INVESTIGATION OF CHRYSANTHEMUM VIRUSES II. VIRUS B (MILD MOSAIC) AND CHRYSANTHEMUM LATENT VIRUS

Chrysanthemum virus B is widespread in English chrysanthemum stocks, but it appears to do little damage, for in thirty-four varieties it caused no or very slight symptoms; the name mild mosaic is proposed for the disease. Virus B infected ten out of eighty-six plant species tested by mechanical inoculation and by grafting. Petunia was used as a local lesion test plant; petunias were much less susceptible to infection during the summer. In addition, some petunias showed genetical resistance at all seasons. The concentration of infective virus B in chrysanthemum sap decreased in the summer. Virus B was transmitted by five out of six aphid species tested; aphids became infective with short acquisition feeds, and lost infectivity within 1 hr. Chrysanthemum was difficult to infect by mechanical inoculation, and the virus was not spread by leaf contact, or by knives used for taking cuttings. The thermal inactivation point of virus B is between 70 and 75 degrees C.; the dilution end-point varied from 1/10 to 1/1000, depending on the host plant tissue from which sap was obtained; the virus inactivated within 24 hr. at 18 degrees C. Infective sap contained rod-shaped particles, about 750 x 17 m mu, not found in sap from uninfected plants. Three strains of virus B were differentiated by their reaction on petunia; one strain prevented infection by a second. Two other, previously unrecorded, viruses were found in chrysanthemums that cause local lesions in petunia: vein mottle virus was similar in some ways to virus B, but virus B did not protect plants against vein mottle. Vein mottle severely damaged some varieties of chrysanthemum, but caused no symptoms in others. Chrysanthemum latent virus was carried without symptoms by eleven varieties of chrysanthemum; thirteen out of twenty-seven plant species were readily infected by mechanical inoculation. Latent virus had a dilution end-point of 1/200 to 1/500, was inactivated after 10 min. at 70 degrees C., or within 48 hr. at room temperature. It was not spread by the cutting knife, root contact, or by two species of aphid. It was found rarely and seems of no commercial importance in English chrysanthemum stocks.