Variations in the “cross protection” effect between two strains of citrus exocortis viroid

Abstract

SUMMARYIn a survey conducted to evaluate the biological properties of several field sources that induced a severe exocortis reaction on citron (Citrus medica L.), a viroid isolate which induced mild symptoms on Gynura aurantiaca was detected. This isolate was characterised as a strain of the citrus exocortis viroid (CEV) by size and homology, and was designated as CEV‐129. Cross protection assays using CEV‐129 as a “protecting” strain against the severe type strain of CEV demonstrated that a mild strain of CEV could provide apparent ‘protection’ against challenge inoculations with the severe strain. The protection effect, however, displayed a variability which ranged from only a brief delay to almost total impairment of symptom expression. The level of protection was dependent upon the length of the interval between the inoculations with the mild and the severe strains. In all cases the effect was temporary since the symptoms and viroid concentration which ultimately prevailed reflected the predominance of the severe strain in the mixed infection. The interpretation of these results and their relationship with previous reports of ‘cross protection’ reactions are discussed along with the consideration of the efficacy and limitations of this term with viroid infection.