Citrus psorosis, ringspot, cristacortis and concave gum pathogens are maintained in callus culture

Abstract

Callus cultures were established from citrus explants infected with several virus-like pathogens of the psorosis group (psorosis A, psorosis B, ringspot, cristacortis, or concave gum), and successively subcultured for up to 16 months. ‘Pineapple’ sweet orange or ‘Duncan’ grapefruit seedlings graft-inoculated with callus pieces, and incubated in a temperature-controlled greenhouse, developed symptoms characteristic of these diseases, whereas similar indicator plants inoculated with callus developed from healthy explants remained symptomless. Calli infected with cristacortis or concave gum pathogens induced in young leaves of indicator plants chlorotic flecking and, occasionally, an oak leaf pattern. Those infected with psorosis (A or B) or ringspot induced a shock 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 in old leaves with gummy pustules in the underside. These results indicate that the agents causing diseases of the psorosis group can be maintained in callus culture. Nevertheless, trials to purify and detect a specific 48K-protein associated with psorosis and ringspot isolates from infected calli were unsuccessful.