Subterranean Clover Red Leaf Virus Research Articles

Genetic improvement of subterranean clover (Trifolium subterraneum L.). 2. Breeding for disease and pest resistance

Subterranean clover (Trifolium subterraneum L.) is the most widely sown pasture legume in southern Australia and resistance to important diseases and pests has been a major plant-breeding objective. Kabatiella caulivora, the cause of clover scorch, is the most important foliar fungal pathogen, and several cultivars have been developed with resistance to both known races. Screening of advanced breeding lines has been conducted to prevent release of cultivars with high susceptibility to other important fungal foliar disease pathogens, including rust (Uromyces trifolii-repentis), powdery mildew (Oidium sp.), cercospora (Cercospora zebrina) and common leaf spot (Pseudopeziza trifolii). Several oomycete and fungal species cause root rots of subterranean clover, including Phytophthora clandestina, Pythium irregulare, Aphanomyces trifolii, Fusarium avenaceum and Rhizoctonia solani. Most breeding efforts have been devoted to resistance to P. clandestina, but the existence of different races has confounded selection. The most economically important virus diseases in subterranean clover pastures are Subterranean clover mottle virus and Bean yellow mosaic virus, while Subterranean clover stunt virus, Subterranean clover red leaf virus (local synonym for Soybean dwarf virus), Cucumber mosaic virus, Alfalfa mosaic virus, Clover yellow vein virus, Beet western yellows virus and Bean leaf roll virus also cause losses. Genotypic differences for resistance have been found to several of these fungal, oomycete and viral pathogens, highlighting the potential to develop cultivars with improved resistance. The most important pests of subterranean clover are redlegged earth mite (RLEM) (Halotydeus destructor), blue oat mite (Penthaleus major), blue-green aphid (Acyrthosiphon kondoi) and lucerne flea (Sminthurus viridis). New cultivars have been bred with increased RLEM cotyledon resistance, but limited selection has been conducted for resistance to other pests. Screening for disease and pest resistance has largely ceased, but recent molecular biology advances in subterranean clover provide a new platform for development of future cultivars with multiple resistances to important diseases and pests. However, this can only be realised if skills in pasture plant pathology, entomology, pre-breeding and plant breeding are maintained and adequately resourced. In particular, supporting phenotypic disease and pest resistance studies and understanding their significance is critical to enable molecular technology investments achieve practical outcomes and deliver subterranean clover cultivars with sufficient pathogen and pest resistance to ensure productive pastures across southern Australia.

Occurrence of virus infection in seed stocks and 3-year-old pastures of lucerne (Medicago sativa)

In tests on seed samples from 26 commercial seed stocks of lucerne (Medicago sativa) to be sown in south-western Australia in 2001, infection with Alfalfa mosaic virus (AMV) was found in 21 and Cucumber mosaic virus (CMV) in 3 of them. Bean yellow mosaic virus (BYMV) and Pea seed-borne mosaic virus (PSbMV) were not detected in any. Incidences of infection within individual affected seed samples were 0.1–4% (AMV) and 0.1–0.3% (CMV), and the infected seed stocks were from 3 (CMV) and at least 11 (AMV) different lucerne cultivars. In a survey of 31 three-year-old lucerne pastures in the same region in 2001, in randomly collected samples, AMV was found in 30 and luteovirus infection in 11 pastures. Pastures in high, medium, and low rainfall zones were all infected. Incidences of AMV within individual infected pastures were high, with 50–98% of plants infected in 20 of them and only 3 having <10% infection, but luteovirus incidences were only 1–5%. In addition to various cultivar mixtures, at least 8 (AMV) and 3 (luteoviruses) different individual lucerne cultivars were infected. When the species of luteovirus present were identified, they were Bean leaf roll virus, Beet western yellows virus ( = Turnip yellows virus), or Subterranean clover red leaf virus ( = Soybean dwarf virus). CMV and legume-infecting potyviruses (BYMV, PSbMV, and Clover yellow vein virus) were not detected in any of the lucerne samples. Acyrthosiphon kondoi infestation was common in the samples collected, and A. pisum and Aphis craccivora were also found. Widespread infection in lucerne stands, and their frequent colonisation by aphid vectors, are cause for concern not only because of virus-induced production losses in lucerne itself but also because they provide virus infection reservoirs for spread to nearby grain legume crops and annual legume pastures.

Further studies on the incidence of virus infection in white clover pastures

Leaf samples of white clover were collected from 19 irrigated white clover (Trifolium repens) pastures in the south-west of Western Australia and tested for virus infection by enzyme-linked immunosorbent assay. Clover yellow vein virus (CYVV) was found in 16 pastures at infection levels of up to 23% and white clover mosaic virus (WCMV) in 10 at levels up to 83%. None of the white clover pastures with a high incidence of WCMV had been resown with white clover within the last 10 years, whereas those resown within the last 5 years had little or no infection. As previously reported in tests on different white clover pastures in the same irrigation area, widespread infection with alfalfa mosaic virus (AMV) and occasional infection with subterranean clover red leaf virus (SCRLV) was also found. Two or more viruses were found within 16 of the pastures with at least 3 having all 4 viruses. AMV and WCMV were detected in flatweed (Hypochaeris glabra) and AMV was detected in clustered dock (Rumex conglomeratus), both commonly occurring weeds in the pastures. In tests on the perennial ryegrass (Lolium perenne) component of 18 white clover pastures, infection with barley yellow dwarf virus was found in 14 at levels up to 5%. In addition, 11 of the pastures contained a virus which reacted with potyvirus-specific monoclonal antibodies, presumably ryegrass mosaic virus (RyMV), which was detected at levels up to 34%. Live aphids were trapped at 8 different times during 1995 in one pasture that was infected with WCMV, CYVV, AMV, and SCRLV. Blue-green aphid (Acyrthosiphon kondoi) and oat aphid (Rhopalosiphum padi) were the only species caught, both reaching peak populations in midwinter, but only the latter was found in summer. No virus transmission was detected when the aphids caught were fed individually on subterranean clover (T. subterraneum) indicator plants. It is concluded that WCMV poses a threat to the productivity of white clover within irrigated pastures, especially when present in mixed infection with AMV. CYVV is also commonly found but normally not at high enough incidences to pose a serious threat. RyMV may pose a threat to the productivity of the perennial ryegrass component within white clover-based pastures.

Molecular evidence for the occurrence of beet western yellows virus on chickpea in Morocco

A luteovirus isolate infecting chickpea in Morocco was experimentally transmitted by Myzus persicae to Physalis floridana, on which it produced mild symptoms. When tested in western blots against antisera to known legume luteoviruses, this isolate reacted strongly to beet western yellows virus (BWYV) antiserum, moderately to bean leafroll virus antiserum, while no reaction was recorded with the antiserum against subterranean clover red leaf virus. In PCR, a fragment of ca. 950 bp was amplified, comprising the 3' end of the open reading frame (ORF) 3, the complete coat protein gene (ORF 4), and the non-translated region in between these ORFs. The nucleotide sequence of the amplified fragment showed high similarity with BWYV (approximately 96%), and lower (50–60%) with other luteoviruses reported to infect legumes. On the basis of these data, the Moroccan isolate was identified as BWYV. This is the first molecular evidence for the occurrence of BWYV on chickpea in Morocco, and on food legumes in general in North Africa.

Occurrence of alfalfa mosaic and subterranean clover red leaf viruses in legume pastures in Western Australia

When leaf samples were collected from 94 Trifolium subterraneum (subterranean clover) pastures from six districts in spring 1993 in the south-west of Western Australia and tested by enzyme-linked immunosorbent assay, no alfalfa mosaic virus (AMV) or subterranean clover red leaf virus (SCRLV) was detected. In contrast, when 21 irrigated T. repens (white clover) pastures from one district (Bunbury) were sampled and tested in January (summer) 1994, AMV was detected in 16, with eight having infection levels >86%, while SCRLV was found in seven at infection levels of <12%. When a further five T. repens pastures were tested for AMV in October (spring) 1994, the virus was found in all with incidences up to 100%. None of the T. repens pastures with high levels of AMV infection had been resown with T. repens within the last 20 years, whereas those resown within the last five years had little or no infection. AMV was detected in 9/91 annual medic (Medicago spp.) pastures from seven wheatbelt districts sampled in spring 1991 or 1993; a single pasture of M. polymorpha (burr medic) cv. Serena was 21% infected, but the other eight infected ones had <3%. AMV seed transmission was detected in 1/19 commercial seed stocks of M. polymorpha harvested in 1991-93. AMV infection was followed over a 12-year period in M. murex (murex medic) cv. Zodiac seed stocks. It persisted readily through successive seed harvests during this period. It is concluded that infection with AMV and SCRLV is currently not a threat to T. subterraneum pastures in the south-west of Western Australia and that AMV seems not to be one in wheatbelt annual medic pastures provided these are sown with healthy medic seed. In contrast, AMV poses a potential threat to the productivity of irrigated T. repens pastures. SCRLV is also sometimes present in T. repens pastures, but was not found at serious levels.

National survey of viruses in pastures of subterranean clover. I. Incidence of four viruses assessed by ELISA

A nationwide survey was made of the time-course incidence of alfalfa mosaic virus (AMV), clover yellow vein virus (CYVV), subterranean clover mottle virus (SCMoV) and subterranean clover red leaf virus (SCRLV) in improved pastures in southern regions of Australia over a 3-year period. Up to 2 500 samples (cvv. Mt Barker or Woogenellup) were collected in winter and spring of 1984, 1985 and 1986 in each of New South Wales, Victoria, South Australia. Western Australia and Tasmania, and assayed by ELISA. Usually, each sample consisted of one or two plants or parts thereof. However, a small number of samples in winter contained parts of up to 10 seedlings. For this reason, incidences are expressed as percentage of infected samples rather than percentages of infected plants. All four viruses occurred in all five states. Averaged over all states, the incidence of infected samples increased progressively from 15.3 to 19.9 to 22.5% from 1984 to 1985 to 1986, respectively, in spring, and from 6.8 to 13.8 to 18.8% respectively in winter. The incidence of samples infected with multiple infections (2-4 viruses) increased from 1.9 to 3.2 to 5.0% from 1984 to 1985 to 1986, respectively, in spring, and from 0.5 to 1.6 to 4.0%, respectively, in winter. Since 1-4 viruses were detected in each infected sample, more individual infections than infected samples were recorded. For example, the incidence of virus infections per paddock increased progressively from 17.5 to 24.1 to 28.6% from 1984 to 1985 to 1986, respectively, in spring. On a state basis, the incidence of infected samples was greatest in W.A., reaching 38.9 and 34.9% in spring of 1985 and 1986, respectively, and in S.A. reaching 30.3% in spring of 1986; these high levels were due largely to SCMoV which reached 97% in one paddock in each of the two states. In N.S.W., Vic. and Tas., the highest incidences of infected samples in either winter or spring were 21.4, 16.2 and 23.1%, respectively. Averaged over all states, the highest mean incidence recorded for samples infected with individual viruses in either winter or spring was 9.4% for AMV, 5.7% for CYW, 10.9% for SCMoV and 7.5% for SCRLV. For AMV and SCRLV, there was an increasing trend from spring 1984 to spring 1986. A similar increasing trend for SCMoV was more evident in winter than in spring. For CYVV, no time-course pattern was evident. These results support the proposition that viruses contribute significantly to 'clover-decline', a well-known problem in pastures of subterranean clover.

National survey of viruses in pastures of subterranean clover. II. Statistical methodology for large scale quantitative ELISA

Statistical methodology was applied to a survey of time-course incidence of four viruses (alfalfa mosaic virus, clover yellow vein virus, subterranean clover mottle virus and subterranean clover red leaf virus) in improved pastures in southern regions of Australia, with samplings in each winter and spring over 3 years. The 100 samples per paddock collected at each time of sampling provided detection probabilities of 0.63 and 0.87 for 1% and 2% infection respectively. A microtitre plate design for ELISA was developed to include 60 field samples, 10 glasshouse-grown healthy control samples and 6 glasshouse-grown samples infected with the virus under examination. This design was used on 816 plates for each of the four viruses tested. The method used for identification of virus in sap of a plant sample from a particular paddock was that the ELISA reading was both significantly greater than healthy control readings, and an outlier in the distribution of readings of all sap samples from that paddock. It is argued that as the identification of uninfected samples as infected was highly unlikely, this double criterion method was superior to the use of each criterion separately. Use of significance above healthy control values as the sole criterion would have increased virus incidences by about 60%; use of outlier identification as the sole criterion would have increased virus incidences by about 30%. A generalized linear model with binomial errors and logit link was used for adjusting the virus incidences reported in the previous paper (Helms et al. 1993) for biases due to paddocks and/or districts not sampled on some occasions. This adjustment slightly increased overall incidences in all but one sampling and confirmed the time-course increase in incidence over the 3 years of the survey. The same model also proved to be the most appropriate for investigating the effects of year, season and district on virus incidence.

Corrigendum - National survey of viruses in pastures of subterranean clover. II. Statistical methodology for large scale quantitative ELISA

Statistical methodology was applied to a survey of time-course incidence of four viruses (alfalfa mosaic virus, clover yellow vein virus, subterranean clover mottle virus and subterranean clover red leaf virus) in improved pastures in southern regions of Australia, with samplings in each winter and spring over 3 years. The 100 samples per paddock collected at each time of sampling provided detection probabilities of 0.63 and 0.87 for 1% and 2% infection respectively. A microtitre plate design for ELISA was developed to include 60 field samples, 10 glasshouse-grown healthy control samples and 6 glasshouse-grown samples infected with the virus under examination. This design was used on 816 plates for each of the four viruses tested. The method used for identification of virus in sap of a plant sample from a particular paddock was that the ELISA reading was both significantly greater than healthy control readings, and an outlier in the distribution of readings of all sap samples from that paddock. It is argued that as the identification of uninfected samples as infected was highly unlikely, this double criterion method was superior to the use of each criterion separately. Use of significance above healthy control values as the sole criterion would have increased virus incidences by about 60%; use of outlier identification as the sole criterion would have increased virus incidences by about 30%. A generalized linear model with binomial errors and logit link was used for adjusting the virus incidences reported in the previous paper (Helms et al. 1993) for biases due to paddocks and/or districts not sampled on some occasions. This adjustment slightly increased overall incidences in all but one sampling and confirmed the time-course increase in incidence over the 3 years of the survey. The same model also proved to be the most appropriate for investigating the effects of year, season and district on virus incidence.

Subterranean Clover Red Leaf Virus Disease: Effects of Light Intensity on Plant Symptoms, Growth, and Virus Content

Subterranean Clover Red Leaf Virus Disease: Effects of Light Intensity on Plant Symptoms, Growth, and Virus Content

Subterranean Clover Red Leaf Virus Disease: Effects of Temperature on Plant Symptoms, Growth, and Virus Content

Subterranean Clover Red Leaf Virus Disease: Effects of Temperature on Plant Symptoms, Growth, and Virus Content

Purification of particles of subterranean clover red leaf virus using an industrial-grade cellulase

Particles of two isolates of subterranean clover red leaf virus were purified by a method in which infected plant tissue was digested with an industrial-grade cellulase, Celluclast ® 2.0 L type X. The yields of virus particles using this enzyme were comparable with those obtained using either of two laboratory-grade cellulases, Cellulase type 1 (Sigma) and Driselase®. However, the specific infectivity or aphid transmissibility of the particles purified using Celluclast ® was 10–100 times greater than those of preparations obtained using laboratory-grade cellulases or no enzyme. The main advantage of using Celluclast® is that at present in Australia its cost is only ca. 1% of laboratory-grade cellulases.

Patterns of spread of the non‐persistently transmitted bean yellow mosaic virus and the persistently transmitted subterranean clover red leaf virus in Vicia faba

SUMMARYPatterns of spread of two aphid borne viruses, the non‐persistently transmitted bean yellow mosaic virus (BYMV) and the persistently transmitted subterranean clover red leaf virus (SCRLV), were compared simultaneously in field plots of Vicia faba minor grown in a Mediterranean climate (winter‐spring growing season, and dry summer).Spread from a primary source was mapped following the artificial introduction of virus alone, or virus with vector, at the centre of the plots. BYMV spread rapidly from the virus source whether or not vectors were introduced with the virus. By contrast, SCRLV spread from the source only when plants were also artificially infested with the vector Aulacorthum solani. An attempt was made to evaluate the importance of secondary spread of both viruses by assessing the degree of clumping of infected plants that occurred outside the primary sites of virus introduction. BYMV‐infected plants were clumped in each treatment irrespective of whether the virus was introduced alone or with vector, as well as in control plots. Clumping of SCRLV occurred only when the vectors were introduced on virus source plants at the beginning of the experiment.Times of spread were determined both by exposing trap plants at 4‐weekly intervals throughout the 30 month trial period, and by analysing the rates of spread in experimental plots between June and November in one growing season. Both viruses spread in the spring when vectors were flying, but negligible spread of the viruses was observed in the autumn despite aphid flight activity.Times of flight of the four main aphid vector species were continuously monitored with yellow water traps. A major spring and a minor autumn flight peak were observed for Aphis craccivora, Macrosiphum euphorbiae, Aulacorthum solani and Myzus persicae. Aphid flights occurred predominantly in weeks when the mean temperature was in the range 13–17°C. Rainfall above 7 mm per week appeared to affect flights only when mean weekly temperatures were outside the range 13–17°C.

Some luteovirus diseases in Tasmania caused by beet western yellows and subterranean clover red leaf viruses

A range of crop plants, pasture legumes and weeds, mostly with yellows symptoms similar to those caused by luteoviruses, were collected from the field around Tasmania and checked for infection with beet western yellows virus (BWYV) and subterranean clover red leaf virus (SCRLV) using aphids and indicator plants. BWYV was recovered from 216 of 897 plants tested, representing 30 different species and including 16 not previously recorded as natural hosts. SCRLV was recovered from 163 of 637 plants, representing twelve species including four not previously recorded as natural hosts. BWYV was isolated most often from composites and crucifers, while SCRLV was recovered most frequently from legumes. Eight plants were found infected with both viruses together. In host range studies, Tasmanian isolates of BWYV caused symptoms in lettuce, subterranean clover and sugar beet like those seen on these plants in the field from which the virus was isolated, and were thus similar to isolates of BWYV from North America. The Tasmanian isolates of BWYV were also closely related serologically to Californian isolates of BWYV; these were serologically quite distinct from isolates of SCRLV, and both these groups were serologically distinct from legume yellows virus and from potato leaf roll virus.

Synthesis of a Complementary DNA Probe Specific for Detecting Subterranean Clover Red Leaf Virus in Plants and Aphids

SUMMARY Two RNA species, and one DNA species, were isolated from subterranean clover red leaf virus (SCRLV) prepared by a modification of previously described methods. 3H-labelled cDNA transcribed from high molecular weight RNA of purified virus was specific for the detection of SCRLV, in that it showed no hybridization with nucleic acids from either healthy plants, or plants infected with the serologically related potato leaf roll virus, but hybridized with homologous RNA and nucleic acids from SCRLV-infected plants of two species. The cDNA detected SCRLV in individuals and groups of the aphid vector, Aulacorthum solani, and the average virus content was greater than 170 pg per aphid. The non-vector, Myzus persicae, contained only trace amounts of SCRLV, a result confirmed by enzyme-linked immunosorbent assay.

Control of Primary Infections of Subterranean Clover Red Leaf Virus, a Luteovirus, in a Broad Bean Crop With the Synthetic Pyrethroid Deltamethrin.

Australasian Plant Pathology Vol. 13 No.4 December 1984 further pods after the symptoms of Infection become apparent (4). The greatest losses have occurred in spring-sown crops. These emerge either during, or Just prior to, the main peak of aphid flight activity and are therefore particularly susceptible to infestation by alate aphids. Alates of the vector of SCRLV (Aulacorthum solani) mainly originate from the most important source of infection, white clover plants (Trifolium repens) in perennial pastures. Primary infections resulting from the activity of vector alatae have not been amenable to control with conventional aphtcldss such as the organophosphates in the past (5). The approach to control has therefore been to advance the sowing date from the spring to late autumn or early winter (4). Although few aphids are flying during the early growth and development of these crops, a few foci of infestation and infection are generally established within them soon after they emerge. Considerable spread from these foci may occur due to movement of apterous aphids but these secondary infections can be controlled with aphicides such as demeton-S-methyl (5). The necessity to sow in the late autumn/early winter period can cause problems in Tasmania. Low temperatures and wet conditions during this period sometimes cause long delays in land preparation and sowing. These conditions also favour the development of chocolate spot, caused by Botrytis fabae, which is another important disease of broad bean in Tasmania (7). A recent report Indicated that the synthetic pyrethrord deltamethrin had a particularly rapid knockdown action or repellant effect against aphids; it substantially reduced infection of plants with a persistent, a sermpersistent and a non-persistent virus transmitted by Myzus persicae (1). An experiment was therefore designed to test whether deltamethrin would control primary infections of SCRLV in spring-sown broad beans in Tasmania. Broad beans (cv. Cotes Dwarf Prolific) were sown at Forth in August 1983 in rows set 500 mm apart to establish 30 plants/m'. Plots measuring 2 m (4 rows) x 8 m were sprayed fortnightly from emergence until harvest with either deltamethrin (25% e.c.; 50 mi per 100 L) or with demeton-S-methyl (25% e.c.; 100ml/100 L); unsprayed plots were also included. The plots were randomized in blocks and replicated four times. Immediately prior to each spray application, counts were made of the number of plants with SCRLV symptoms in a buffered zone (6 m long) in the central two rows of each plot. Randomly selected plants were also sampled from each plot to assess aphid mtestation

Purification and properties of subterranean clover red leaf virus

Abstract Subterranean clover red leaf virus (SCRL V) was purified from infected peas using cellulase to assist in virus extraction. Yields of 0.5–3.0 mg/kg tissue were obtained. The virions were isometric with a diam. of 27 nm and sedimented as a single component of 114 S. The mean A260/A280 of purified preparations was 1.85. Apterae of Aulaeorthum solani (Kltb.) when fed on purified preparations through parafilm membranes were able to acquire and transmit virus. Purified virus had a dilution end point of 1.36 µg/ml, a thermal inactivation point between 55 and 60°C, and longevity in vitro of greater than 14 days. Gel electrophoresis of disrupted virions revealed that the coat protein was a single polypeptide with a molecular weight of 22 600. An antiserum with a homologous titre of 11256 in gel diffusion tests was prepared. SCRL V was shown to be serologically very closely related to soybean dwarf virus and less closely to bean leafroll, legume yellows, potato leaf roll, and tomato yellow top viruses. A m...

Purification of a Tasmanian isolate of subterranean clover red leaf virus, and its serological interactions with a New Zealand isolate and other luteoviruses

A Tasmanian isolate of subterranean clover red leaf virus (SCRLV) was purified and concentrated from pea (Pisum sativunz L. cv. Puget) by tissue extraction with cellulase followed by heat clarification, precipitation with polyethylene glycol in salt, and differential and density gradient centrifugation. Virus particles were isometric and 27 nm in diameter. Aulacorthum solani (Kalt.) acquired virus particles from the preparations through Parafilm membranes and transmitted them to healthy subterreanean clover test seedlings, causing disease symptoms of SCRLV. Isolates of SCRLV from Tasmania appeared identical with those from New Zealand on the basis of particle morphology and serological tests. Tasmanian SCRLV was distantly related serologically to Californian isolates of beet western yellows (BWYV) and legume yellows (LYV) viruses. Tests indicated that the relationship was closest with a potato isolate of BWYV and most distant with LYV. Virus particles in sap extracts from infected pea plants were readily detected by electron microscopy following adsorption onto grids coated with SCRLV antiserum, but not on grids coated with antisera to BWYV and LYV. These data confirm that SCRLV is a new, distinct member of the luteovirus group.

Luteovirus-like Particles Associated with Subterranean Clover Red Leaf Virus Infection

SUMMARY Small isometric virus-like particles have been detected in thin sections of phloem transfer cells of subterranean clover plants infected with subterranean clover red leaf virus (SCRLV). Virus-like particles similar in size and appearance but serologically distinct from those of potato leafroll virus, were also detected in purified preparations from SCRLV-infected plants. The morphology of the particles and their distribution in infected cells is consistent with SCRLV being a member of the luteovirus group, as previously suggested by its biological properties.

Control of subterranean clover red leaf virus in broad bean crops with aphicides

SUMMARYTrials at three sites in north‐western Tasmania monitored the spread of subterranean clover red leaf virus into and within broad bean crops with movement of its vector, Aulacorthum solani. Records indicated that almost all infections within the crops resulted from secondary spread by apterae following the establishment by migrant alatae of a very few foci of infestation and infection soon after crop emergence. Spread of the virus within the crops was reduced by sprays of demeton‐S‐methyl. Incidence of infection in sprayed plots at harvest was 23%, 13% and 23% at the three sites compared with 80%, 31% and 84% respectively, in unsprayed plots. Yield increases as a result of spraying were 980, 420 and 540 kg/ha.

Yellows of Sugar Beet Caused by Subterranean Clover Red Leaf Virus.

Yellows of Sugar Beet Caused by Subterranean Clover Red Leaf Virus.