CITRUS PSOROSIS, RINGSPOT, CRISTACORTIS AND CONCAVE GUM PATHOGENS AREMAINTAINED IN CALLUS-CULTURE

Callus cultures were established from citrus explants infected with se veral virus-like pathogens of the psorosis group (psorosis A, psorosis B, ringspot, cristacortis, or concave gum), and successively subcultu red for up to 16 months. 'Pineapple' sweet orange or 'Duncan' grapefru it seedlings graft-inoculated with callus pieces, and incubated in a t emperature-controlled greenhouse, developed symptoms characteristic of these diseases, whereas similar indicator plants inoculated with call us developed from healthy explants remained symptomless. Calli infecte d with cristacortis or concave gum pathogens induced in young leaves o f indicator plants chlorotic flecking and, occasionally, an oak leaf p attern. Those infected with psorosis (A or B) or ringspot induced a sh ock reaction in the first flush, and chlorotic flecks in young leaves developed in successive flushes. Calli from psorosis B-infected plants caused in addition blisters in mature shoots and chlorotic blotches i n old leaves with gummy pustules in the underside. These results indic ate that the agents causing diseases of the psorosis group can be main tained in callus culture. Nevertheless, trials to purify and detect a specific 48K-protein associated with psorosis and ringspot isolates fr om infected calli were unsuccessful.