Virology Laboratory, ICARDA, PO Box 5466, Aleppo, Syria

Background and objectives
Pea early-browning tobravirus (PEBV) was originally described from western Europe as a nematode- and seedborne virus able to cause severe disease in pea and often symptomlessly infecting other plant species, particularly legumes. Later, a virus from faba bean in southern Italy, broad bean yellow band tobravirus (BBYBV) was identified and considered a distinct tobravirus, serologically unrelated to PEBV. Soon thereafter, it was found that BBYBV is a serotype of PEBV, designated PEBV:BBYB, rather than a distinct tobravirus. Recently, two tobravirus isolates were recovered from faba bean in Libya and Algeria. The Libyan isolate (LYV66-91) was closely related to the Dutch type strain of PEBV (El 16) and the Algerian isolate (AigRl0) was related to, but biologically different from, the Italian type strain of PEBV:BBYB serotype of PEBV . LYV66-91 reacted in ISEM with antiserum to El 16 but not with that to BBYB, but AlgRl0 reacted with antiserum to BBYB only.

This paper describes the production of antisera against the LYV66-91 and Algerian isolates meant for further surveying in the region, for germplasm testing for seed infection and for resistance screening. It further characterizes the serological relationship between these isolates and strains of PEBV and tobacco rattle tobravirus (TRV), and evaluates the usefulness of these antisera for sensitive detection of those two isolates using various ELISA variants.

Results and conclusions

Antisera against purified particles of tobravirus isolates from Libya (LYV66-91) and Algeria (AlgRl0) were produced and used for their serological comparison with other isolates of pea early-browning tobravirus (PEBV), including the broad bean yellow band serotype (PEBV:BBYB), and with the PRN strain of TRV (TRV:PRN). DAS-ELISA, DAC-ELISA and dot-blot ELISA showed that the Algerian and Libyan isolates represented different serotypes. In DAS-ELISA, the Libyan and Algerian isolates of PEBV hardly cross-reacted, if at all. The Libyan isolate of PEBV (LYV66-91) reacted strongly with antisera to the homologous virus or to El 16, but not with that to AlgRl0. In contrast, AlgRl0 reacted strongly with its homologous antiserum but not with LYV66-91 or El 16 antisera. These reactions suggest that the Libyan isolate is serologically closely related to the Dutch PEBV isolate, but that the Algehan isolate is not. When five tobravirus antisera were used in a comparative study using DAC-ELISA, reactions obtained again showed that the Libyan isolate is serologically closely related to the Dutch isolate but not to Algeria, PEBV:BBYB or the type strain of TRV (TRV-PRN). In contrast, the Alg10 isolate reacted strongly with the homologous antiserum and with antisera to PEBV:BBYB and TRV-PRN, and no reaction was obtained against antisera to the Libyan isolate or the Dutch strain of PEBV (El 16). Dot-blot ELISA also clearly showed that the Algerian and the Libyan isolates of PEBV strongly differ serologically. However, the Algerian isolate of PEBV was serological close to PEBV:BBYB but not identical to it. AigRl 0 antiserum reacted strongly with the homologous antigen and with PEBV:BBYB, but not as strongly with TRV-PRN. Likewise, PEBV:BBYB antiserum reacted with AigR10 and PEBV:BBYB antigens but not with TRV-PRN. These reactions suggest that AlgRl0 has some serological relatedness to the type strain of TRV, but PEBV:BBYB does not. Some differences were also reported in their host reactions: PEBV:BBYB and AlgRl0 both infect faba bean systemically but they differ appreciably in their reaction on Nicotiana rustica, N. tabacum and Petunia hybrida, and the symptoms observed in faba bean were less severe than those described earlier for BBYBV.