Effect of high and low temperatures on the survival of wound-tumor virus in sweet clover

Abstract

Sweet clover plants, Melilotus officinalis L. (Lam.), infected with the vectorless (VL) strain of wound-tumor virus (WTV), were grown for various intervals at 35° or 40°. Cuttings from the treated plants were grown under conditions favorable for tumor development in the resulting plants. Successive vegetative generations propagated by cuttings from symptomless plants yielded only progressively higher proportions of symptomless plants. After four succesive vegetative propagations from symptomless plants under conditions optimal for symptom expression, cuttings from plants treated at 35 and 40° were, respectively, 98 and 100% symptomless. Plants remaining symptomless for two successive vegetative generations under optimal conditions were designated “90% virus-free” because tests indicated that 82–90% of them remained free of symptoms when tested for the third and fourth vegetative generations. Growth of cuttings at a low temperature (14°) markedly increased the frequency of 90% virus-free plants. When plants infected with a vectorial strain of WTV were subjected to identical 14° exposures, the increase in proportion of 90% virus-free cuttings was much less. Incidence of symptom expression after growth for one generation at 14° was different for three strains of WTV.