Three viruses were studied which produce diseases in French beans, Phaseolus virus I (PV1), Phaseolus virus 2 (PV2) and a virus isolated from white clover (WKV). Included are symptoms, host plants, properties in vitro, occurrence and spread in the field. Special attention was given to the transmission of PV1 by the aphids Myzuspersicae Sulz. and Aphisfabae Scop. From the results, it was postulated that the virus becomes attached to the outer surface of the aphid's stylets when it punctures an infected plant and that the virus may be removed from the stylets only by washing or elution; differences in vector efficiency between M.persicae and A.fabae might be due to differences in the fine structure of the outer surface of the stylets. A paper-chromatographic method succeeded in demonstrating the presence of characteristic thread-like particles in the sap of bean plants infected with the viruses. The particles in the sap of plants infected with PV1 on PV2 tended to aggregate and associate with amorphous material. The particles in the sap of plants infected with WKV hardly showed a tendency to aggregate.
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