CORESTA Meeting, Agronomy/Phytopathology, Budapest, 1993

The tobacco bushy top disease in the Republic of South Africa

Tobacco and Cotton Research Institute, Rustenburg, South Africa
The Bushy Top disease, affecting late plantings of tobacco, can cause serious losses, thus limiting the growing season. Previous research suggested two viruses involved, one of which is mechanically transmitted and only aphid transmissible in the presence of the other, which is always aphid transmissible. Previous attempts to separate them were unsuccessful. The casual organism(s), associated with the disease in South Africa has not yet been identified. The purpose of this study was to separate and characterise the casual agent(s) of the disease. Field infected plants were collected, kept in a growth chamber and used as source plants for further experiments. These plants were tested with a F(ab')2 ELISA against known tobacco viruses, but none reacted positively. dsRNA was extracted from these plants and fractionated on polyacrylamide gels to obtain dsRNA profiles of the virus(es), possibly implication a dsRNA virus(es). Separation of the components was attempted with transmission studies, which included vector transmission with the aphid Myzus persicae, dodder transmission, grafting and mechanical inoculations. Tobacco plants exposed to differential inoculation feeding times with aphids, developed veinal distortion symptoms and a distinct crinkling of the leaves, but no bushy appearance. A bushy appearance yet without veinal distortion and crinkling of the leaves developed after mechanical inoculations. dsRNA was successfully extracted from these plants, and had different dsRNA profiles than the field infected source plants. These different symptom types were once again tested with F(ab')2 ELISA, but none reacted positively. Electron microscopy showed ultrastructural changes of phloem tissue, typical of a viral infection, but no virus particles could be observed.