Studies on lettuce big vein disease
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. Lettuce big vein is an important disease of lettuce (Lactuca sativa L.) wherever the crop is grown in cool-temperature areas. It has caused economically significant losses in New Zealand, Particularly in crops grown during the late autumn to early spring periods. The causal agent is known to be transmitted by the soil fungus Olpidium brassicae, an obligate parasite. Although several types of microorganism have been implicated in LBV disease, noon has been proven to be the causal agent. In vitro culture experiments showed that the causal agent is distributed systemically throughout the plants. The in vitro cultured plants continued to exhibit LBV symptoms for six successive generations in the absence of the fungal vector. It has found that the lettuce big vein diseased state is associated with elevated rates of ethylene synthesis. Whether the disease symptoms are due to the elevated ethylene biosynthesis or the cause of it, was not determined. Nucleic acids from healthy and LBV-diseased plants were analysed by molecular hybridisation and gel electrophoresis. No evidence was obtained to suggest that a viroid was involved in LBV disease. However, gel electrophoretic analysis consistently detected double-stranded RNA in extracts from LBV-diseased tissue, but only when plants had been grown at low temperatures. One dsRNA pattern comprising 11 components in approximately equimolar concentrations, resembles the polyacrylamide gel electrophoretic patterns of the dsRNA genomes of members of the Phytoreovirus genus. A second pattern, comprising up to 13 components, was more frequently isolated. Virus or virus-like particles were not detected consistently in LBV-diseased tissue or in the O. brassicae vector. Numerous cytoplasmic inclusions were fond in LBV-diseased tissue but as they were also found occasionally in plants known to be free of LBV they were not considered to represent the causal agent.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1985