Studies of pelargonium leaf curl virus

SUMMARY A virus consistently isolated from pelargoniums showing symptoms of leaf curl infected fifty‐four out of ninety‐nine plant species by sap inoculation. The virus was cultured in Nicotiana clevelandii , and assayed in Chenopodium amaranticolor, Gomphrena globosa or Phaseolus vulgaris; C. amaranticolor, P. vulgaris and Antirrhinum majus produced diagnostic symptoms. During the summer, the virus could not be isolated from pelargoniums, when they show no symptoms, although it is readily transmissible between other plants at all times. Pelargoniums proved immune to infection between March and September, but were readily infected during the winter by sap inoculation from N. clevelandii though not from infected pelargoniums, in which the virus titre is very low. Many infected pelargoniums no longer contained the virus after being grown for a summer in a warm glass house, and plants grown for 4 weeks at 37° C. in a heat chamber were freed from virus. Leaf curl virus was not transmitted by handling, plant leaf or root contact, or by the cutting knife, nor when healthy and infected pelargoniums were exposed together outdoors. Experiments did not detect any virus transmission through the seed or soil, nor by four species of aphid or the glasshouse whitefly. Infectivity was destroyed by heating infective sap for 10 min. at 90° C. and, often, at 85° C.; expressed sap was still infective after dilution to 1:100 (pelargonium) or 1:40,000 ( N. clevelandii ), and infectivity persisted for at least 21 days in sap at c. 18° C. and more than 3 years at 0° C. The virus was fairly stable in vitro and could be partially purified and concentrated in several ways: the butanol‐centrifugation method proved best. Specific antisera were made by intramuscular injection of such preparations into rabbits, and were successful in tests by precipitin tube, micro‐precipitin drop and gel‐diffusion methods. Electron micrograms of virus preparations contained icosahedral particles about 28 mμ diameter. The virus produced one, two or more distinct light‐scattering zones when centrifuged to equilibrium in density‐gradient sucrose columns; these zones were associated with maximum serological activity, infectivity, and concentration of particles. Leaf curl disease seems to spread slowly in Britain and is not a threat to commercial cultivation of pelargoniums; it can be controlled by selection of material for propagation, or heat treatment of any badly‐affected varieties.