Reaction to the potato leafroll virus (PLRV) in diploid potatoes

Summary Diploid parents with some resistance to PLRV, were intercrossed to give 3 families with 191 clones which were evaluated for reaction to PLRV and yielding ability. After inoculation with PLRV the clones could be separated into those: 1) resistant, 2) susceptible, 3) intolerant, reacting with low virus concentration, 4) tolerant and 5) intermediate in reaction. Both the ELISA test and the evaluation of external disease symptoms were necessary to separate the clones. No correlation was found between resistance to PLRV and tuber yielding ability.     

Transmission of potato virus Y by cereal aphids

Summary The cereal aphidRhopalosiphum padi transmitted PVY^O and PVY^N from and to both tobacco and potato. However, it transmitted much less frequently thanMyzus persicae, even when aphids making single probes were compared.R. padi andM. persicae retained PVY for a similar duration. Both alatae and apterae ofR. padi transmitted infrequently, as also did progenies of individual aphids that had transmitted PVY.Metopolophium dirhodum also transmitted PVY infequently butSitobion avenae did not transmit.

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Zusammenfassung In Laborversuchen übertrugen ca, 1% der Getreideblattl?useRhopalosiphum padi undMetopolophium dirhodum, nicht jedochSitobion avenae das Kartoffel-Y-Virus (PVY). 65% vonMyzus persicae übertrugen PVY.R. padi übertrugauch dann weniger h?ufig alsM. persicae, wenn die Aphiden bei Einzelentnahmen untersucht wurden.R. padi übertrug PVY^O und PVY^N von und auf die Tabaksorte White Burley und die Kartoffelsorte King Edward. Zwei oder zehn Minuten Einwirkung waren optimal fürM. persicae, um PVY zu erlangen, w?hrendR. padi PVY h?ufiger nach l?ngeren Entwirkungszeiten übertrug (Tabelle 1).R. padi undM. persicae beinhalteten PVY für eine ?hnliche Zeitdauer (Tabelle 2). Sowohl alatae als auch apterae vonR. padi übertrugen unregelm?ssig, ebenso Nachkommen von individuellen Aphiden, welche PVY übertragen hatten.

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Résumé En tests de laboratoire, environ 1% des pucerons des céréalesRhopalosiphum padi etMetopolophium dirhodum, aucun deSitobion avenae et environ 65% deMyzus persicae transmettent PVY.R. padi transmet également moins fréquemment queM. persicae lorsque les pucerons sont mis en comparaison par piq?re unique.R. padi acquiert PVY^O et PVY^N à partir des variétés de tabac White Burley et de pomme de terre King Edward et les transmet à ces deux mêmes variétés. Deux ou 10 minutes d'acquisition sont optimum pour queM. persicae acquiert PVY tandis queR. padi transmet PVY plus fréquemment après une plus longue période d'acquisition (tableau 1).R. padi etM. persicae conservent PVY dans les mêmes délais (tableau 2). Les ailés et les aptères deR. padi transmettent rarement PVY comme les descendants de pucerons ayant transmis PVY.

     

Transmission of potato virus Y by aphids trapped in potato crops in southern England

Summary Harrington et al. (1986) assessed the relative efficiency with which different aphid species transmitted potato virus Y (PVY), by trapping flying aphids on a net downwind of a plot of infected potatoes and transferring the aphids to tobacco seedlings. This paper reports the extension of that work over further seasons and identifies three more vector species,Aphis sambuci, Myzus cerasi andDysaphis sp. A method for determining the proportion of those aphids caught on the net that had landed and probed in the crop is suggested. The use of such information to improve a model of virus spread is discussed.     

The infection pressure of potato leafroll virus and potato virus Y in relation to aphid populations in Cyprus

Summary The infection pressure of two viruses, potato leafroll (PLRV) and potato virus Y (PVY), both common in seed potatoes grown in Cyprus, was determined in three experiments in 1982–83. Virus-free bait plants, of potato and four other species, were exposed weekly to field infection during the growing season (March–June), and then returned to an aphid-free glasshouse for symptom expression. Only tobacco plants produced clear symptoms enabling reliable assessment of PVY infection pressure. When assessed with ELISA or by tuber indexing, the potato plants were efficient baits for both viruses whose infection period commenced at emergence (mid March to early April) and ended within 6–7 weeks. The seasonal trend of aphid populations, determined with Moericke traps or 100-leaf counts, correspond to that of virus spread. Correlation and regression analysis of aphid and virus data implicated the alate form ofMyzus persicae as the principal vector of both viruses.     

Acquisition of potato leafroll virus byMyzus persicae from secondarily-infected potato plants of different genotypes

Summary The acquisition of potato leafroll virus (PLRV) byMyzus persicae nymphs from the top leaves of potato plants was studied throughout a growing season in relation to the antigen titre in those leaves and the feeding behaviour of the aphid. Secondarily-infected plants of eight potato genotypes with different levels of field resistance served as virus sources. Early in the growing season, plants were efficient sources for virus acquisition. The amount of viral antigen detected inM. persicae nymphs fed on the top leaves was strongly correlated with the titres of viral antigen in these leaves. Virus acquisition from the top leaves of older potato plants was markedly impaired and could not be correlated with their virus titre. With increasing age of the potato plants and the development of virus symptoms, the virus titre in the leaves declined and the initial weak correlation between the virus titre and field resistance ratings disappeared. Thus, screening secondarily-infected potato plants for field resistance to PLRV based on the concentration of viral antigen in leaves or in aphids fed on them should be avoided later in the growing season. The feeding rate ofM. persicae, measured by the number of honeydew droplets excreted, did not account for the reduced uptake of virus from older plants since it was not influenced by the age of the plant. Throughout the growing season, the feeding rate ofM. persicae nymphs on PLRV-infected plants was higher on genotypes with low levels of field resistance to PLRV than on genotypes with high ones.     

Host genes and transgenes that confer resistance to a Scottish isolate of potato leafroll virus are also effective against a Peruvian isolate

Summary Potato genotypes with host gene-mediated resistance (host-MR) and coat protein-mediated transgenic resistance (CP-MR) to potato leafroll virus (PLRV), were inoculated with a Scottish and a Peruvian isolate of PLRV. The coat protein transgene had been cloned from the Scottish PLRV isolate which had also been used during the screening and selection of genotypes with host resistance. Significantly less PLRV accumulated in plants with either host-MR or CP-MR than in plants of susceptible genotypes or in non-transformed control plants, but the two forms of resistance were equally effective against both PLRV isolates.     

Evidence that resistance to potato leafroll virus accumulation in tetraploidsolarium tuberosum L. is controlled by one or few major genes that are not complementary

Summary Previous investigations into the inheritance of resistance to accumulation of potato leafroll virus indicated a dominant major-gene effect, but the segregation ratios in progenies of crosses were a closer fit to a model involving two complementary genes (both required for resistance and one present in the susceptible parent cultivar, Maris Piper) than to a single gene model. In this study, we tested the complementary gene hypothesis by backcrossing susceptible segregants from one of these progenies to their susceptible parent, Maris Piper. No resistant segregants were found in the five backcross progenies examined, so the complementary gene hypothesis was not supported. There was significant variation between susceptible progeny-members in these backcrosses. The progeny of another, highly resistant parent clone, G. 8107(1), selfed, was also examined: all members were resistant. Whilst there is evidently a dominant major-gene effect involved, this is not the whole picture and there are other unidentified genetic effects.     

Effect of applying nitrogen fertilizer to a potato seed crop on the susceptibility of the daughter plants toErwinia carotovora subsp.atroseptica

Summary Seed potatoes of cvs Record and Bintje were grown with 30, 80 or 130 kg N/ha. In the subsequent growing season the seed tubers, which were inoculated or not with two concentrations ofErwinia carotovora subsp.atroseptica (Eca), were subjected to the same fertilization treatment. The incidence of blackleg and the level of seed tuber decay increased from the uninoculated treatment to the higher inoculum level, but the degree varied considerably from one year to another. Cv. Bintje was more susceptible to blackleg and yield loss caused by Eca than cv. Record. In 1990, when disease incidence was high, the proportion of blackleg stems increased with increasing doses of nitrogen fertilizer which had been applied to the seed crop, but was not significantly affected in 1988 and 1992. Seed tuber decay was retarded by the lowest nitrogen dose. The yield of the subsequent crop was not affected by the nitrogen dose applied to the seed crop.     

Sporulation ofPhytophthora infestans (Mont.) de Bary on the surface of diseased potatoes and tuber to tuber spread of infection during handling

Summary Laboratory experiments and observations in commercial stores showed that viable sporangia ofPhytophthora infestans can be produced on infected potato tubers of different cultivars. Infection was also found to spread from diseased to healthy tubers during handling.     

The feasibility of predicting the number of stems per tuber produced by potato seed stocks from measurements made before planting

Summary The number of stems per seed tuber produced by 17 seed stocks of cv. Record was determined in glasshouse and field experiments to assess the feasibility of predicting the number of stems produced in the field. Multiple regression analysis showed that seed tuber weight alone gave the most satisfactory fit to stems produced in the field, which was not improved by adding terms involving the number of stems produced in the glasshouse. Quadratic relationships between the number of above-ground stems per tuber and tuber weight were most appropriate for each stock, with the fitted curves for individual stocks differing only in the constant term. However, there were significant rank correlation coefficients between the constant terms for relationships between field stems and tuber weight and glasshouse stems and tuber weight, suggesting that in other cultivars and seed stocks a predictive glasshouse test might still be useful.     

Penetration of potato tuber lenticels by bacteria in relation to biological control of blackleg disease

Summary In a glasshouse experiment three potato cultivars were grown to maturity in wet or dry compost. The number of lenticels per tuber was greater with larger tubers but was unaffected by the cultivar or soil moisture regime. The frequency of lenticel penetration by a water soluble stain, safranin O, remained high as tubers developed to maturity in wet soil. In dry soil penetration declined markedly as the tubers matured. When tubers, harvested just before the onset of haulm senescence, were inoculated withErwinia carotovora ssp.atroseptica, the frequency of lenticel rotting was less than the rate of stain penetration, indicating that additional factors were involved in the bacterial infection process. Inoculating mature tubers with bacteria and incubating them in anaerobic conditions frequently resulted in rotting of the lenticels. Comparable tubers incubated in aerobic conditions showed no rotting or invasion by bacteria. The results are discussed in relation to the biological control of blackleg.     

The response of potatoes to a single transient heat or drought stress imposed at different stages of tuber growth

Summary The effects of a single treatment of heat or drought stress imposed at different stages of tuber growth, were investigated in two potato cultivars, Désirée and Cara. Both stresses impaired dry matter accumulation in tubers, and tuber yields. Stress imposed at early stages of tuber growth, when tubers were less than 20 mm in diameter, had little effect on dry matter accumulation, whereas stress imposed later resulted ingnificant losses of dry matter in tubers. Stress imposed during later tuber growth increased the percentage of tubers sprouting in the soil. Tuber malformation was less affected. The adverse effects of heat on tuber yield and quality seemed to be greater than those of drought. Drought increased the content of total soluble solids (TSS) and lowered the osmotic potential in the tuber tissue of both cultivars. These effects were detectable even 55 days after stress relief. Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel No 984-E, 1984 Series. This investigation was sponsored by a grant from the Ministry of Foreign Affairs, Technical Assistance Department, Netherlands.     

Use of the polymerase chain reaction to clone the potato leafroll virus coat protein gene directly from the total RNA of infected plants

Summary The potato leafroll virus (PLRV) coat protein (CP) gene was directly cloned from the total RNA extracted from virus-infected plants. First strand cDNA synthesis was not necessarily specific; it was equally efficient using either random or CP-specific primers. The viral sequence encoding the coat protein was specifically amplified from the total population of cDNA molecules by polymerase chain reaction (PCR), using specific primers bordering the CP gene. The unique amplified product thus obtained was cloned blunt-end into the pT7T318U plasmid vector, and the authenticity of the cloned gene verified by sequence analysis. This cloning strategy obviates the need for virus purification. Sequence comparison of the CP gene of the Italian isolate and those of five other PLRV isolates revealed a close similarity to the three European and the Canadian isolates, and a more distant relationship with the Australian one.     

Comparative detection of mild strains of potato spindle tuber viroid from the dormant potato tubers by Return-polyacrylamiee gel electrophoresis and nucleic acid hybridization

Summary Using a modified procedure of Return-polyacrylamide gel electrophoresis (R-PAGE) mild strains of potato spindle tuber viroid (M-PSTV) were detected reliably from dormant tubers. The sensitivity of R-PAGE detection of M-PSTV was equivalent to that of nucleic acid hybridization. Both methods detected M-PSTV when infected tissue was mixed with healthy tissue in a ratio of 1 to 100. When extracted nucleic acid was diluted with buffer, R-PAGE detected PSTV at a dilution of 1:256 and nucleic hybridization only up to 1:64 PSTV was readily detected from 18 potato cultivars. In addition, mild, ‘intermediate’ and severe strains were separated by R-PAGE, on the basis of their mobility on the electrophorogram.     

Oxygen and carbon dioxide treatment to break potato tuber dormancy for reliable detection of potato virus Y (PVY) by enzyme-linked immunosorbent assay (ELISA)

Summary Dormant tubers of the potato cv. Bintje were treated for 7 or 14 days in an atmosphere enriched to either 40% O₂ plus 1% CO₂, or 40% O₂ plus 20% CO₂, or they were stored in closed plastic bags for identical periods. Rindite-treated and untreated tubers served as references. Treatment with 40% O₂ plus 20% CO₂ for 7 days was nearly as efficient as rindite for inducing sprouting. Both the O₂ plus CO₂ treatments, for 7 and 14 days, considerably increased virus concentration in tubers and had an effect similar to that of rindite 40 days after treatment, but the plastic bag treatment was not as efficient. It is concluded that O₂ plus CO₂ enriched atmospheres could be used for breaking tuber dormancy in order to detect reliably PVY in tuber extracts.     

Progeny testing for resistance to potato virus Y: a comparison of susceptible potato cultivars for use in testcrosses with resistant parents

Summary For potato breeding or genetical research purposes, the number of copies of a dominant major gene for resistance to potato virus Y in a parent clone can be determined by test-crossing with a susceptible parent and observing the segregation ratio of resistant to susceptible seedlings in the progeny. In a comparison of different susceptible cultivars for use in test-crosses for this purpose, their progenies differed in the clarity of symptoms and parental phenotype proved an unreliable guide. Cvs Maris Piper, Arran Peak and Dr MacIntosh were found to be suitable tester parents, Pentland Squire less so and cv. Désirée was confirmed as having a major gene conferring incomplete resistance. It was found advisable to test-cross with more than one susceptible parent.