Developpement de Leveillula taurica en Fonction des Facteurs Climatiques et Sensibilité de Capsicum annuumà Differents Stades Végétatifs

Influence of environmental factors on the development of L. taurica on pepper genotypes To our knowledge, the effects of certain environmental factors, as well as those of host genotype on the development of Leveillula taurica on pepper have not been extensively investigated. To study effects of the above factors on the development of this parasite, two Capsicum genotypes (‘Yolo Wonder' and ‘line 815’) have been tested and compared under different light and relative humidity conditions. In vitro, optimal light intensity for spore germination was situated between 20 and 80 μE/m²/s, and for the growth of germ tubes it was 20 μE/m²/s. In vivo, under continous darkness during 20 days, the different organs of the hosts were only slightly affected. Under light conditions, during 20 days after inoculation, the highest infection level on leaves was observed at 20 μE/m²/s, (60 % of leaves) while with the cotyledons it was observed between 20–80 μE/m²/s. Effects of relative humidity on the development of this parasite were also studied on ‘Yolo Wonder’ and ‘line 815′. With relative humidity below 50 %, 60 % of ‘Yolo Wonder’ plants and 33 % of ‘line 815’ were infected. Under conditions of saturated relative humidity, the level of infection was the inverse, i. e. 13 and 55 %, respectively. Moreover, ‘Yolo Wonder’ plants were found more or less susceptible to infection with L. taurica during their different development stages. At the cotyledon stage all plants were infected. After this stage, up to flowering, susceptibility depended on the physiological age of the leaves. First leaves (older leaves) were not infected, while young ones near the apex became progressively susceptible.     

The isolation and identification of rhubarb viruses occurring in Britain

Virus-like symptoms were common in British crops of rhubarb. All plants tested of the three main varieties, ‘Timperley Early’, ‘Prince Albert’ and ‘Victoria’, were virus-infected. Turnip mosaic virus and a severe isolate of arabis mosaic virus (AMV) were obtained from ‘Timperley Early’; and ‘Prince Albert’ contained turnip mosaic virus, cherry leaf roll virus (CLRV), a mild isolate of AMV and, infrequently, cucumber mosaic virus (CMV). The main commercial variety ‘Victoria’ contained turnip mosaic virus, CLRV, a mild isolate of AMV and, infrequently, strawberry latent ringspot virus (SLRV). All the viruses were identified serologically. The rhubarb isolates did not differ markedly from other isolates of these viruses in herbaceous host reactions, properties in vitro or particle size and shape. A rhubarb isolate of CLRV was distinguished serologically from a cherry isolate of the virus. Turnip mosaic virus, CLRV and SLRV, were transmitted with difficulty, but AMV isolates were readily transmitted by mechanical inoculation. Turnip mosaic virus was also transmitted to rhubarb by Myzus persicae and Aphis fabae. CLRV was transmitted in 6–8% of the seed of infected ‘Prince Albert’ and ‘Victoria’ rhubarb and in 72% of the seed of infected Chenopodium amaranticolor. Mild isolates of AMV were also transmitted in 10–24% of the seed of infected ‘Prince Albert’ and ‘Victoria’ plants.     

The distribution of poplar mosaic virus in hybrid poplars and virus detection by ELISA

Purified poplar mosaic virus (PMV) at a concentration of 8 ng/ml was readily detected by enzyme-linked immunosorbent assay (ELISA). Bioassay in Nicotiana megalosiphon was more sensitive (detecting 1–4 ng/ml) and latex flocculation less sensitive (c. 25 ng/ml) than ELISA assays. While the foliar sap of fresh, naturally-infected poplars (e.g. Populus. euramericana cv. Robusta) was not infective at dilutions greater than 2 . 10–², ELISA easily detected PMV antigen when sap was diluted 4 . 10–³ in buffer or when one part of infected tissue was triturated with 99 parts healthy leaf. Furthermore, although sap from poplar leaves stored at -20 °C for 6 months was not infective, PMV was still detectable in ELISA tests. PMV antigen in poplar leaves was not all pelleted after centrifugation for 2.5 h at 130 000 g yet parallel tests using unbuffered sap from systemically infected Nicotiana megalosiphon foliage showed that infectivity was restricted to the pellet. In poplar foliage, the concentration of PMV antigen was generally greatest where symptoms were most obvious; least antigen was detected in the overwintering leaves located at the bases of long shoots. In winter, when root and inner bark tissue in the trunk was an erratic source of PMV, the virus was readily detected in buds, the concentration being greatest in the bases, including the meristem, of terminal buds. Propagation from single node cuttings of P. euramericana cv. Regenerata allowed the selection of clones that consistently showed either ‘severe’ or ‘mild’ foliar symptoms. The associated virus isolates also infected another poplar clone causing symptoms characteristic of their source. ELISA consistently detected less PMV antigen in field-grown cv. Regenerata than in cv. Robusta foliage, but this was reversed when the associated virus isolates were propagated in Nicotiana glutinosa at 24 °C. During 6 yr, 21 out of 127 poplars at a site in Western England, became infected with PMV. By contrast, in Eastern England, none of 46 were infected. The aphids Pterocomma populea and Myzus persicae did not transmit PMV.     

Carnation Italian ringspot virus

Carnation Italian ringspot virus (CIRV) was obtained only twice in tests on several thousand carnations in Britain during 15 yr. The two isolates, from cultivars ‘Dusty Sim’ imported from Italy and ‘Orchid Beauty’ from the U.S.A., were indistinguishable serologically and in host reactions. CIRV was cultured in Nicotiana clevelandii and assayed in Chenopodium amaranti-color; it was readily transmitted by leaf-rubbing inoculation to 62 of 104 plant species tested. Virus-free carnations were infected only by injecting purified preparations into the stem, and developed chlorotic spots and oval rings in the younger leaves. CIRV was eliminated from Nicotiana clevelandii plants grown for 8 weeks at 36°C. CIRV presents no threat to carnation growing in Britain. In N. clevelandii sap, CIRV was infective at a dilution of 1/50000 to 1/100000, after heating 10 min at 85 °C (but not 90 °C), and after 16 weeks at 16 °C or 23 weeks at 2 °C. After freeze-drying, the virus survived at least 7 yr storage under vacuum at room temperature. CIRV was still infective and antigenic after treatment for 30 min at 18 °C with ultraviolet radiation (750 μW/cm²), ultrasound, 2% formaldehyde or 0.2% tri-sodium ortho-phosphate (TSP). Infectivity was not wholly abolished in 30 min by 2% TSP. The virus was readily purified by overnight maceration of N. clevelandii leaves extracted in phosphate buffer + butanol, followed by differential centri-fugation. Purified preparations contained abundant isometric particles c. 29 nm diameter, and like other serotypes of the tomato bushy stunt-pelargonium leaf curl group, gave three or four specific bands in density-gradient centri-fugation. The bands corresponded to four Schlieren peaks in analytical centrifugation. Virus from the lower bands was usually less invasive in N. clevelandii than from the upper bands, although the material in the different bands contained similar amounts of nucleic acid. Only one antigenic component was found by Immunoelectrophoresis; different serotypes of the TBSV-PLCV group differed widely in immunoelectrophoretic behaviour. The present cryptogram of CIRV is */*:*/*:S/S:S/*.     

Systemic Infection of Tobacco by Pepper Veinal Mottle Potyvirus (PVMV) Depends on the Presence of Potato Virus Y (PVY)

In single inoculations, both PVY and PVMV replicated in inoculated leaves of Nicotiana tabacum cv. ‘Xanthi nc’ plants, but only PVY infected the tobacco plants systemically, whereas PVMV caused localized infection. A mixed infection by the PVY-To72 and PVMV-type strains was experimentally realized in ‘Xanthi nc’ plants. In the presence of PVY, PVMV migrated systemically into the upper leaves of the tobacco plant, as was proved by back inoculation. It would appear that in tobacco, PVY acts as a “helper” virus, providing PVMV with the necessary component factor for migration. In extracts from the co–infected leaves. Immune Electron Microscopy (IEM) revealed phenotypic mixed particles which contained a mixture of coat proteins of PVY and PVMV. The role of the structural and functional interactions between the two viruses, which enable PVMV to migrate systemically in tobacco plants, is discussed.     

Transient expression of human papillomavirus type 16 virus-like particles in tobacco and tomato using a tobacco rattle virus expression vector

The major capsid protein L1 of human papillomavirus type 16 (HPV16) was transiently expressed in tobacco (Nicotiana benthamiana and Nicotiana tabacum) and tomato (Lycopersicon esculentum) leaves using Agrobacterium tumefaciens. The expression vector pTV00 was derived from tobacco rattle virus (TRV). The highest L1 expression 15 μg g⁻¹(f.m.) was achieved when the coding sequence of L1 was optimized for expression in humans that caused an increase of the guanine and cytosine (GC) content from 38.2 % in wild type HPV16 to 64.1 % in optimized sequence. L1 monomers readily self-assembled into capsomeres and further into virus like particles (VLPs). Immunological characterization and electron microscopy showed that 89 % of L1 retained VLP structure also in extracts prepared from freeze-dried leaves. Plant expressed L1 in crude extracts was highly immunogenic without any additional adjuvant as vaccinated mice developed strong humoral and cellular immune response, comparable to that elicited by purified VLPs derived from insect cells. Further, the induced antibodies effectively neutralized infection of 293TT cells with pseudovirions. This finding demonstrates that the TRV expression system is comparable to other plant expression systems and due to the broad host range of TRV is particularly attractive when expression in plants with low content of toxic alkaloids is desired. Moreover, a monoclonal anti-L1 antibody E2 raised in the course of immunization with crude extract from freeze-dried leaves expressing L1 is specific preferentially against HPV VLPs and could be used in direct ELISA for monitoring of VLPs assembly and VLP purification protocols.     

Antiphytovirale Verbindungen mit nichtzyklischer Azin Struktur

Antiphytoviral compounds with noncyclic azine structutre The antiphytoviral activities of variously substituted compounds with noncylic azine structures have been studied f a total of 90 tested compunds 42 had the effect of more or less strong inhibiting the concentration of potato virus X (PVX) in inoculated and / or secondarily infected leaves of Nicotiana tabacum L. cv Samsun (Table.1) An effect on the virion of PVX in vitro couldn't be observed (Table.2) Thus the substances may interact with the virus replication. Some of them also reduced the number of local lesions caused by tabacco mosaic virus on leaves of Nicotiana glutinosa L. (Table-3) Several Compounds proved exellent synergists of 2,4- dioxohexahydro- 1,3,5- triazine (DHT = 5 azadihydrouracil; Table 4) Pyridine- 3- aldehyde S ethyl isothiosemicarbazone and the Cu complex of 1- ethyl-isatine S- ethyl isothisosemicarbazone when used in combination with 2,4 dioxohexahydro- 1,3,5, triazine, greatly increased the mass of potato tubers produced from plantlets grown up from patato eye cutting as compared with the indentical control Simultaneously the mentioned substances reduced the number of symptom bearing eye cutting plants Quinoline 2- aldehyde- N- oxide S- allyl-isothiosemicarbazone had the effect of very greatly reducing the number of symptom-bearing plants without substantially influencing the mass of tubers (Table 5). Thus some compounds with noncyclic azine structure especially when used in combinitation with DHT, may be of high interest for practical application Comparing the structures of compounds with noncyclic azine structure active against plant or human viruses it should be noted that antiphytoviral compounds active in these two different virus host systems often are closely related structurally.     

The Effects of Co-Infection by Sunn-Hemp Mosaic Virus (SHMV) and Fusarium oxysporum on the Growth of French Bean

In the seedlings of three cultivars of French bean (Phaseolus vulgaris), ‘Prince,’‘Masterpiece’ and ‘Pinto’, co-infection by Sunn-hemp mosaic virus (SHMV) with either of the vascular wilt pathogens, Fusarium oxysporum f. sp. phaseoh (Fop) or f. sp. tracbeiphilum (Fot) caused greater losses in total fresh weight and in leaf area compared with uninfected plants or plants infected singly with any one of these pathogens. Co-infection of a fourth cultivar (‘Canadian Wonder’) had no greater effects on growth reduction than single infection. The concentration of SHMV in the leaves of ‘Prince’ and to a greater extent in the leaves of ‘Masterpiece’ increased more after double infection than with infection by the virus alone. The nature and possible mechanisms of the pathogenic effects in French bean are discussed.     

Viruses Associated with a Mosaic Disease of Crotalaria juncea in Nigeria

Cowpea mosaic virus was one of the viruses isolated from Crotalaria juncea showing mosaic, distortion and puckering symptoms. The virus induced local necrotic lesions in Canavalia ensiformis, Cassia occidentalis, Nicotiana debneyi, N. occidentalis, N. repanda and N. sylvestris and some cultivars of Phaseolus vulgaris 4–6 days after inoculation. It induced local chlorotic spots, which later became necrotic, as well as systemic chlorotic spots, vein chlorosis, distortionand puckering in Chenopodium amaranticolor. Sap from systemically infected cowpea cv. ‘C20-55’was infective after dilution to 10^?5 but not 10^?6, after 10 min at 65°C but not 70°C, or after 4 days, but not 5 days, at a room temperature of 15–25°C. Infectious virus was recovered from fresh reproductive tissues of infected cowpea cvs ‘C20-55’and ‘Ife Brown’ plants but not after dehydration.     

Physiology of Rice Tungro Virus Disease: Involvement of Abscisic Acid-Like Substance in Susceptible Host-Virus Interactions

Stunting was severe in susceptible rice (Oryza saliva L.) cultivar ‘Taichung Native 1’ infected with tungro virus (RTV) compared to less-susceptible cultivar ‘IR 20’. The senescence of detached leaves of RTV-infected susceptible cultivar incubated in water in dark was accelerated compared to the healthy leaves as measured by the loss of total chlorophyll content. The transpiration rate of RTV-infected leaves of the susceptible cultivar was much lower than the healthy and RTV-infected leaves of the less-susceptible cultivar. Partially purified extracts obtained from RTV-infected leaves effectively inhibited GA-induced α-amylase synthesis in barley endosperms, and rice seedling growth, and they accelerated senescence of detached rice leaves. In all the three bibassays the ABA-like activity was significantly greater in the extracts from the RTV-infected susceptible cultivar than in extracts from the less-susceptible cultivar.     

Physiology of Rice Tungro Virus Disease: Increased Cytokinin Activity in Tungro-Infected Rice Cultivars

Tungro virus infection decreased the chlorophyll content in the susceptible rice (Oryza sativa) cultivar ‘Taichung Native 1’, while it increased the pigment in the less-susceptible cultivar ‘IR 20’ during the later stages of infection. The virus was recovered from the infected susceptibile cultivar as early as 3 days after inoculation. Relatively low amounts of virus were recovered from the less-susceptible cultivar between 12 and 30 days after inoculation. The senescence of detached leaves of virus-infected less-susceptible cultivar incubated in water in darkness was considerably delayed compared to healthy leaves. The cytokinin activity in the extracts from inoculated less-susceptible cultivar was 69% higher than in the extracts from the healthy plants.     

Effect of thermal shock treatments on symptom expression in test plants inoculated with potato aucuba mosaic virus

Potato aucuba mosaic virus (PAMV) has few reliable local lesion assay hosts. However, lesions formed when PAMV-inoculated leaves were exposed to thermal shock (dipping for 40 s in water at 50 or 2 °C). Leaves of Nicotiana tabacum (cv. Xanthi-nc, Samsun or Samsun NN), Hyoscyamus niger and Datura metel consistently developed necrotic lesions, leaves of Chenopodium amaranticolor developed whitish rings, and leaves of N. glutinosa developed diffuse cream-coloured rings and spots. In PAMV-inoculated leaves of Xanthi-nc tobacco, C. amaranticolor and D. metel, lesions formed only in areas exposed to light. Thermal shocks applied to systemically infected leaves of Xanthi-nc tobacco induced necrotic vein banding patterns. In inoculated Xanthi-nc tobacco leaves, PAMV seemed confined to local lesions. The rate of lesion enlargement was therefore a measure of rate of virus spread. Lesion size increased as the interval between inoculation and shock treatment increased. The mean rates of increase in lesion radius were 17 and 27 Cμm/h at 15 and 25°C respectively. ‘Target’ lesions, composed of concentric necrotic rings, formed when inoculated Xanthi-nc tobacco leaves were given two or more 50°C shocks. The first of two 50°C treatments decreased subsequent rates of lesion enlargement.     

Transmission by Laodelphax striatellus Fallen of Rice black‐streaked dwarf virus from Frozen Infected Rice Leaves to Healthy Plants of Rice and Maize

Rice black‐streaked dwarf virus (RBSDV) is transmitted naturally to important crops such as rice, maize, barley and wheat in a persistent manner by the planthoppers, Laodelphax striatellus, Unkanodes sapporona and Unkanodes albifascia. Insect vector transmission tests are the basis for identifying viral incidence, evaluating the resistance of varieties and selecting resistance sources for rice and maize breeding. A simple, rapid and reliable method is described by which virus‐free small brown planthoppers (L. striatellus) acquired RBSDV from frozen infected rice leaves and transmitted it to healthy rice and maize plants. After feeding on frozen infected rice leaves, the planthoppers were tested by RT‐PCR for the presence of virus after 10, 15, and 22 days, respectively. The percentages of RBSDV‐containing insects were 0, 25 and 71.43% of L. striatellus fed on frozen infected rice leaves compared to 0, 28.25 and 71.43% of L. striatellus fed on fresh infected rice leaves, respectively. In transmission tests, three of eight rice seedlings (37.5%) and four of eight maize seedlings (50%) were inoculated by the planthoppers that had fed previously on frozen leaves and had allowed a 22 days latent period and showed typical disease symptoms. As a positive control, four of eight rice seedlings (50%) and four of six maize seedlings (66.67%) became infected. All rice and maize plants expressing disease symptoms were identified as virus‐positive by RT‐PCR. These results indicated that the planthoppers acquired RBSDV from frozen infected leaves and transmitted the virus to healthy plants.     

Agrobacterium rhizogenes—transformed plant roots as a source of grapevine viruses for purification

Agrobacterium rhizogenes-mediated transformation was applied toVitis spp. andNicotiana spp. infected by different grapevine phloem-limited viruses (grapevine fleck virus, grapevine virus A, grapevine virus B) to obtain root cultures for virus purification. All plant species were successfully transformed, and several clones were established in liquid culture. Transformed grapevine roots contained as much virus as non transformed roots and more than leaves, as assessed by ELISA and thin sectioning. Likewise, transformed roots ofNicotiana benthamiana Domin. contained in average more GVA than leaves, especially those at the base and the top of the plant, whereas withNicotiana occidentalis wheel., GVB was apparently less concentrated than in leaves.Nicotiana root grew faster than those ofVitis. All viruses multiplied and persisted in root cultures, which were successfully used for purification. Virus yields were the same (GFkV and GVB) or higher (GVB) than those reported in the literature. Grapevine roots may prove useful for culturing and purifying other non-mechanically transmissible grapevine viruses.     

Some hosts, properties and possible affinities of a labile virus from Hypochoeris radicata (Compositae)

Hypochoeris mosaic virus (HMV) is common in Hypochoeris radicata (‘cat's ear’) in western Canada. It infected 10 of 53 mechanically inoculated species in five of twelve families, but was not transmitted by aphids or through seed or soil. Sap from infected Nicotiana clevelandii was sometimes infective after dilution to 10^-1 and occasionally 10², after 10 min at 45 but not 50°C, and after 1 but not 2 days at 20°C. Infectivity of crude nucleic acid extracts from infected leaves was rapidly abolished by RNase but not by DNase. Host sap contained very few rod-shaped particles or particle fragments mostly 21.0–22.5 nm in diameter, and up to 420 nm long but with predominant lengths of 120–140 and 240–260 nm. Many rods in purified virus preparations were less than 240 nm long, and the majority were c. 140 nm or shorter. The particles had a helical substructure with a pitch of 2.58 nm and contained a single type of protein of estimated mol. wt 24.5 × 10³. HMV showed no serological relationship to eight morphologically similar viruses (beet necrotic yellow vein, broad bean necrosis, barley stripe mosaic, peanut clump, potato mop-top, Nicotiana velutina mosaic, wheat soil-borne mosaic and defective strains of tobacco mosaic). It is probably a hitherto undescribed tobamovirus.     

Aphid behavioral responses to virus‐infected plants are similar despite divergent fitness effects

We compared the settling preferences and reproductive potential of an oligophagous herbivore, the pea aphid, Acyrthosiphon pisum Harris (Hemiptera: Aphididae), in response to pea plants, Pisum sativum L. cv. ‘Aragorn’ (Fabaceae), infected with two persistently transmitted viruses, Pea enation mosaic virus (PEMV) and Bean leaf roll virus (BLRV), that differ in their distribution within an infected plant. Aphids preferentially oriented toward and settled on plants infected with PEMV or BLRV in comparison with sham‐inoculated plants (plants exposed to herbivory by uninfected aphids), but aphids did not discriminate between plants infected with the two viruses. Analysis of plant volatiles indicated that plants inoculated with either virus had significantly higher green leaf volatile‐to‐monoterpene ratios. Time until reproductive maturity was marginally influenced by plant infection status, with a trend toward earlier nymph production on infected plants. There were consistent age‐specific effects of plant infection status on aphid fecundity: reproduction was significantly enhanced for aphids on BLRV‐infected plants across most time intervals, though mean aphid fecundity did not differ between sham and PEMV‐infected plants. There was no clear pattern of age‐specific survivorship; however, mean aphid lifespan was reduced on plants infected with PEMV. Our results are consistent with predictions of the host manipulation hypothesis, extended to include plant viruses: non‐viruliferous A. pisum preferentially orient to virus‐infected host plants, potentially facilitating pathogen transmission. These studies extend the scope of the host manipulation hypothesis by demonstrating that divergent fitness effects on vectors arise relative to the mode of virus transmission.     

Roditis Leaf Discoloration - A New Virus Disease of Grapevine: Symptomatology and Transmission to Indicator Plants

The symptoms and some characteristics of an unreported disease of grapevine, which was observed during the last years on the cv. ‘Roditis’ in central Greece, are described. The disease is transmissible to V. vinifera‘Mission’ and to some herbaceous test plants. The constant association of a virus to naturally infected plants cv. ‘Roditis’ and chip-budding infected indicator plants cv. ‘Mission’ supports the evidence that the disease is caused by a virus.     

Purification and properties of closterovirus-like particles associated with a whitefly-transmitted disease of sweet potato

A virus causing sunken veins on ‘Georgia Jet’ sweet potato, and yellow brittle leaves and stunting on Ipomoea setosa, was purified and a specific antiserum was prepared. Flexuous particles with a normal length of 850 nm and a diameter of 12 nm with an open helical structure typical of closteroviruses were observed. The virus particle protein has an apparent mol. wt of c. 34 kD. Double-stranded RNA isolated from SPSVV-infected I. setosa and subjected to electrophoresis in agarose consisted of one major band with an estimated M_r of 10.5 kbp and two minor bands with M_r of 9.0 and 5.0 kbp. Fibril-containing vesicles in phloem cells were observed in ultrathin sections of infected leaf tissues. The virus was transmitted by the whitefly Bemisia tabaci in a semi-persistent manner and by grafting, but not mechanically. The virus could be transmitted to various Ipomoea species, to Nicotiana clevelandii, N. benthamiana and Amaranthus palmeri. The virus did not react with an antiserum to lettuce infectious yellows virus. Based on particle morphology, serology and symptom expression, the virus appears unique and different from all other reported whitefly-transmitted closteroviruses. We propose it be named “sweet potato sunken vein virus” (SPSVV).     

Hemmung von Pflanzenviren durch Membranlipidanaloga unter besonderer Berücksichtigung von Alkanmonosulfonaten

Inhibition of plant viruses by membrane lipid analogs under special regard of alkane monosulfonates Under investigation was the inhibition of plant viruses by compounds, which could be comprehended as membrane lipid analogs because of some structural similarities to these lipids. Among these analogs, alkane monosulfonates (AMS, preparation “Emulgator E 30”) exhibit distinctly marked antiphytoviral activities, which are reported in detail. The mentioned AMS reduced the concentrations of PVX (potato virus X) as well as of TMV (tobacco mosaic virus) markedly in inoculated leaves of Nicotiana tabacum L. ‘Samsun’ and remarkably in secondarily infected ones. The formation of local lesions on leaves of hypersensitive hosts is influenced strongly by AMS. Likewise, the activity of the free virion is reduced in vitro. The mentioned effectsproved dependent on the length of the alkane chains and the concentration of AMS. Innertherapeutical effects of AMS have been pointed out. Several substances proved excellent coergists of AMS and vice versa, above all DHT (2,4-dioxohexahydro-1,3,5-triazine), several guanidines, a thiourea compound, a thiadiazole compound and ribavirin. In part full synergistic effects were obtained. AMS as well as combinations of AMS with some of the mentioned coergists also influence potato viroses. Using so-called identical potato eye cuttings, it was demonstrated that the mentioned treatments resulted in a significant and in part considerable reduction of the number of potato eye cutting plants with virus symptoms and in a partially high increase in the weight of tubers formed by the eye cutting plants. Likewise, the number of plants with virus symptoms was reduced and the tuber mass was increased by newly developed granulated AMS. A combined treatment with granulated AMS and granulated DHT resulted in a strong reduction of the number of potato eye cutting plants with virus symptoms, but the tuber mass was not augmented.     

A strain of cassava latent virus occurring in coastal districts of Kenya

A strain of cassava latent geminivirus (CLV) was isolated from mosaic-affected cassava plants from coastal districts of Kenya. This virus (CLV-C) did not infect Nicotiana clevelandii, a diagnostic host of the type strain (CLV-T); experimental host range was very restricted and CLV-C only infected N. benthamiana and N. rustica out of several solanaceous hosts readily infected by CLV-T. CLV-C was also isolated from naturally infected Jatropha multifida (Euphorbiaceae) and Hewittia sublobata (Convolvulaceae). CLV-C was propagated in N. benthamiana with difficulty and only those isolates derived from cassava plants infected with severe mosaic symptoms were maintained more or less successfully; these sources usually contained a higher concentration of CLV than plants with mild symptoms. Symptom variants generally remained unchanged when grafted into a highly susceptible South American cassava variety. CLV-C and CLV-T seemed to occur respectively only in coastal and western districts but their ranges overlapped in central Kenya where they could have been introduced in infected material. CLV-C could be purified satisfactorily with the method used for CLV-T but only after modifying the procedure by substituting phosphate for borate in the extraction buffer, n-butanol for n-butanol/chloroform in clarification of extracts, and phosphate for borate buffer when resuspending concentrated virus. A virus serologically indistinguishable from CLV-T was isolated from mosaic- affected material obtained from Nigeria; East African and Nigerian isolates were essentially similar in host range and symptomatology. In gel-diffusion serology tests, pronounced precipitation spurs developed between CLV-T and CLV-C indicating that the isolates were related but not identical serologically. Symptoms typical of cassava mosaic disease appeared in only three of 105 plants in experiments on transmission of CLV-C and CLV-T by whiteflies, when attempted acquisition of either clarified CLV-infective sap or purified CLV was made through ‘Parafilm’ membranes. Because it is possible that the three infections resulted from contamination, they cannot constitute proof of transmission. The presence of CLV in relation to the etiology of cassava mosaic thus remains unresolved.