CHRYSANTHEMUM STUNT: A VIROID DISEASE
Brierley (1953) found that of 76 species and cultivars, mostly Compositae, 39 were susceptible, but that only seven (including five species or cultivars of Chrysanthemum and two species of Senecio) developed discernible symptoms. Brierley made many attempts to use the local lesions response in cineraria for assaying the causative agent of the disease, but concluded that local lesion development is too variable for this purpose (Brierley, 1953). Lawson (1968a) detected local starch lesions on inoculated leaves of Senecio cruentus (florists' cineraria) after inoculation with the stunt agent, but concluded that variation in lesion counts among cineraria plants and among leaves on a single plant precludes their use for detecting small differences in stunt agent concentration. Lack of a suitable assay host has been a major limitation in efforts to determine the nature of the causative agent.
Brierley (1952) reported that the stunt agent (CSV) is unusually heat-stable; infectivity was not destroyed after boiling of the extracts from stunt-infected Mistletoe chrysanthemum. He also found that infectivity was retained in stunt extracts treated with alcohol. Hollings and coworkers (1964) attempted to purify the stunt agent by precipitation with ammonium sulphate and differential centrifugation of buffer extracts treated with organic solvents. Phenol extracts were also prepared to determine if the agent could be transmitted by this method. Hollings and Stone (1965) later reported that none of these extracts were infectious. Although flattened particles similar to those described for tomato spotted wilt virus were found in diseased, but not in healthy plants, there is no evidence that these particles were infectious.
Hollings and Stone (1968) extracted stunt-infected Mistletoe chrysanthemums in 0.1 M phosphate buffer and treated the extract with chloroform-butanol. Highly infectious supernatants were obtained. Extracts prepared in 0.02 M phosphate buffer were not infectious. Infectivity was present in the interface pellets of re-extracted pulp homogenized in a blendor, treated with chloroform-butanol and clarified by low-speed centrifugation. Supernatants of the re-extracts were not infectious.
Hollings and Stone (1969) reported that CSV infectivity was lost in crude extracts made in 0.005 M phosphate buffer treated with RNase (3 μg/ml), but not in extracts made in 0.5 M buffer treated with the same