The workshop courses in Plant tissue culture previously referred to has been offered each summer by the Department of Plant Sciences, University of California at Riverside. This course is offered in cooperation with the California Association of Nurserymen and is sufficiently general to be applicable to most horticultural varieties. Leading Plant Tissue authorities seem to agree that the overall technique of MTC is sound and workable and requires only refinement for each variety of plants. The workshop consists of both formal instruction and practical experience into taking and culturing of various plant tissues. The cultures thus established are propagated for a time in the culture rooms at the University. The research program of the resident students take precedence over workshop programs, so when the academic year begins, the workshop cultures must be removed to make room for controlled research program materials.
The author was fortunate to visit the University when the "house cleaning" was taking place and able to salvage some of his cultures. Out of 24 dahlia meristem tips placed in culture, 8 were recovered and brought home. Because of their immature nature and the lack of an adaquate culturing environment, only three of those grew to grafting size. Different types of grafts were used for each propagule.
One graft was established but later the root stock damped off. Efforts to re-graft failed so all propagules of the initial program were lost. Considering the lack of facilities and the conditions under which these first propagules were grown and grafted the loss was understandable. Some improvement in facilities was obtained and the first of these programs was initiated. The nutrient medium used was made up of a combination of both inorganic mineral salts and organic substances. Each is included to promote a particular function and to facilitate organic response in the plant. Since the days of Dr. Morel's and Dr. Martin's experiments much has been learned regarding these substances and their effect on organ growth and differentiation. The second program was conducted employing a process and nutrient medium designed to promote rooting.
After the propagules develop into plants and grow to maturity, they require indexing to see if they are truly free of the specified virus. At the present time, California does not have a program for indexing dahlias nor, in so far as the author has been able to determine, does any other state or agency. Such programs do exist for other crops and according to the District Supervising Biologist of the Department of Agriculture for this part of California, a program could be set up for dahlias with sufficient demand and funding.
In the absence of a State-sponsored program for indexing for the disease-free nature of recovered plants, a blue-ribbon panel of local dahlia experts would probably be a suitable temporary alternative. The dahlias could not in this way carry the designation of being certified by the D.of A, but in the author's opinion would be an acceptable stop gap. Certification by a panel of this type would probably be equally if not more meaningful to those who know dahlias best, the dahlias hobbyists. This seems axiomatic since there are several enthusiasts probably more familiar with the manner in which dahlias virus is manifested than anyone else in the country, and several of them reside in the local area. Various methods of indexing have been developed and reported which would assist the panel in determining the pathogenic condition of the recovered stock. Some of these methods are presently being investigated and can be reported on at a later date.
The experience of the author to date has convinced him that the recovery of virus free dahlias from plants which are completely infected is not only possible but feasible under home conditions, providing proper care and precautions are exercised. Some specialized equipment is required and considerable time will be needed to develop a stock of virus free dahlias and to perfect the technique which will result in an improved success rate of the overall process. The author believes this is possible and is presently continuing experimentation along such lines. Space suitable for culturing is rather at a premium but at the time of writing something over 1100 cuttings are in various phases of culture.
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