Induced Tolerance to Mal Secco Disease in Etrog Citron and Rangpur Lime by Infection with the Citrus Exocortis Viroid. Z. SOLE, Department of Plant Pathology, Department of Virology, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel. N. MOGILNER, R. GAFNY, and M. BAR-JOSEPH, The S. Talkowsky Laboratory, Department of Virology, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel. Plant Dis. 79:60-62. Accepted for publication 18 July 1994. Copyright 1995 The American Phytopathological Society 60. DOI: 10.1094/PD-79-0060.

Etrog citron and Rangpur lime plants were each inoculated with one of four isolates of the citrus exocortis viroid (CEVd) and incubated for 1 yr, during which time the viroid spread systemically. CEVd caused severe leaf curling and short internodes in Etrog citron but not in Rangpur lime. After leaf inoculation with Phoma tracheiphila, typical mal secco disease symptoms developed on leaves of both cultivars, regardless of prior infection by CEVd, but growth of the mycelium from the infected leaves into the branches was greatly affected by CEVd infection. In Etrog citron, P. tracheiphila was isolated from only 9.1% of the branches sampled from CEVd-infected plants, compared with 100% from CEVd-free plants. In Rangpur lime, the incidence of branches containing P. tracheiphila varied in CEVd-infected plants from 16.7 to 50% among CEVd isolates, compared with 70% in the viroid-free controls. This is believed to be the first report of systemic resistance induced in a woody plant by prior inoculation with a viroid.

Keyword(s): biological disease control, induced systemic resistance