Viruses involved in chickpea stunt

Chickpea stunt is the most important virus disease of chickpea ( Cicer arietinum L). This disease is characterized by leaf chlorosis or leaf reddening (depending on the chickpea cultivar), plant stunting, internode shortening, reduction in size of tip leaves, and phloem discoloration. Infected plants decline and premature death mostly follows. Till recently, the disease was ascribed in India to bean leafroll luteovirus (BLRV) only. Also in other countries, luteoviruses were reported to be associated with similar stunt symptoms. In India, however, indications emerged that more viruses were involved in the disease. Therefore, the viruses associated with chickpea stunt, and their role in the etiology of the disease were now studied.Surveys were undertaken in India, Pakistan, Syria, Turkey and Lebanon. No saptransmissible viruses could be isolated from plants with chickpea stunt. In India and Pakistan, 1804 samples from plants with stunt-like symptoms were collected and tested with poly- and monoclonal antibodies (Chapter 3). In plants with chickpea stunt from farmers' fields, a geminivirus (newly described in this thesis as chickpea chlorotic dwarf virus; CCDV), isolates reacting with an antiserurn to a luteovirus isolate from chickpea (tentatively named chickpea luteovirus; CpLV), and BLRV-Iike isolates were detected. CCDV and CpLV- like isolates occurred widely, whereas the BLRV-Iike isolates were found at very low incidences in two regions in India only. In Syria, Turkey and Lebanon, the recently characterized, unclassified faba bean necrotic yellows virus (FBNYV) was predominantly associated with chickpea stunt (Chapter 4). More than 50% of the 313 plant samples collected reacted with FBNYV antiserum. Luteoviruses (beet western yellows virus-like, CpLV-like and BLRV-Iike) were also detected in ELISA using poly- and monoclonal antibodies, but their incidences were lower than those of FBNYV. The geminivirus was not detected in these countries.The reaction patterns with the Mabs of the luteovirus isolates from these five countries differed from those of a number of known luteoviruses (Chapters 3 and 4). New luteoviruses may well have been traced here. The surveys showed that the etiology of chickpea stunt is much more complex than originally thought. More viruses seem to be involved, but their role in the etiology is not yet clear.To further identify the luteoviruses obtained from chickpea, three isolates from a single field-infected chickpea plant with stunt symptoms in India were studied (Chapter 5). One of them was characterized by host range, ELISA and Western blot analysis, and was identified as a distinct strain of beet western yellows luteovirus (BWYV). When chickpea plants were inoculated with this isolate, infection was obtained very rarely, and in case of infection no symptoms were observed. The second isolate was serologically similar to potato leafroll luteovirus (PLRV), and the third to BWYV. Only the latter was associated with stunt symptoms in greenhouse-grown chickpea plants inoculated with Myzuspersicae. These orientational experiments suggest that luteoviruses occur in mixtures in chickpea. Not all components may be able to cause chickpea stunt on their own, and the behaviour of luteoviruses, and their involvement in the etiology of chickpea stunt appears to be complex.The Indian geminivirus from chickpea was first detected by electron microscopy after partial purification from extracts of chickpea plants from the field (Chapter 6). This virus was not sap transmissible and could not be transferred with aphids or whiteflies. The leafhopper Orosius orientalis was identified as a vector of the virus. The virus particles were geminate, and contained a coat protein of 32 kD and a circular, ssDNA of 2900 nucleotides. The virus was shown to be serologically distinct from the known leafhopper-transmitted geminiviruses, and was considered to be a new, hitherto undescribed, geminivirus. It was named chickpea chlorotic dwarf geminivirus (CCDV) and was shown to cause symptoms in chickpea similar, if not identical, to those described for chickpea stunt.Plants that became infected with CCDV in the field during or before flowering, suffered from yield losses of 75 - 100% (Chapter 8). Plant densities in fanners' fields are likely to be too low to allow uninfected plants around infected ones to compensate the yield losses of infected plants. Therefore, crop loss is likely to equal percentage of disease incidence.To better understand the ecology of CCDV, its relationships with the leafhopper O. orientalis were studied (Chapter 7). The leafhopper can acquire and introduce the virus in very short feeding periods. The minimum latency period was also very short (2 h). This indicates that the vector can easily transfer the virus. Single leafhoppers could transfer the virus for 10 - 15 consecutive days when they were transferred daily to healthy plants. Application of ELISA to single leafhoppers showed that the amount of virus in the vector decreased when the insects were kept on a non-host of the virus. Thus, the virus is transmitted by O.orientalis in a persistent, non-propagative manner.When a number of chickpea genotypes, identified as disease resistant in field experiments, were inoculated in the greenhouse with CCDV, these genotypes showed slower symptom development than the vulnerable control 'WR 315' (Chapter 9).