Abstract
1. Some isolates of barley yellow dwarf virus (BYDV) differing in vector transmission characteristics and in host plant reactions were studied in single and mixed inoculations in glass-house trials. 2. The symptoms obtained depended on the isolate of BYDV, and the interval between the protective and test inoculation and the variety of host plant. 3. Two of the isolates showed complete protection in Macrosiphum avenae (F.) aphids and in Clintland 60 oats plants. These 2 isolates differed in pathogenicity and in vector specificity. 4. Although interference (protection and synergism) was found in these tests, it appears to be of little value in determining relations between isolates of BYDV. 5. It is thought best at present to assume that there is only one barley yellow dwarf virus with a wide host range in grasses and cereals and differing in adaptation for transmission by several aphid species. 6. Several glass-house trials confirmed that oat plants with an early, mild infection of BYDV were then protected from the severe effects of late infection by the same and other isolates of BYDV.