The reactions of fifteen plant species in families related to the Caryophyllaceae (Hutchinson's classification) when mechanically inoculated with sixty-eight isolates of fifty-two plant viruses are described. No general relationship could be detected between susceptibility to particular viruses and taxonomic relationships of the host plants. Some of the species were susceptible to many more viruses than were others. Chenopodium amaranticolor, Gomphrena globosa and Tetragonia expansa were hosts for many viruses and their reactions make them suitable as standard test plants. Amaranthus caudatus, Celosia argentea, Beta vulgaris and Spinacia oleracea had limited use in diagnostic or quantitative work. Dianthus barbatus and Primula malacoides were convenient plants for maintaining stock cultures of some viruses. Phytolacca americana, Stellaria media, Fagopyrum esculentum and Plantago lanceolata had little use, and Portulaca oleracea and Lythrum salicaria were useless as test plants. All fifteen species contained inhibitors of infection; these did not prevent infection on the fifteen species, but many did so in Solanaceous and Leguminous plants. Systemic invasion in some hosts depended on such factors as temperature, virus concentration in the inoculum, virus strain and type of local lesion. Strains of one virus often had different host ranges or caused different kinds of symptoms, while similar symptoms were sometimes caused by different viruses. But some viruses that caused similar symptoms on several of the test plants also shared other properties. Host-range studies can be used to separate similar viruses or virus strains, and can suggest similarities between viruses for further study. But host-range studies are unlikely to provide a useful basis for a natural classification of viruses.
