Epidemiology of beet necrotic yellow vein virus in sugar beet at different initial inoculum levels in the presence or absence of irrigation: Dynamics of inoculum

Using field plots where rhizomania had not previously been detected, different inoculum levels of beet necrotic yellow vein virus (BNYVV) were created by application of infested soil. A susceptible sugar beet cultivar (cv. Regina) was grown for two consecutive years (1988 and 1989), in the presence or absence of drip irrigation. In soil samples taken in spring 1989, the different initial inoculum levels of 1988 could be distinguished using a quantitative bioassay estimating most probable numbers (MPNs) of infective units per 100 g dry soil. The first sugar beet crop resulted in a tenthousandfold multiplication of inoculum of BNYVV (viruliferousPolymyxa betae). Mean MPNs of BNYVV ranged from 0.6 and 7 per 100 g soil for the lowest inoculum level to 630 and 1100 per 100 g for the highest level, in plots without and with irrigation, respectively. In spring 1990, MPNs had again increased. In both years, the initial inoculum level of 1988 had a significant linear effect on log-transformed MPNs of BNYVV determined. Log-transformed MPNs for 1990 and 1989 showed a positive linear correlation, despite a decreasing multiplication ratio at higher inoculum levels. Drip irrigation during one or two years enhanced the increase in MPN of BNYVV, which was reflected by the enhancement of multiplication ratios at all inoculum levels. The totalP. betae population was also higher after growing two irrigated crops than after growing two non-irrigated ones.     

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.     

Dissemination of rhizomania by soil,beet seeds and stable manure

Polymyxa betae, the vector of beet necrotic yellow vein virus (BNYVV), (the causal agent of rhizomania of sugar-beet), forms cystosores which are very persistent and might be dispersed by soil, beet seeds, plant material and stable manure. Research has been carried out into the risk of dissemination; relative importance was not determined. Inoculation with diseased soil in a field caused rhizomania in sugar-beets within one year. This implies that even small amounts of soil adhering to plant roots constitute a potential danger. Direct transmission of BNYVV by sugar-beet seeds could not be demonstrated, but, after processing and cleaning seed lots originating from infested fields, the seed waste proved to be contaminated. Cystosores ofP. betae and, to a lesser extent, BNYVV could pass through the intestine of sheep in fodder experiments carried out with heavily infested sugar-beet tails.Samenvatting De vector van het bieterhizomanievirus,Polymyxa betae, vormt zeer persistente ruststructuren (cystosoren) die verspreid worden met grond, bietezaad, plantmateriaal en stalmest.Onderzoek is uitgevoerd naar de risico's van verspreiding, maar door het ontbreken van kwantitatieve methoden kon de relatieve belangrijkheid niet worden vastgesteld.Wanneer een gezond perceel werd besmet met geïnfecteerde grond (20 ng dm^–3 op bouwvoor), werd binnen één jaar aantasting door rhizomanie waargenomen. Dit wijst erop dat geringe hoeveelheden grond, die meekomen met wortelmateriaal van plantgoed, een potentieel gevaar vormen.Directe overdracht van het bieterhizomanievirus door bietezaad kon niet worden aangetoond, maar na het schonen en bewerken van zaadpartijen afkomstig van zieke percelen bleek dat schoningsafval en in het bijzonder de grondfractie daarvan, wel was besmet.Cystosoren vanP. betae en in mindere mate het bieterhizomanievirus konden het maag-darmkanaal van schapen passeren, hetgeen werd aangetoond in enkele voederproeven, uitgevoerd met zwaar besmette suikerbietestaartjes.     

Epidemiology of beet necrotic yellow vein virus in sugar beet at different initial inoculum levels in the presence or absence of irrigation

A field experiment was set up in 1988 to study the development of rhizomania disease of sugar beet at different inoculum levels of beet necrotic yellow vein virus (BNYVV) in soil. Five, tenfold different, inoculum levels were created by addition of the approximate amounts of 0, 0.5, 5, 50 and 500 kg infested soil per ha (the latter corresponding to 0.01% v/v calculated to the tillage layer). A drip irrigation treatment was applied to study the influence of soil moisture on disease. Susceptible sugar beet, cv. Regina, was grown for three consecutive years.In the first year, root symptoms were not observed, but BNYVV-infected plants were detected by ELISA in low numbers at all inoculum levels at harvest. After late drilling in 1989, high numbers of infected plants, up to 90–100% in plots with the highest inoculum level, were detected already in June. Root symptoms were also observed from June onwards. In both these years disease incidence increased in time and was significantly influenced by the initial inoculum level. In the third year, the whole field was heavily diseased, and only for the non-irrigated plots incidence differed for different initial inoculum levels. The expression of symptoms by BNYVV-infected plants was influenced by initial inoculum level, thus by the amount and timing of primary infection.Root weight at harvest was not affected, but sugar content decreased with increasing inoculum level already in 1988, leading to a reduction in sugar yield of 10% at the highest inoculum level. In 1989, both root weight and sugar content decreased progressively with increasing inoculum level, resulting in sugar yield reductions of 11–66% (down to approximately 3000 kg ha^–1) for low to high inoculum levels compared to the control. As the control plots became contaminated, all yields were low in 1990, still showing a decrease with increasing inoculum level in the non-irrigated plots, but an overall mean sugar yield of 3323 kg ha^–1 for the irrigated ones.Sodium and -amino nitrogen content of the root, additional quality parameters determining extractability of sucrose, showed an increase and decrease, respectively, with increasing initial inoculum level already in the first year. The relative differences in contents compared to those from the control were largest for Na content. A significant negative correlation was found between Na (mmol kg^–1 root) and sugar content (% of fresh weight); linear for 1988, exponential for 1989 and 1990.In spring 1989, the infestation of individual plots was assessed using a quantitative bioassay estimating most probable numbers (MPNs) of infective units of BNYVV per 100 g dry soil. The relationship between the MPns determined and root weight, sugar content and sugar yield at harvest could be described by Gompertz curves. The increase in disease incidence with increasing MPN in 1989 was adequately fitted with a logistic equation.     

Breaking of <Emphasis Type="Italic">Beet necrotic yellow vein virus</Emphasis> resistance in sugar beet is independent of virus and vector inoculum densities

Beet necrotic yellow vein virus (BNYVV) is transmitted by Polymyxa betae to sugar beet, causing rhizomania disease. Resistance-breaking strains of BNYVV, overcoming single (Rz1) or double (e.g. Rz1  +  Rz2) major resistance genes in sugar beet have been observed in France and recently in the USA and Spain. To demonstrate if resistance-breaking is dependent on inoculum density, the inoculum concentration of BNYVV and P. betae in soil samples where resistance-breaking had been observed was estimated using the most probable number (MPN) method. The MPN-values obtained displayed highly significant differences with respect to the virus concentration in various soils and did not correlate with the ability to overcome resistance. Virus quantification in susceptible plants demonstrated that soils containing resistance-breaking isolates of BNYVV did not produce higher virus concentrations. The MPN assay was repeated with Rz1  +  Rz2 partially-resistant sugar beets to see if the resistance-breaking is concentration-dependent. There was no correlation between soil dilution and increased virus concentration in Rz1  +  Rz2 plants produced by BNYVV resistance-breaking strains. Determination of the absolute P. betae concentration by ELISA demonstrated that all resistance-breaking soil samples contained elevated concentrations. However, the calculation of the proportion of viruliferous P. betae did not show a positive correlation with the resistance-breaking ability. Finally resistance-breaking was studied with susceptible, Rz1 and Rz1  + Rz2 genotypes and standardised rhizomania inoculum added to sterilised soil. Results from these experiments supported the conclusion that resistance-breaking did not correlate with virus concentration or level of viruliferous P. betae in the soil.     

Effect of sugar beet cultivars with different levels of resistance to beet necrotic yellow vein virus on transmission of virus byPolymyxa betae

The effect of resistance of sugar beet cultivars to beet necrotic yellow vein virus (BNYVV) on virus content of resting spore clusters of the vectorPolymyxa betae was studied in controlled environments and in naturally infested fields. The total number of resting spore clusters formed in roots of a partially resistant and a susceptible cultivar did not differ when assessed 6 and 12 weeks after inoculation with viruliferous resting spores. Transmission experiments showed that in partially resistant plants, having a low virus content in the roots, the population of resting spores formed was less viruliferous than that in susceptible plants with a high virus content. Consequently, growing a resistant cultivar can be expected to delay the build-up of virus inoculum in soil.In a trial field sampled in 1991, the inoculum potential of BNYVV (most probable number of viruliferousP. betae propagules) in soil was lower after growing a partially resistant cultivar than after growing a susceptible one. On the other hand, in four sites sampled in 1990, inoculum potential in soil was hardly increased by growing sugar beet and was not significantly affected by the cultivar grown.     

Epidemic progress of beet necrotic yellow vein virus: Evidence from an investigation in Japan spanning half a century

Beet necrotic yellow vein virus (BNYVV) is the causal agent of rhizomania, the most serious sugar beet disease worldwide. Since the first finding in Japan in 1969, BNYVV became widespread throughout Hokkaido in a few decades and led to the introduction of Rz1-resistant sugar beet cultivars in the 1990s. Here, we report the historical progress of the BNYVV epidemic in Hokkaido from 1969 to 2019. Previous analysis on samples from 1991 showed that BNYVV isolates were classified into three strains (named O, D, and T) based on the RNA3-encoded p25 gene. The O-type viruses were widely detected in Hokkaido, while the D- and T-type viruses were detected in limited areas. The RNA5, encoding the p26 gene, was initially contained in some D- and O-type isolates but not in any T-type isolates. Interestingly, recent sample analysis revealed that RNA5-containing T-type viruses, seemingly more virulent than the other two strains, were widely detected in Hokkaido. Additionally, a small group of virus isolates harbouring a new p25 gene (named C) was found in limited areas. These results suggest that the T-type viruses, which accompanied RNA5, have been preferentially spread from a limited area to other districts over the last few decades and that this spread might be strongly associated with the recent introduction of Rz1-resistant sugar beet cultivars. BNYVV-positive samples also contained mainly beet soil-borne virus and traces of beet virus Q, both of which are the first to be recorded in Japan.     

Fluorescent staining of resting spores of Polymyxa betae as a fungal vector of rhizomania disease of sugar beet in soil

When the resting spores of Polymyxa betae were pretreated with 2% sodium dodecyl sulfate (SDS) and then stained with various fluorochromes including 3,3′-dihexyloxacarbocyanine iodide [DiOC6(3)], calcofluor, and a fluorescein isothiocyanate (FITC)-conjugated lectin, such as wheat germ lectin (WGA) or caster bean lectin, most spores fluoresced brightly. FITC-WGA mainly stained the cell surface, while DiOC6(3) stained the cytoplasm. After pretreatment with SDS and addition of FITC-WGA or DiOC6(3) to a soil suspension containing resting spores, the resting spores were distinguishable from the soil particles. This staining method is easy to use for the detection of resting spores in the soil.     

Effect of conditions during storage of infested soil on infection of bait plants by Polymyxa betae and beet necrotic yellow vein virus

Infectivity of resting spores ofPolymyxa betae in soil stored air-dry or moist was determined by assessing infection of bait plants that were exposed to the soil. Storage of soil under air-dry conditions at room temperature resulted in a delayed onset of germination of resting spores compared to germination in soil stored under moist and cool conditions, as inferred from the infection of the bait plants. Bait plants had to be exposed for more than 12 h to flooded infested soil before germination and infection had occurred. However, when soil was prewetted for 24 h before exposing bait plants, germination, infection and transmission of beet necrotic yellow vein virus (BNYVV) were accomplished within 12 h, but only with the moistly stored soil. When resting spores isolated from roots were stored for 4 and 8 weeks under dry conditions at 22°C, germination of viruliferous spores, as measured by detection of BNYVV in bait plants exposed for 48 h to the spores, was less than that of spores stored in moist soil at 22°C. Approximately 100% of bait plants were infected after exposure to resting spores that were frozen in demineralized water or stored cool (5°C) in water or moist soil for 42 weeks. Air-dry cool storage for 42 weeks resulted in a low percentage of infection. Storage conditions of soil influence the results of bioassays for detection of rhizomania when short baiting periods are applied, whereas differences in infectivity were not detected using a bioassay with long duration.     

The development of rhizomania in two areas of the Netherlands and its effect on sugar-beet growth and quality

Since detection of beet necrotic yellow vein virus (BNYVV) by serological techniques was not possible 10 years ago, there is no relaible information about the occurrence of the disease in the Netherlands prior to 1983. By evaluating historical information on the sugar production of fields, which have been proved to be infested by rhizomania recently, the time of occurrence of the first infections could be estimated roughly.Analysis of root samples from diseased fields and comparison with uninfected fields, revealed a correlation between sugar and sodium content and the occurrence of rhizomania. Therefore both quality parameters can be used as indicators for the disease under Dutch conditions.Differences between infected and uninfected fields were detected from mid-June onwards. Only a weak correlation between sugar and -amino N content could be established. At later stages of crop growth the -amino N level of diseased fields was always lower than that of healthy ones. However sugar content remained at a low level and did not increase substantially.Samenvatting Aangezien tien jaar geleden geen serologische technieken beschikbaar waren, kon het voorkomen van bieterhizomanie in Nederland niet worden aangetoond. Door het beoordelen van de suikeropbrengsten van percelen die met bieterhizomanie besmet bleken te zijn, over een reeks voorafgaande jaren, kon een periode waarin de infectie vermoedelijk had plaatsgevonden worden vastgesteld.Door analyse en vergelijking van bietmonsters van een reeks aangetaste proefplekken te een aantal gezonde referentiepercelen, kon worden aangetoond dat onder Nederlandse omstandigheden suiker- en Na-gehalten de meest betrouwbare indicatoren voor de aanwezigheid van bieterhizomanie zijn.Afwijkingen in de gehalten van deze kwaliteitsparameters werden in de zwaar aangetaste percelen vanaf half juni gevonden. Er werd een zwakke relatie tussen de gehalten aan suiker en -amino N aangetoond. Gedurende de tweede helft van de zomer was het gehalte aan -amino N in de zieke percelen lager dan dat van bieten in de gezonde. Het suikergehalte bleef op een laag niveau en vertoonde geen of een geringe toename.     

Identification of rhizomania-infected soil in Europe able to overcome <Emphasis Type="Italic">Rz1</Emphasis> resistance in sugar beet and comparison with other resistance-breaking soils from different geographic origins

Rhizomania, caused by Beet necrotic yellow vein virus (BNYVV), is vectored by Polymyxa betae. The disease can only be controlled by growing partially resistant sugar beets, which quantitatively reduce virus replication and spread. None of the known major resistance genes (Rz1, Rz2, Rz3), alone or in combination, are able to prevent BNYVV infection entirely. Here we report for the first time the identification of a Spanish soil, containing an A-type BNYVV with RNA 1-4, displaying Rz1 resistance-breaking abilities comparable to soils from the USA and to those from France containing the French (Pithiviers) P-type BNYVV with RNA 5. A resistance test with several soil samples vs. different sugar beet cultivars was conducted under standardised conditions. Sugar beets were analysed after 12 weeks of greenhouse cultivation for taproot weight, BNYVV and relative P. betae content. The soil samples from Spain, France and the USA produced high virus contents and strong rhizomania symptoms in Rz1 plants, indicative of resistance-breaking abilities. In addition, all resistance-breaking soil samples produced detectable virus concentrations in plant lateral roots of the Rz1 + Rz2 cultivar, and plants grown in the Spanish soil sample also had reduced taproot weight and displayed severe rhizomania disease symptoms. Additionally, the main pathogenicity factor P25, responsible for the formation of BNYVV symptoms, showed high sequence variability in the amino acid tetrad at position 67–70. The results suggest the geographically independent selection of BNYVV resistance-breaking isolates following the uniform cultivation of Rz1-containing sugar beet cultivars.     

Pathogenetic roles of beet necrotic yellow vein virus RNA5 in the exacerbation of symptoms and yield reduction,development of scab-like symptoms,and Rz1-resistance breaking in sugar beet

Beet necrotic yellow vein virus (BNYVV) generally has a four-segmented positive-sense RNA genome (RNAs 1–4), but some European and most Asian strains have an additional segment, RNA5. This study examined the effect of RNA5 and RNA3 on different sugar beet cultivars using a Polymyxa-mediated inoculation system under field and laboratory conditions. In field tests, the degree of sugar yield served as an index for assessing the virulence of BNYVV strains. Japanese A-II type isolates without RNA5 caused mostly 15%–90% sugar yield reductions, depending on the susceptibility of sugar beet cultivars, whereas the isolates with RNA5 induced more than 90% yield losses in the seven susceptible cultivars, but small yield losses in one Rz1-resistant and Rizor cultivars. However, a laboratory-produced isolate containing RNA5 but lacking RNA3 caused higher yield losses in Rizor than in susceptible plants, and induced scab-like symptoms on the root surface of both susceptible and resistant plants. In laboratory tests, A-II type isolates without RNA5 had low viral RNA accumulation levels in roots of Rizor and Rz1-resistant plants at early stages of infection, but in the presence of RNA5, viral RNA3 accumulation levels increased remarkably. This increased RNA3 accumulation was not observed in roots of the WB42 accession with the Rz2 gene. In contrast, the presence of RNA3 did not affect RNA5 accumulation levels. Collectively, this study demonstrated that RNA5 is involved in the development of scab-like symptoms and the enhancement of RNA3 accumulation, and suggests these characteristics of RNA5 are associated with Rz1-resistance breaking.     

Use of zoospores of Polymyxa betae in screening beet seedlings for resistance to beet necrotic yellow vein virus

A system to culture viruliferousPolymyxa betae and to produce zoospores is described. The zoo spores were used for inoculation of beet seedlings, grown in nutrient solution, in tests for resistance to beet necrotic yellow vein virus (BNYVV). On most occasions in a time course experiment, and with various zoospore cultures, the partially resistant cultivar Rima and the accession Holly-1–4 had virus concentrations similar to the susceptible cultivar Regina, but the virus concentration inBeta vulgaris ssp.maritima accession WB42 was significantly lower (P<0.05). ‘Regina’ could be distinguished from various resistant accessions by a significantly higher virus concentration (P<0.05) shortly after inoculation, or after transplanting the seedlings from the nutrient solution into sand. Results of screening for resistance to BNYVV, using zoospores for inoculation, did not correspond with results of a test in which infested soil was used.Tests in which seedlings are grown in nutrient solution and inoculated with zoospores are suitable for the detection of accessions with a high level of resistance to BNYVV. To obtain virus infection in all plants, the optimal density of the zoospore suspension should first be determined and plants should be assayed shortly after inoculation.     

First Record of A and B Type Beet necrotic yellow vein virus in Sugar Beets in Sweden

Natural infections of sugar beet with Beet necrotic yellow vein virus (BNYVV), the causal agent of rhizomania, have been detected for the first time in Sweden in two small areas, one on the island of Öland and one in the Southeastern part of Scania. Single strand conformation polymorphism analyses of PCR products revealed that the infections on Öland were produced by A type BNYVV, whereas those in Scania were caused by the B type. This suggests that BNYVV has been introduced into Sweden at least twice. Alternatively, the virus may have invaded sugar beet from unknown native hosts. BNYVV RNA 5 was not detected in the samples investigated.     

A method for detecting Polymyxa betae and beet necrotic yellow vein virus in soil using sugar-beet as a bait plant

Samenvatting Door in Petrischalen met grond bietezaailingen als lokplant voor de obligaatparasitaire schimmelPolymyxa betae, een vector van het rhizomanievirus, te laten fungeren, kunnen zowel kwalitatieve als kwantitatieve gegevens over de besmetting van de grond met de schimmel en het virus worden verkregen. De methode en enkele ermee bereikte resultaten worden beschreven.     

Analysis of the resistance-breaking ability of different beet necrotic yellow vein virus isolates loaded into a single Polymyxa betae population in soil

The genome of most Beet necrotic yellow vein virus (BNYVV) isolates is comprised of four RNAs. The ability of certain isolates to overcome Rz1-mediated resistance in sugar beet grown in the United States and Europe is associated with point mutations in the pathogenicity factor P25. When the virus is inoculated mechanically into sugar beet roots at high density, the ability depends on an alanine to valine substitution at P25 position 67. Increased aggressiveness is shown by BNYVV P type isolates, which carry an additional RNA species that encodes a second pathogenicity factor, P26. Direct comparison of aggressive isolates transmitted by the vector, Polymyxa betae, has been impossible due to varying population densities of the vector and other soilborne pathogens that interfere with BNYVV infection. Mechanical root inoculation and subsequent cultivation in soil that carried a virus-free P. betae population was used to load P. betae with three BNYVV isolates: a European A type isolate, an American A type isolate, and a P type isolate. Resistance tests demonstrated that changes in viral aggressiveness towards Rz1 cultivars were independent of the vector population. This method can be applied to the study of the synergism of BNYVV with other P. betae-transmitted viruses.     

Occurrence of pepper yellow vein in the Netherlands

Samenvatting Het verschijnsel van geelnervigheid bij paprika (Capsicum annuum) kon door verenting en met de schimmelOlpidium brassicae worden overgebracht. Hierdoor bleek de ziekte identiek te zijn met een eerder in Engeland gemelde ziekte. Door inoculatie met zoösporen of rustsporen vanO. brassicae werden ook symptomen verkregen inC. baccatum, C. chacoense enC. frutescens. S. villosum bleek een waardplant te zijn voor het geelnervigheidsagens, waarbij zwakke nerfchlorosen werden waargenomen.Seconded to the Glasshouse Crops Research Station, P.O. Box 8, 2670 AA Naaldwijk, the Netherlands.     

Management of the yellow beet cyst nematode with crop rotation,soil fumigation and granular nematicides

A trail field was managed for six years to test effects of short crop rotations, soil fumigation and granular nematicides on the population dynamics of the yellow beet cyst nematode (Heterodera trifolii f. sp.beta) and sugar-beet yields.In the two-year rotation, the cyst nematode population before planting varied from about the tolerance level (5 eggs per millilitre of soil) to 25 eggs per millitlitre of soil, leading to losses of sugar yield. Soil fumigation with metam-sodium effectively reduced the nematode density before planting, resulting in a 15 and 25% increase in sugar yield in the first two crops, respectively, but was insufficient to protect the third sugar-beet crop from yield-reducing nematode attack. This was attributed to the wet soil at the time of application and accelerated disappearence of the chemical in the soil through biological adaptation to repeated fumigation. Oxamyl or aldicarb granular nematicides applied as a side-dressing to the rows had insufficient effect to protect the sugar-beet from yield-reducing nematode attack. An overall treatment with aldicarb rotavated into the soil, alone and in addition to soil fumigation, increased sugar yield significantly. However in the two-year rotation, yield of the third sugar-beet crop treated with soil fumigation and granular nematicide was still lower than that of the untreated second crop in the three-year rotation.In the three-year rotation, thecyst nematode population before sugar-beet varied from hardly detectable to about the tolerance level. Here sugar-beet could be protected from yield-reducing nematode attack by soil fumigation or an overall treatment with granular nematicide. In the threeyear rotation with soil fumigation an increase ofRhizoctonia crown rot was observed in the second sugar-beet crop.Samenvatting Om de effecten na te gaan van korte rotaties, grondontsmetting en nematicide granulaten op het populatieverloop van de gele bietcystenematode (Heterodera trifolii f.sp.beta) en de opbrengsten van suikerbieten werd gedurende zes jaren een proef uitgevoerd op een natuurlijk besmet perceel van de proefboerderij Vredepeel.In de tweejarige rotatie variëerde de cystenematodepopulatie vóór het bietegewas boven de schadedrempel, wat resulteerde in verlaging van de suikeropbrengst. Grondontsmetting met metam-natrium veroorzaakte een effective vermindering van de dichtheid van de nematoden vóór de bieten en een verhoging van de suikeropbrengst van respectievelijk 15 en 25% bij de eerste twee gewassen, maar was onvoldeende om het derde bietegewas te beschermen tegen een opbrengst verminderende aantasting door nematoden. Dit was een gevolg van natte bodemomstandigheden bij het injecteren en het versneld verdwijenen van het middel door biologische adaptatie van de grond na herhaalde ontsmetting. Nematicide granulaten (oxamyl of aldicarb) naast de rij toegepast hadden onvoldoende effect om de bieten te beschermen tegen een opbrengstverminderende aantasting door nematoden. Een volvelds toepassing van in de grond gefreesde aldicarb alleen en toegevoegd na grondontsmetting verhoodge de suikeropbrengst beduidend. De opbrengst van het derde suikerbietegewas in de tweejarige rotatie was echter na toepassing van grondontsmetting en nematicide granulaten nog beduidend lager dan die van het onbehandelde tweede suikerbietegewas in de driejarige rotatie.In de driejarige rotatie variëerde de cystenematodepopulatie vóór het bietegewas van nauwelijks aantoonbaar tot nabij de tolerantiegrens. Hier kon het suikerbietegewas worden beschermd tegen een eventueel opbrengst verminderende nematodenaantasting door grondontsmetting of volveldstoepassing van nematicide granulaten.     

RNA 3 deletion mutants of beet necrotic yellow vein virus do not cause rhizomania disease in sugar beets

ABSTRACT Two mutant strains of beet necrotic yellow vein virus (BNYVV) containing deletions in RNA 3 were obtained by single lesion transfers in Tetragonia expansa. The deleted regions encode either 94 or 121 amino acids toward the C-terminal part of the 25-kDa protein (P25). Wild-type and mutant virus strains were inoculated by Polymyxa betae to sugar beet seedlings of susceptible and partially resistant cultivars. No differences were found in virus content in rootlets between mutant and wild-type viruses or between susceptible and resistant cultivars after culture for 4 weeks in a growth cabinet. However, when virus-inoculated seedlings were grown in the field for 5 months, the wild-type virus caused typical rhizomania root symptoms (69 to 96% yield loss) in susceptible cultivars, but no symptoms (23% loss) developed in most plants of the resistant cultivar, and BNYVV concentrations in the roots were 10 to 20x lower in these plants than in susceptible plants. In contrast, the mutant strains caused no symptoms in susceptible or resistant cultivars, and the virus content of roots was much lower in both cultivars than in wild-type virus infections. Wild-type RNA 3 was not detectable in most of the taproots of a resistant cultivar without any symptoms, suggesting that replication of undeleted RNA 3 was inhibited. These results indicate that the P25 of BNYVV RNA 3 is essential for the development of rhizomania symptoms in susceptible cultivars and suggest that it may fail to facilitate virus translocation from rootlets to taproots in the partially resistant cultivar.