|Title||Viruses of faba bean (Vicia faba L.) in Morocco : surveying, identification, and ecological aspects|
|Source||Agricultural University. Promotor(en): R.W. Goldbach; L. Bos. - S.l. : Fortass - 123|
|Department(s)||Laboratory of Virology|
|Publication type||Dissertation, externally prepared|
|Keyword(s)||plantenziekten - plantenvirussen - vicia faba - tuinbonen - marokko - plant diseases - plant viruses - vicia faba - faba beans - morocco|
A systematic virus survey covering the main areas where faba bean ( Viciafaba L.) is grown in Morocco was conducted in 1988 and 1990. From the 240 leaf samples collected on the basis of symptoms suggestive of virus infection from 52 fields, the following viruses were detected by means of electron microscopy, biological indexing, and serology, and their incidence and geographical distribution were assessed: alfalfa mosaic virus (AMV), bean yellow mosaic virus (BYMV), broad bean mottle virus (BBMV), broad bean stain virus (BBSV), broad bean true mosaic virus (BBTMV), peaearly browning virus (PEBV), pea enation mosaic virus (PEMV), pea seed-borne mosaic virus (PSbMV), and luteoviruses. (BBTMV), PEMV, PSbMV, and luteoviruses had not previously been reported in the country. BBMV, considered earlier of limited distribution and the luteoviruses were found to be prevalent (in 50 and 56% of the surveyed fields, respectively; and with field incidences of 20 and 33%, respectively), whereas the opposite held for BBSV and BYMV. More detailed studies concentrated on the actually important BBMV and the luteoviruses, and on the potentially important BYMV-like isolates.
The biological indexing of samples revealed considerable variation in symptom severity on test plants among BBMV isolates. Comparison of seven selected Moroccan isolates with isolates from Algeria, Sudan, and Tunisia, revealed a pathogenic variability of the virus on a number of food-legume genotypes. Clusters of isolates differing in virulence could be distinguished as mild, intermediate, and virulent, although they an reacted similarly to the antisera to a Moroccan and a Syrian isolate. When a number of promising ICARDA breeding lines and accessions of faba bean, chickpea, lentil, and pea were tested with the BBMV isolates, different interactions were observed, but all genotypes were found vulnerable to all the isolates investigated, and no immunity could be detected. Some isolates were more pathogenic (often even necrotic) on other food legumes, such as pea and chickpea, than on faba bean. BBMV was found to be seed transmitted in faba bean (at a rate of 1.2%) when occurring on its own, and was detected to be so in chickpea and pea at transmission rates of 0.9 and 0.1%, respectively. Besides transmission by seed, BBMV was found to be transmissible by the curculionid weevils Apion radiolus, Hypera variabilis, Pachytychius strumarius, Smicronyx cyaneus, and the previously reported Sitona lineatus. The latter appeared to be an efficient vector since the first bite was sufficient for acquisition and transmission of the virus, virus retention lasted for at least seven days, and the transmission rate was estimated to be 41%. S.lineatus turned out to transmit BBMV not only from faba bean to the same species, but also to lentil and pea. When searching for natural sources of the virus by testing of 351 samples of food legumes from 24 fields and 102 samples of wild legumes, it was found to occur naturally in chickpea, lentil, and pea, as well as in common bean (Phaseolus vulgaris) in 16, 11, 19, and 16% of the tested samples, respectively; but it was not detected in the samples of wild legumes reported in literature as potential hosts.
The problem of virus variability emerged and gradually evolved during these studies. It was encountered with BBMV showing a variation of isolates. It particularly holds for the cluster of potyviruses related to BYMV but also for the luteoviruses, where it leads to difficulties in virus identification.
When testing faba-bean samples, showing luteovirus-like symptoms, in DAS-ELISA with polyclonal antisera to bean leafroll virus (BLRV), beet western yellows virus (BWYV), and subterranean clover red leaf virus (SCRLV), and in TAS-ELISA with two monoclonal antibodies discriminating between BLRV and BWYV, various serological reaction patterns were obtained. This pointed to a considerable variation among the luteovirus isolates which could not be identified as one of the known legume luteoviruses. To enable reliable detection of this group of viruses by the polymerase chain reaction (PCR), a pair of designed oligonucleotide primers were found to specifically amplify a 535-bp fragment of the coat- protein gene of known luteoviruses and of all Moroccan isolates tested. In molecular hybridization tests, selected field isolates showed nucleotide sequence homology among them, and with BLRV, but not with BWYV, although some of them behaved BWYV-like serologically. These results support the idea of the involvement of either deviant strains of known luteoviruses or of (a) completely new faba-bean luteovirus(es).
On the other hand, BYMV and the closely related clover yellow vein virus (CYVV) could be distinguished by host-range studies including non-legume test species, and by cytopathology and polyacrylamide-gel electrophoresis of their coat proteins. Both viruses could not be distinguished by the N-terminal serology claimed to discriminate between potyviruses. Conflicting reports as to the taxonomic status of the potyviruses infecting legumes showed the need to develop more reliable tools to unambiguously identify these viruses. Recent molecular studies, including the elucidation of nucleotide sequences of legume-potyvirus RNAs, appeared to provide a rational basis for the identification and classification of potyviruses in general. The use of such molecular data in PCR for the distinction of BYMV and CYVV was investigated, and preliminary results showed that a pair of primers could be used in PCR to distinguish between both viruses.