen van Noord-Amerikaansche soorten van Melampsora naar Europa en van Europeesche soorten naar Noord-Amerika niet denkbeeldig is (Afb. 1).Bij proeven aan het Laboratorium voor Mycologie en Aardappelonderzoek te Wageningen over de vatbaarheid van populieren voor eenige soorten van Melampsora, traden in 1941 verschillende rassen en een witte variant vanMelampsora Larici-populina Kleb. op (Afb. 4, 5 en 6), die in de eerste twee jaren zeker niet voorkwamen (Tabel 1). De physiologische rassen zijn met behulp van een speciale laboratorium-methode te onderscheiden door de verschillende reactietypen van een sortiment van populieren (Afb. 2 en 4). Het optreden van de rassen wordt toegeschreven aan het feit, dat in het najaar van 1940Larix sp. tusschen de populieren van het proefveld werden geplant. Op deze Larix kunnen nl. door kruising nieuwe combinaties van genetische factoren tot stand komen.Ook bij andere parasitaire zwammen o.a. bijPhytophthora infestans de Bary (de aardappelziekte) en bijSynchytrium endobioticum (Schilb.) Perc. (de wratziekte) op aardappel, zijn onverwachte complicaties van het resistentie-onderzoek ontstaan door het verschijnen van voordien onbekende rassen van deze parasieten. Of door deze de mycoflora werkelijk met iets nieuws werd verrijkt, is moeilijk te beslissen. Zij kunnen er reeds zijn geweest, maar in een zoo gering percentage, dat de kans op een isolatie practisch nul was. Pas de vermeerdering door het gebruik van speciale rassen van hun waardplanten brengt hun bestaan aan het licht.Met het oog op de roesten van populieren is een verrijking van de mycoflora mogelijk door invoer van elders en door nieuwe rassen, die uit kruising ontstaan. Beide kansen ontnemen aan het resistentie-onderzoek zijn absolute waarde. Dat zulke teleurstellingen groot kunnen zijn, blijkt uit de hevige aantasting vanPopulus candicans Aiton door één der nieuwe rassen en de witte variant vanM. Larici-populina, die alles overtreft, wat bij jarenlange observatie werd waargenomen.Summary Only the appearance of new elements by introduction from abroad, by new hybrids or by new mutations of fungi may be called an enrichment of the mycoflora. Discovery of formerly unknown fungi is no proof of such an enrichment. It may be that till then the unknown fungus merely escaped attention.Examples of enrichment of the mycoflora by transport of parasitic fungi from one continent to another are easy to give. From the distribution of poplar rust fungi on earth it is evident, that the danger of introducing North-American Melampsora's on poplar into Europe and European species into North-America is not imaginary (Afb. 1).Experiments on the susceptibility of poplars to infection by some species of Melampsora, carried out at the Laboratorium voor Mycologie en Aardappelonderzoek at Wageningen, proved the appearance in 1941 of several physiologic races and one white variety ofM. Larici-populina Klebahn (Afb. 4, 5, 6). These were certainly not present during the first two years of the experiments, 1939 and 1940 (Tabel 1). The physiologic races can be distinguished with the aid of a special laboratory-method by the reaction-types of a collection of poplars (Afb. 2, 4).The appearance of the races is supposed to be due to the planting in the autumn of 1940 of larches between the poplars of the experimental plot. The formation of new races has become possible by hybridizing on these larches, on whichM. Larici-populina forms its 0 and I stages.Unexpected complications in breeding of potatoes for resistance have been raised by the appearance of formerly unknown races ofPhytophthora infestans de Bary (potato blight) andSynchytrium endobioticum (Schilb.) Perc. (wart-disease). It is, however, difficult to decide, whether in these cases the mycoflora has been enriched by some new races. They may have existed already before, but in such a low percentage that practically there was no chance to isolate them. It is only by using special races of hostplants that their existence can be stated.In relation to the rusts of poplars the possibility exists of an enrichment of the mycoflora by introduction from abroad and by the formation of new races by hybridization. Both possibilities make the results of breeding for resistance less absolute. To what extent such disappointments may develop is evident from the heavy attack ofPopulus candicans Aiton by one of the new races and by the white variety ofM. Larici-populina. This attack surpasses all earlier ones observed during several years.Voordracht gehouden op 4 Maart 1944 voor de Nederl. Plantenziektenkundige Vereeniging.     

Enkele gegevens omtrent sclerospora in Indonesië

Samenvatting De gevoelige periode van de maisplant voor infectie doorSclerospora maydis (Rac.) Butler werd bepaald, alsmede de incubatietijd (grafieken 1 en 2). Behalve deze soort komt sinds 1947 op Noord CelebesSclerospora philippinensis Weston voor, die daar aanzienlijke schade aan mais toebrengt. De verschillen van deze schimmel metSclerospora sacchari zijn slechts gering en een nader onderzoek dienaangaande is dan ook zeer gewenst.De eerste oögoniën-vorm vanSclerospora in Indonesië werd gevonden en wel vanSclerospora northii Weston op eenErianthus spec. op het eiland Soemba.Summary The nomenclature ofSclerospora maydis (Rac.) Butler is discussed. The susceptible period of the host and the time of incubation of this fungus (graphs 1 and 2) were determined.Since 1947 a species ofSclerospora has caused considerable damage on maize in North Celebes. It is stated to beSclerospora philippinensis Weston. The differences between this fungus andSclerospora sacchari T. Miyake are very slight. AsSclerospora sacchari is still unknown in Indonesia, it is very important for the culture of sugarcane on Java to investigate by means of inoculation trials, ifSclerospora philippinensis can attack sugarcane. Sclerospora northii Weston has been found onErianthus spec. on the isle of Sumba; it is the only knownSclerospora species in Indonesia which forms oögonia.Vroeger verbonden aan het Instituut voor Plantenziekten te Bogor, Indonesia, thans aan het Inst. v. Plantenziektenkundig Onderzoek (I.P.O.) te Wageningen.     

Grondonderzoek op phytophthora sabdariffae

Summary For the successful cultivation ofHibiscus sabdariffa for fiberproduction in Java a preceding determination is necessary whether either soil or organic manure are contaminated byPhytophthora sabdariffa or not. To this end a method has been elaborated analogous to the method to determined soil contamination byPhytophthora nicotianae.According toGarrett's scheme ofWaksman's classification of soil fungi into soil inhabitants and soil invaders,Phytophthora sabdariffa has to be classified as soil invader.To test the presence of this fungus in soil or in organic manure leaves ofRicinus communis prove to be useful as a detector, since leaves ofHibiscus sabdariffa are too small and too quickly given tot decay. A method is to spread a paste of the soil or of the organic manure on the leaves and to keep these leaves contaminated during 24 hours (spreading method). Afterwards the paste is washed away and the leaves are kept in humid conditions during another 24 hours or 2×24 hours.The number of spots caused by Phytophthora is a measure of soil- or manureinfection (Table I). A more sensitive method is obtained by making a suspension of the soil or of the manure and by streaming this suspension (submersion method) during two hours over Ricinus leaves (Thung, 3). Afterwards the leaves are kept in humid air to give the fungus occasion to form spots (Table 2). Since Ricinus leaves are also susceptible to tobacco- and Ricinus-Phytophthora, these fungi will also be detected by these leaves. Owing to the fact that these fungi are also a parasite ofHibiscus sabdariffa, these method prove to be useful to detect suspected soil or manure regarding the cultivation of this fiberplant.     

Virusziekten van de komkommer in Nederland

Samenvatting Van de in Nederland bij komkommer optredende virusziekten worden het ziektebeeld en de belangrijkste eigenschappen beschreven.Aantasting door komkommervirus 1 vindt voornamelijk plaats bij een nateelt van komkommers. Het virus wordt overgebracht door de groene katoenbladluis (Aphis gossypii Glover) bij het opkweken en tijdens de teelt. De komkommerplanten hebben hiervan ernstiger te lijden naarmate de temperatuur gedurende de incubatieperiode lager is. De planten kunnen dan verwelken en afsterven.Bij de teelt onder glas wordt komkommervirus 2 verreweg het meest aangetroffen. Vers komkommerzaad is een belangrijke infectiebron. Tijdens de bewaring van het zaad loopt het infectievermogen snel achteruit. Toch kan een deel van het op het zaad aanwezige virus gedurende meer dan een half jaar zijn activiteit behouden. De in Nederland veel gebruikte onderstamCucurbita jicifolia kan worden beschouwd als een carriër voor dit virus. Bij het enten kan het virus gemakkelijk worden overgebracht, ook met behulp van besmet sap, dat op het hierbij gebruikte mes is achtergebleven. Er zijn aanwijzingen, dat het virus in de grond kan overblijven.Het optreden van komkommervirus 2 A is veel minder algemeen, hoewel het ziektebeeld veel ernstiger is. De eigenschappen stemmen overeen met die van komkommervirus 2.Het komkommernecrosevirus kan grote schade te weeg brengen aan de teelt, doordat de vruchten van de aangetaste planten onverkoopbaar zijn en de planten soms geheel afsterven. De inactiveringstemperatuur van dit virus bedraagt ruim 80°C gedurende 10 minuten. Een groot aantal waardplanten reageert met plaatselijke necrose op de geïnoculeerde bladeren. Een enkele maal kan tijdelijk een systemisch beeld worden waargenomen, zo b.v. opNicotiana tabacum var. White Burley. In de grond behoudt het virus zijn infectievermogen minstens een half jaar. Aantasting vindt van de grond uit plaats. Zij wordt bevorderd door een lage temperatuur van bodem en lucht. Tijdens een periode van warm, zonnig weer kan algeheel herstel plaats vinden. OpCucurbita ficifolia geënte planten zijn veel minder gevoelig en herstellen gemakkelijker.Summary The symptoms and the properties of the virus diseases occurring on cucumber in Holland, are described in this article. Cucumis virus 1 occurs mainly on cucumbers grown in autumn. It is transmitted byAphis gossypii Glover, both during the propagation period and during the remainder of the season. Cucumber plants suffer most when the temperature during the incubation period is low. The plants may then wilt and die. Cucumis virus 2 is found very frequently in cucumber growing under glass. Freshly harvested seed is an important source of infection. Although the percentage of infection diminishes fairly rapidly with time, the virus in the seeds may retain its activity for more than half a year.Cucurbita ficifolia, used in Holland on a large scale as a root stock for cucumber, is a carrier ofCucumis virus 2. The virus is transmitted very easily during grafting, as well as by cultural measures such as pruning etc. There are indications, also, that the virus may survive in the soil. Cucumis virus 2 A occurs very rarely. The symptoms it causes are more severe than those caused byCucumis virus 2. Its physical properties are the same as those ofCucumis virus 2.Cucumber necrosis virus is the most serious of the viruses affecting cucumber, as the fruits may be badly damaged and the plants sometimes die. This virus is inactivated for the most part by exposure to a temperature of 80°C for 10 minutes. A large number of host plants react with local necrosis on the inoculated leaves. In a few cases systemic symptoms were observed temporarily, e.g. onNicotiana tabacum var. White Burley. In the soil this virus can survive for more than half a year. Infection of the plants takes place from the soil. The disease is favoured by low soil- and air-temperatures. During a period of warm sunny weather complete recovery is possible. Plants grafted onCucurbita ficifolia are less susceptible to an attack of cucumber necrosis virus and if infected frequently recover.     

Thielaviopsis basicola (Berk. et Br.) Ferraris als veroorzaker van een voor Nederland nieuwe ziekte van de tabak

Summary Tobacco black root rot, caused byThielaviopsis basicola (Berk. etBr.) Ferraris has been reported for the first time in the Netherlands. It occurred in tobacco seed beds in the Maas and Waal district, especially in the variety Kentucky Italiana.A part of the literature on this subject is briefly discussed and methods for control are given.     

Korte overdrachttijd van bladrolvirus

Conclusie Uit de bovenbeschreven infectieproeven is dus wel duidelijk aan het licht gekomen, dat de overdracht van het bladrolvirus in zeer korte tijd kan plaats vinden, naarPhysalis in ieder geval in een half uur. Hiermee zal rekening moeten worden gehouden bij de virusbestrijding. Hoe men deze snelle overdracht zal moeten interpreteren in verband met de verdere eigenschappen van het bladrolvirus, dat in allerlei opzichten overeenkomt met andere persistente viren, zal nader moeten worden onderzocht.Summary Groups of 5 virus freeMyzus persicae (Sulz.), which were not starved, were used in experiments to transmit leafroll virus. An acquisition period of 15 minutes on leafroll-infectedPhysalis floridana, followed immediately by an inoculation-period of 15 minutes onP. floridana seedlings, proved to be long enough for the transmission of leaf-roll virus: 31 out of 45 test plants ofP. floridana became infected. In the same way 11 out 18 of healthyP. floridana plants were infected when potato was used as a leafroll-source.     

De overbrenging van een virusziekte van zure kers op komkommer

Summary Cucumber seedlings were infected with a virus from sour cherry by rubbing the cotyledons with sap from young sour cherry leaves, showing shock effect symptoms of a virus infection (probably necrotic ringspot). The symptoms on cucumber consisted of necrosis of vegetation point, yellow spots in the leaves and rosette formation, thus showing the resemblance of this infection to the oneMoore, Boyle andKeitt effected in cucumber by using a mixture of necrotic ringspot and yellows virus.     

De rijpingsvraat van de eikensp intkever,scolytus intricatus ratzeb

Summary The oak barkbeetle,Scolytus intricatus Ratz., is, in normal times, a rather uncommon insect in the Netherlands. After the outbreak of the war, however, it developed in large numbers in oak trees, damaged bij direct war action. The presence of the beetle is indicated bij its maturation feeding, consisting in affecting and killing a large number of spur shoots during May. Bij their boring activity the young beetles nearly cut off these shoots near their base, soon they wither and fade becoming clearly visible in the green canopy. Stormy weather causes them to drop off green so that the ground may become densily covered with these shoots.A rectification is added concerning the maturation feeding ofSc. laevis Bechst., as described formerly by the same author. The identification of the beetle in question proved to be wrong. The maturation feeding described refers to the commonSc. rugulosus Ratz.     

Influence of altitude on adult emergence and dynamics ofKaltenbachiola strobi (Winnertz) (Diptera: Cecidomyiidae) populations and its parasites in the Tatra National Park, Poland

A field study onKaltenbachiola strobi (Winnertz) (Diptera: Cecidomyiidae), was carried out between 1987 and 1993 in the highest mountain range in the Western Carpathians located within the Tatra National Park, in southern Poland. Rearing and analysis of 5 780 cones ofPicea abies (L.)Karst., collected from trees growing at different altitudes, yielded 30,478 individuals ofKaltenbachiola strobi, which infested 89.10% of the cones. In case of 29.81% of cones there were more than 20 larvae ofK. strobi per cone. It was shown thatK. strobi in the Tatra Mts. is more abundant in spruce stands of the upper mountain forest zone. Significant connections between the time of cone infestation byK. strobi and its parasitoids and characteristic phenophases in spruce foliage and cone development, and also the development of selected vegetation of the forest floor at different heights above sea level were determined. It was shown that the population dynamics ofK. strobi had tendency to alternate the increase and decrease in numbers every two years. It was also discovered that every two years spruce cones were inhabited by the population ofK. strobi with increased percentage of individuals having prolonged diapause. In total, the parasitoids reduced the population ofK. strobi by about 16–22.63%. Parasitism ofK. strobi in the lower mountain forest zone was higher than in the upper zone.Triplatygaster contorticornis Ratz. (Hymenoptera: Platygasteridae),Torymus azureus Boheman (Hymenoptera: Torymidae) andTetrastichus strobilanae Ratz. (Hymenoptera: Eulophidae) were most effiecient in reduction of the population ofK. strobi, and in the lower mountain forest zoneT. azureus was the dominant parasitoid species, while in the upper zoneT. contorticornis andT. strobilanae. It was shown that a considerable part of the populations ofT. azureus, T. contorticornis andT. strobilanae have their diapause prolonged and highly synchronized with the diapause of their host.     

Het toetsen van stammen vanNectria cinnabarina (Tode) fr. op levende takken in vitro

Summary Some types of elm hybrids resistent to the Dutch elm disease turned out to be susceptible toNectria cinnabarina (Tode) Fr. This fungus may behave as a parasite, killing branches. Virulent and non-virulent strains are known. A method is described for cultivatingNectria-strains in vitro on living desinfected branches ofAcer and ofUlmus. Three weeks after inoculation with a virulent strain the branches were killed. They were covered all over by a thick layer of mycelium. The mycelium of non-virulent strains could not kill the branches. It developed only sparsely. By this method it will be possible to select the most virulent strains out of a collection of isolates in order to use them for testing elm hybrids on their resistence toNectria cinnabarina.     

Geelnervigheid,een virusziekte van luzerne

Samenvatting Door middel van de erwtebladluis (Acyrthosiphon pisum Harris) werd het topvergelingsvirus van de erwt op luzerne (Medicago sativa L.) overgebracht. De besmette planten vertoonden na verloop van tijd het verschijnsel van geelnervigheid, te weten een regelmatige, lichtgroene tot gele nerftekening op de bladeren. Dit verschijnsel blijkt veelvuldig in luzernepercelen in het zuidwesten van het land (het gebied waar topvergeling in erwt algemeen voorkomt) op te treden. Het gelukte uit vier van vijf van verschillende plaatsen van West-Brabant en Zuid-Beveland afkomstige monsters geelnervige luzerne het topvergelingsvirus te isoleren.Uit waarnemingen bij planten, groeiende in het veld en in de kas, is gebleken, dat de omstandigheden een grote invloed hebben op het te voorschijn komen van het ziektebeeld. In het voor- en najaar treedt de geelnervigheid doorgaans zeer duidelijk op; in de zomermaanden verdwijnt dit verschijnsel veelal tijdelijk bij de besmette planten en onderscheiden deze zich niet van virusvrije luzerne.In een veldproef bleek het topvergelingsvirus, gezien het voorkomen van geelnervigheid in de loop van het seizoen, vrij snel in luzerne verspreid te worden.Summary Tip yellows (leaf roll) virus of pea was transmitted to lucerne (Medicago sativa L.) by means of the pea aphid (Acrythosiphon pisum Harris). After some months the infected lucerne plants showed a regular lightgreen to yellow discoloration of the veins and the adjacent tissue (fig. 1). No differences in growth and in shape of the leaves between infected and healthy plants were found to exist.This disease—for which we have chosen the name of vein yellowing of lucerne—is very prevalent in lucerne fields in the southwestern part of The Netherlands where tip yellows is often found in peas. We succeeded in isolating tip yellows virus from four out of five samples of lucerne showing vein yellowing collected in western Brabant and Zuid-Beveland.In carrying out transmission tests, it was found that the pea aphid often hardly sucks on lucerne in the greenhouse. Although it generally is present in large numbers in lucerne fields, it refuses to feed on lucerne after having been reared on peas for a prolonged period. This behaviour of the vector impedes the experiments. It may be due to the fact that the different generations of the insect differ in host plant requirements (Evans & Gyrisco, 1956) but also to the fact that morphologically indistinguishable forms of the insect exist, which may differ in host plant preferences (Hille Ris Lambers, 1947).Furthermore, in our experiments seldom a 100% infection of pea plants was obtained, even when using pea aphids which fed on a tip yellows diseased pea plant for a period of 48 hours or longer, and using five to ten aphids per test plant.Observations on infected plants growing in the field and in the greenhouse indicate that the environment has great influence on the appearance of the vein yellowing symptom. Generally, vein yellowing occurs very strikingly during the spring-time and the late autumn but disappears in the summer months.According to observations on the rate of occurrence of vein yellowing at different times of the year in a field experiment at Wageningen (fig. 2), it appears that tip yellows virus is capable of spreading fast in a lucerne field.The experiments and observations described in the present paper confirm the idea expressed byde Fluiter & Hubbeling (1955) that lucerne fields are important as a reservoir of the tip yellows virus of pea in The Netherlands.     

Effects of Host and Microbial Factors on Development of Clonostachys rosea and Control of Botrytis cinerea in Rose

Development of Clonostachys rosea in rose leaves and petals and control of Botrytis cinerea by the agent were investigated. C. rosea germinated, established endophytic growth, and sporulated abundantly whether the tissues were mature, senescent or dead when inoculated. Germination incidence was moderate on mature and senescent leaves (47% and 35%) and petals (31% and 43%), and high (>98%) on dead tissues. Sporulation of C. rosea in tissues inoculated when mature, senescent or dead averaged 41%, 61%, and 75% in leaves, and 48%, 87% and 53% in petals. When leaves were wounded with needles before inoculation, germination of C. rosea increased from 45–56% to 90–92%, but sporulation became high (>75%) regardless of wounds. When leaves were inoculated with C. rosea at 0–24h after wounding and subsequently with B. cinerea, germination of the pathogen was reduced by 25–41% and sporulation by 99%. A humid period prior to inoculation of senescent or dead leaves promoted communities of indigenous fungi, reduced sporulation of C. rosea and B. cinerea, and, in dead leaves, increased control of the pathogen associated with C. rosea. Applied at high density, isolates of indigenous Penicillium sp. and Alternaria alternata from rose interacted with C. rosea and reduced control of the pathogen by 16% and 21%, respectively. In conclusion, C. rosea markedly suppressed sporulation of B. cinerea in rose leaves and petals regardless of developmental stage, minor wounds, and natural densities of microflora. This versatility should allow C. rosea to effectively control inoculum production of B. cinerea in rose production systems.     

De invloed van enige virussen op de concentraties van organische zuren in een aantal planten

Samenvatting In dit onderzoek werden de concentraties van organische zuren in gezonde en in even oude, met virus geïnfecteerde planten vergeleken. Deze proefplanten warenPhysalis floridana, Nicotiana tabacum var. White Burley enLycopersicum esculentum var. Ailsa Gray. Partijen van 40 of 60 planten werden mechanisch geïnoculeerd, respectievelijk met X-virus, Y-virus, tabaksmozaïekvirus en een virus, dat rozetziektesymptomen op de kers veroorzaakt. Bovendien werd bladrolvirus, overgebracht doorMyzus persicae, bijPhysalis en tomaat in het onderzoek betrokken.Zowel op de dag van de inoculatie als 7, 14 en 21 dagen later werden 20 gezonde planten en 20 exemplaren van elke genoemde partij geoogst. Elk monster werd met water geëxtraheerd. Na zuivering van het extract door een sterke kationwisselaar Imac C 12 en concentratie in vacuo werden de organische zuren met behulp van silicagelzuilen en mengsels van chloroform en n-butanol als solvents kwantitatief bepaald. Twee dimensionale papierchromatografie maakte verificatie van de identificatie van de in de extracten voorkomende zuren mogelijk.Behoudens een enkele uitzondering werd in het algemeen gevonden, dat de zieke planten hogere zuurgehalten bezitten dan de gezonde. Het wordt waarschijnlijk geacht, dat door de virusinfectie het evenwicht tussen de fotosynthese, de ademhaling en de eiwitsynthese zodanig verschuift, dat het gehalte van de organische zuren, speciaal dat van appel- en citroenzuur, verhoogd wordt.Summary A number of publications, concerning special aspects of the relationship between virus synthesis and the metabolism of organic acids, showed a reciprocal influence. However a more general investigation into the influence of the virus infection on the changes in organic acids seemed necessary.In the present paper the concentrations of organic acids in healthy plants and in those of the same age, infected with virus, are compared. For this purpose plants ofPhysalis floridana, Nicotiana tabacum var. White Burley andLycopersicum esculentum var. Ailsa Gray were used. Groups of 40 or 60 plants were inoculated with potato virus X, potato virus Y, tobacco mosaic virus and a virus, causing symptoms of rozet ziekte in cherry respectively. The potato leaf roll virus, transmitted byMyzus persicae, was used withPhysalis and tomato.On the day of inoculation as well as 7, 14 and 21 days later 20 healthy plants and 20 specimen of each group, above mentioned, were harvested. These samples were boiled for five minutes in an amount of water of twice the fresh weight. After mixing, each suspension obtained was subjected to 10,000 atm. in a hydraulic press, the sap then centrifuged (8000 r.p.m. during 10 min.) and the residue washed once. The combined supernatants were percolated through a strong cation-exchanger Imac C 12 (columns of 30×1.2 cm). The effluent contained the organic acids and the carbohydrates. Further purification appeared not to be necessary. The solution was concentrated in vacuo at 40–50°C and dried in a warm stream of air.The organic acids in the dried preparations were analyzed according to the method ofBulen, Varner & Burrell (1952) on silicagel columns of 4 grams, the preparation of which is described byNijkamp (1954). AsBulen et al. worked with columns of 8 grams, we used proportional amounts of solvent mixtures of chloroform and n-butanol, recommended in their publications. After placing the column on a fraction collector, the fractions were titrated against 0.01 n NaOH, using bromothymolblue as an indicator.Although each organic acid leaves the column at a definite fraction-number we verified the identifications of the acids by means of paper chromatography (Wolfgang, 1957) on Ederol-202 filter paper. A mixture of 80 volumes of 96% ethanol, 4 vol. of ammonia and 16 vol. of water as the first solvent and a mixture of 77 vol. of n-butanol, 12 vol. of formic acid and 11 vol. of water as the second solvent were used. After dipping the paper in a mixture of 16 ml aniline, 5 g glucose, 30 ml 96% ethanol and 470 ml light petroleum (b.p. 40–60°C) and heating for 15 min. at 120°C the acids became visible as brown spots on a white background. Especially oxalic, malic and citric acids were demonstrated on the papers.In general the tables show that during the development of the plants the content of organic acids, particularly of the oxalic, malic and citric acids, is lower in normal plants than in diseased ones. ThePhysalis plants, infected with the viruses X and Y and the tomatoes, inoculated with leaf roll virus and virus X are an exception to this. Their contents of organic acids being the same during the first week after the virus-inoculation as those of normal plants. The higher concentrations of the acids in tobacco could be shown very clearly already one week after the infection, whereas the symptoms were not yet visible. The same occurs in the case of tomatoes infected with virus Y, tobacco mosaic virus and the cherry virus.Investigations by several authors have shown an increased respiration activity and a decrease in photosynthesis. It is assumed that the substrate for the synthesis of organic acids will be present at a lower rate. As the synthesis of proteins would be decreased, a higher content of organic acids would result.     

Verticillium albo-atrum as a parasite ofSenecio vulgaris

Many weeds, includingSenecio vulgaris, are virtually symptomless hosts ofVerticillium albo-atrum. Plants ofSenecio vulgaris were inoculated with spore suspensions ofV. albo-atrum by two different methods: root-dipping or injection. Infected plants rarely wilted but were stunted, their lower leaves died prematurely and numbers of lateral shoots were decreased.V. albo-atrum was isolated from all parts of the host including achenes. When placed on agar, infected achenes germinated more rapidly than healthy ones. Only small amounts of mycelium developed in the vessels. Severe wilting was possibly avoided because there is insufficient time for damaging amounts of mycelium to develop inSenecio which has a short period of vegetative growth. On the other hand its tolerance to infection may be attributed to the morphology of its vascular system with its narrow vessels and sparse parenchyma cells.In co-operation withH. van Geest, J. P. Overdulve andC. J. J. Richter.