A Unique, Infectious RNA Associated with Citron Showing Symptoms Typical of Citrus Exocortis Disease

Abstract

A pathogenic RNA with a molecular weight of approximately 105,000 was detected in extracts from citron (C. medica) by polyacrylamide gel electrophoresis. Under denaturing conditions, 2 bands can be observed that migrate faster than the respective circular and linear molecules of the citrus exocortis viroid (CEV). Leaf curling symptoms in citron are similar to those produced by CEV and are favored by high temperature and high light intensity. The isolate was readily transmitted from citron to citron by mechanical inoculation, but not to the known herbaceous hosts of CEV. These properties suggest that the isolate may be a new viroid. Since the most characteristic feature is the irregular or variable appearance of the leaf curling symptoms in citron, the name citron variable viroid (CVaV) is suggested. Eight isolates of what have been characterized as mild forms of the exocortis disease agent principally by their reaction on citron did not protect citron plants against infection by CVaV or severe CEV. Practical implication for the efficacy of citron and gynura (Gynura aurantiaca) as indicator plants for CEV are discussed. Furthermore, the question is raised whether CEV is the only causal agent of the syndrome known as citrus exocortis disease.