RELATIONSHIPS BETWEEN BARLEY YELLOW DWARF VIRUS TITER AND SYMPTOM EXPRESSION IN BARLEY

To investigate relationships between barley yellow dwarf virus (BYDV) titer and symptom expression in barley, enzyme-linked immunosorbent assay (ELISA) was used to compare the titers of specific virus isolates in artificially infected plants of cultivars previously assessed for symptom expression in field tests. Four Mexican isolates of BYDV were used: Mex-PAV, Mex-MAV, Mex RPV, and Mex-RMV. The near-isogenic lines 'CM67' (Yd2+) and 'California Mariout' (Yd2-) were compared in most detail. The titer of Mex-PAV was significantly lower in shoots and roots of the relatively tolerant line CM67 3 to 29 d after inoculation than in the relatively sensitive California Mariout. Similar differences were observed, but in roots only, for Mex-MAV infections. For the Mex-RPV and Mex-RMV isolates, no significant differences in virus content occurred between these lines. With each isolate and cultivar, maximum virus content occurred 11 d after inoculation. Assessments at this time indicated relatively low levels of some of the virus isolates used in the relatively tolerant cultivars 'Shyri', 'Laurel', 'Agave', and 'Zarza'. The results with other cultivars were less consistent, but in general their virus contents were relatively high whether they had shown tolerance or sensitivity in field tests. The trends noted remained the same when the data were compared using a ''susceptibility index'' designed to provide a convenient comparison of cultivars across serotypes by compensating for differences in the reactivities of different antisera used for ELISA. The need for identification of the specific strains of BYDV involved in tests for tolerance and resistance is emphasized.