Groundnut Rosette Disease Research Articles

First Report of the Green Rosette Variant of Groundnut Rosette Disease in Kenya.

Groundnut (Arachis hypogaea L.) is an important food crop in sub-Saharan Africa. One of the major production constraints is groundnut rosette disease, which is caused by a complex of two viruses, groundnut rosette assistor luteovirus (GRAV) and groundnut rosette umbravirus (GRV) together with the associated satellite RNA (satRNA) (1). Two main forms of the disease have been described: chlorotic and the green rosette. Variants of the satRNA have been shown to be largely responsible for the different forms of the disease (1). Chlorotic rosette has been the predominant form in all of sub-Saharan Africa while green rosette has been reported in the western and southern regions of Africa (2). During the 1997-1998 crop season, disease surveys conducted in Kenya showed the incidence of the rosette disease in farmers' fields to be 24 to 40% in a total of 23 fields surveyed in the western regions of the country (Homabay, Kendubay, Kisumu) and 30% in 8 fields sampled in the Rift Valley (Cheplamus, Marigat) regions. Representative peanut plants showing rosette symptoms were analyzed for the presence of GRV by reverse transcription polymerase chain reaction (RT-PCR). With primers specific to a portion of ORF4 of GRV RNA (3), RT-PCR gave a product of expected size (approximately 300 bp). The PCR product was cloned in pGEM-T vector and sequenced. The sequenced region showed 89% nucleotide sequence identity with published GRV sequences. Green rosette was observed on groundnut cultivars Nyaela Red and Homabay Local in the Kendu Bay region. The incidence of the green rosette was 5.3% of the plants with rosette symptoms.

Satellite RNA Is Essential for Encapsidation of Groundnut Rosette Umbravirus RNA by Groundnut Rosette Assistor Luteovirus Coat Protein

Groundnut rosette disease is caused by a complex of agents comprising groundnut rosette umbravirus (GRV), GRV satellite RNA (sat-RNA)groundnut rosette assistor luteovirus (GRAV). Both GRAV and GRV sat-RNA are needed for GRV to be aphid transmissible. To understand the role of GRAVGRV sat-RNA in the aphid transmission of GRV, encapsidation of GRV genomicsatellite RNAs has been studied using transgenicNicotiana benthamianaplants expressing GRAV coat protein (CP). GRAV CP expressed from a transgene was shown to package GRV genomicsatellite RNAs efficiently, giving a high yield of transcapsidated virus particles. GRV sat-RNA was absolutely essential for this process. GRV genomic RNA was not encapsidated by GRAV CP in the absence of the sat-RNA. Using different mutants of GRV sat-RNA, it was found that some property of full-length satellite RNA molecules, such as size or specific conformation rather than potential open reading frames, was required for the production of virus particles. A correlation between the ability of sat-RNA to stimulate encapsidation of GRV RNA by GRAV CPits capacity to promote aphid transmission of GRV was observed.

Detection of each of the causal agents of groundnut rosette disease in plants and vector aphids by RT-PCR

Detection of the three agents of groundnut rosette disease (groundnut rosette assistor virus, groundnut rosette virus and its satellite RNA) in plants and vector aphids by reverse transcription-polymerase chain reaction (RT-PCR) is reported. Three procedures for extraction of total RNA from groundnut were tested, of which two were found to be useful in giving RNA of sufficient quality for RT-PCR. Of these two, the total RNA extraction kit supplied by Qiagen was found to be the most versatile for extraction of all three agents from individual vector aphids ( Aphis craccivora). Both groundnut rosette assistor virus and groundnut rosette virus could be detected from total RNA extracted from a single aphid that had been exposed to either green or chlorotic rosette-infected groundnut plants. They could be detected in aphids stored in 70% ethanol for up to 30 days at room temperature. However, satellite RNA could be amplified only when total RNA extracted from two or more aphids was used. Groundnut rosette assistor virus, groundnut rosette virus and its satellite RNA were detected by RT-PCR in aphids that had been exposed only to groundnut rosette diseased plants containing all three agents. The potential of RT-PCR in studying certain key issues of rosette disease epidemiology is discussed.

Sources of resistance to groundnut rosette disease in global groundnut germplasm*

SummaryAbout 6800 groundnut germplasm accessions originating from South America, Africa, and Asia were evaluated for resistance to rosette disease using an infector row technique between the 1990/91 and 1996/97 growing seasons. Of these, 116 germplasm accessions, including 15 short‐duration Spanish types, have shown high levels of resistance to rosette disease. A high percentage of these resistant accessions were from West Africa and a few were from Asia and southern Africa. Only one out of 1400 accessions from South America showed resistance to rosette disease. All disease‐resistant accessions were susceptible to groundnut rosette assistor virus. This is the first report to identify sources of resistance to rosette disease in groundnut germplasm from Asia and South America. These additional sources of resistance provide an opportunity to broaden the genetic base of resistance to rosette disease. The origins of rosette resistance in groundnut are discussed.

The satellite RNAs associated with the groundnut rosette disease complex and pea enation mosaic virus: sequence similarities and ability of each other's helper virus to support their replication.

Pea enation mosaic virus (PEMV) and the causal agents of groundnut rosette disease are diverse examples of disease complexes involving two RNA species, one of which is related to the genomes of luteoviruses and the other to those of umbraviruses. In both complexes, these viral RNA components may be supplemented with satellite RNAs that are dependent on the umbravirus component for replication and systemic movement, and on the luteovirus component for encapsidation and vector transmission. Sequence analysis identified regions of similarity between the satellites of groundnut rosette virus (GRV) and PEMV, particularly at the 5' and 3' termini and around duplicate sequence repeats present in each satellite RNA. The umbravirus GRV and the umbravirus-like PEMV RNA-2 were each able to support the replication and systemic spread of homologous and heterologous satellites. The presence of the PEMV satellite in infections with GRV had no effect on symptom expression in Nicotiana spp. or in Arachis hypogaea. Likewise, in Pisum sativum, the GRV satellite had no effect on the symptoms induced by PEMV. However, the intense yellow blotch symptoms induced in Nicotiana benthamiana by the YB3 GRV satellite in conjunction with GRV were also manifested when PEMV was the helper. Although PEMV RNA-1 was capable of supporting the encapsidation and aphid transmission of the GRV satellite, no evidence was obtained that the essential role of the GRV satellite in the aphid transmission of GRV could be supplied by the PEMV satellite. These data further strengthen the hypothesis of an evolutionary relationship between PEMV and the luteovirus-umbravirus complexes.

Replication of the satellite RNA of pea enation mosaic virus is controlled by RNA 2-encoded functions.

The helper virus mediating replication of the satellite RNA (RNA 3) of pea enation mosaic virus (PEMV) consists of two autonomously replicating, taxonomically unrelated viral RNAs with ties to the luteovirus (RNA 1) and the newly proposed umbravirus (RNA 2) genera. The following study dissects the relative contribution of each of the genomic RNAs of PEMV to the subsistence and dissemination of this satellite RNA. Infectivity assays in a pea protoplast system demonstrate that RNA 2 alone is responsible for the replication of RNA 3, an observation that is supported in part by shared regions of sequence homology at the 5' and 3' termini of both RNAs. In pea seedlings, infectivity assays also demonstrated that the presence of RNA 2 alone is necessary for the systemic invasion of RNA 3. In contrast, the luteovirus-like phase of PEMV (RNA 1) is solely responsible for the encapsidation and aphid transmission of both RNA 2 and the satellite RNA. In a manner comparable to several other virus-satellite systems, the satellite of PEMV also displays a differential response in its capacity to attenuate symptom expression in selected host species. Thus, the satellite RNA of PEMV exists in a trilateral arrangement with its host and two viral RNAs, comparable in many respects to the satellite-virus-host interaction occurring with groundnut rosette disease.

A variant of the satellite RNA of groundnut rosette virus that induces brilliant yellow blotch mosaic symptoms in Nicotiana benthamiana

SUMMARYSome Malawian cultures of groundnut rosette virus (GRV) give rise to variants that, although still causing symptoms of the chlorotic type of rosette in groundnut, induce brilliant yellow blotch mosaic symptoms, instead of the usual veinal chlorosis and mild mottle, in Nicotiana benthamiana. One such isolate (YB) induced the formation in infected plants of a 0.9 kbp dsRNA having extensive sequence homology with molecules of similar size in other naturally occurring isolates of GRV. These dsRNA molecules were shown to be double‐stranded forms of single‐stranded satellite RNA molecules. Experiments in which the satellite was removed from and restored to isolate YB, or exchanged with those from other GRV isolates, showed that it carries the determinant for yellow blotch mosaic symptoms. Plants inoculated with the 0.9 kbp dsRNA (denatured or undenatured) developed yellow blotch mosaic even when the satellite‐free GRV helper was not inoculated until 11 days later. The satellite RNA is therefore a very stable molecule. Prior infection of N. benthamiana with a GRV isolate containing a normal form of the satellite protected against expression of yellow blotch mosaic symptoms when the plants were later inoculated with isolate YB, whereas prior infection with satellite‐free isolates did not. This provides a simple method of determining whether a GRV isolate has an associated satellite RNA. The YB satellite seems to be a newly recognised variant additional to those known to cause the chlorotic, green and other forms of groundnut rosette disease.

Different variants of the satellite RNA of groundnut rosette virus are responsible for the chlorotic and green forms of groundnut rosette disease*

SUMMARYGroundnut plants with symptoms of rosette disease contain groundnut rosette virus (GRV), but GRV is transmitted by Aphis craccivora only from plants that also contain groundnut rosette assistor virus (GRAV). Two main forms of rosette disease are recognised, ‘chlorotic rosette’ and ‘green rosette’. GRV cultures invariably possess a satellite RNA and this is the major cause of rosette symptoms: satellite‐free isolates derived from GRV cultures from Nigerian plants with chlorotic or green rosette, or from Malawian plants with chlorotic rosette, induced no symptoms, or only transient mild mottle or interveinal yellowing, in groundnut. When the satellite RNA species from GRV cultures from Nigerian green or Malawian chlorotic rosette were reintroduced into the three satellite‐free isolates in homologous and heterologous combinations, the ability to induce rosette symptoms was restored and the type of rosette induced was that of the cultures from which the satellite RNA was derived. Thus different forms of the satellite are responsible for the different forms of rosette disease. Other forms of the satellite induce only mild chlorosis or mottle symptoms in groundnut. Individual plants may contain more than one form of the satellite, and variations in their relative predominance are suggested to account for the variable symptoms (ranging from overall yellowing to mosaic) seen in some plants graft‐inoculated with chlorotic rosette.

A Satellite RNA of Groundnut Rosette Virus that Is Largely Responsible for Symptoms of Groundnut Rosette Disease

Summary Groundnut (Arachis hypogaea) plants with rosette disease contain a manually transmissible virus, groundnut rosette virus (GRV), which depends on a luteovirus, groundnut rosette assistor virus (GRAV), for transmission by the aphid Aphis craccivora. No virus-like particles have been reported for GRV but infected plants yield infective ssRNA. Infected leaves also contain dsRNA with prominent electrophoretic species of 4.6 kbp (dsRNA-1) and 1.3 kbp (dsRNA-2), a very abundant species of 900 bp (dsRNA-3), and numerous minor species of intermediate mobility. In studies with GRV(C), an isolate from groundnut plants with a chlorotic form of rosette, cDNA to dsRNA-1 reacted with dsRNA-1 and dsRNA-2 but not with dsRNA-3 or any of the minor dsRNA species. In contrast, cDNA to dsRNA-3 reacted with dsRNA-3 and several of the minor dsRNA species but reacted only weakly or not at all with dsRNA-1 or dsRNA-2. An isolate lacking dsRNA-3 (isolate G96) was derived from GRV(C) by passage through Gomphrena globosa. When dsRNA-3 recovered from agarose gels was melted and inoculated to Nicotiana benthamiana plants, it was not infective on its own but multiplied in plants that were also infected with G96. Similar results were obtained with sucrose density gradient fractions containing RNA molecules of the size expected for ssRNA-3. These results show that dsRNA-3 represents a satellite RNA. Addition of dsRNA-3 to the G96 culture resulted in a slight amelioration of symptoms in N. benthamiana and N. clevelandii. However, only cultures containing RNA-3 induced rosette symptoms in groundnut, though the symptoms were intensified by further addition of GRAV. The results show that the satellite RNA is largely responsible for rosette disease symptoms in groundnut.

A Simple Procedure to Detect a dsRNA Associated with Groundnut Rosette

AbstractA low‐molecular weight double‐stranded (ds) RNA [900 base pairs (bp)] associated with groundnut rosette disease can be used as a diagnostic tool. A simple procedure has been developed that is rapid, reliable, and requires only standard electrophoresis equipment and ultraviolet light for detection of nucleic acid bands. Using this procedure, the dsRNA was detected only in groundnut plants with green rosette of chlorotic rosette symptoms. It was not found in uninoculated groundnut plants, in symptomless groundnut plants with groundnut rosette assistor virus, or in groundnut plants infected with several other known groundnut viruses. In studies with northern blots of extracts from rosette‐diseased and healthy plants, 5′‐endlabeled dsRNA only hybridized to a 900 bp dsRNA from diseased plants. The 900 bp dsRNA was not infectious and its origin remains obscure.

Groundnut Crinkle, a new Virus Disease observed in Ivory Coast

A virus causing crinkling and stippling symptoms on groundnut plants (Arachis hypogaea L.) was observed in 1976-1977, near Dabou, Abidjan and Bingerville, all along the lagoons of the south of.Ivory Coast, often with a high frequency (90% of diseased plants). Symptoms were very discrete and easily observed when plants were young and growing vigorously. The disease could not be observed three months after sowing, in the fields where it was first noticed and marked on young seedlings one month after sowing; old plants were chlorotic and attacked by fungi, mites and groundnut rosette disease.

Insecticides for the Control of the Spread of Groundnut Rosette Disease in Uganda

Abstract Trials described show that control of the secondary spread of groundnut rosette virus, which is transmitted by Aphis craccivora Koch, is possible with insecticides. In six trials carried out in the period 1965–1970 insecticides tested included dimethoate, phosphamidon and dicrotophos as emulsion concentrates and ulv formulations, menazon as a wettable powder, endosulfan and phenthoate as an emulsion concentrate and aldicarb as a granular formulation. Menazon, at a rate of 294 g a.i./ha applied four times at ten day intervals starting ten days from the germination (emergence) of the crop, was the most efficient insecticide overall in reducing rosette attack. In two trials it was shown to be very effective at rates of 196 and 147 g a.i./ha. Endosulfan and dimethoate and to a lesser extent phosphamidon were also successful as emulsion concentrates and in the instance of dimethoate and phosphamidon as ulv's. Dicrotophos was promising in the two trials in which it was used, but phenthoate was not. Aldicarb, used as a side dressing, was inadequately tested. Ulv application was quick, simple and efficient and has much to recommend it in Uganda conditions. Insecticides significantly reduced the percentage plants attacked in growth up to harvest. There were improvements in kernel quality, shelling percentage and yield. It was concluded that in most seasons even on well grown and well tended crops spraying was worthwhile.

Groundnut rosette and its assistor virus

SUMMARYChlorotic rosette from Malawi (isolate CR1), passed through Stylosanthes gracilis and S. juncea, was not subsequently transmissible from groundnuts (Arachis hypogaea) by Aphis craccivora or A. gossypii, but with S. mucronata transmissibility was occasionally regained after a period of time. Aphid transmissibility was similarly lost after passage of two isolates (a chlorotic rosette from Rhodesia, CR2, and a green rosette from Nigeria, GR) through soybean (Soja max) and after manual inoculation to groundnuts. Groundnut plants that remained symptomless after exposure to rosette infection by aphids often contained a virus that restored aphid transmissibility when introduced into groundnuts containing the vectorless virus from that isolate. Groundnut rosette disease therefore consists of a symptom‐inducing virus that we call groundnut rosette virus (GRV) and a symptomless assistor virus (GRAV) that must be present for aphid transmission. The interactions between the GRV and GRAV of chlorotic and green rosette, and their transmission by different vector races, are described.

The effect of planting date and spacing on the incidence of groundnut rosette disease and of the vector, Aphis craccivora Koch, at Mokwa, Northern Nigeria

SUMMARYAt Mokwa in Northern Nigeria early planting and close spacing increased the yield of groundnuts and decreased rosette incidence. With late planting, increasing the plant population reduced both the rosette incidence and the number of aphid‐infested plants. It is suggested that winged Aphis craccivora Koch bring the virus into the crop, and that spacing and planting date affect rosette incidence because of the influence of groundcover on the optomotor landing response of the winged aphids.

VIRUSES CAUSING ROSETTE AND OTHER DISEASES IN GROUNDNUTS

From plants with a form of groundnut rosette disease, characterized by discrete areas of green and chlorotic tissue on the leaflets and here designated ‘mosaic rosette’, a virus was separated that produced only a mild mottle or sometimes a mottle with rare chlorotic flecks. It was separated by leaf grafts, by mechanical inoculation and by Aphis craccivora.Plants inoculated simultaneously with the mottle virus and normal rosette virus usually developed the mosaic‐rosette symptoms. When the mottle virus was introduced 14–35 days before the rosette virus, the plants failed to develop the severe chlorosis of rosette; the mottle virus thus protected the plant from rosette, and this was true whether the rosette virus was inoculated by aphids or by grafting.Plants showing two other forms of mild mottle were collected in the field; viruses from them were readily transmitted by grafting or by mechanical inoculation, but not by A. craccivara. In plant‐protection tests with one of these, it failed to protect plants from developing chlorotic symptoms when later inoculated with the rosette virus, although a form of interaction was evident and the doubly‐infected plant was less severely chlorotic and less stunted than one infected with the rosette virus alone.