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Abstract: It was reported in 1960 by Gandy that mushroom virus affected the rate of growth of mycelial cultures, and that treatment of the diseased mycelium at 33°C for a period of 2 weeks resulted in a faster growth, when the cultures were subsequently transferred and grown at 25°C. Edwards in 1968 referred to the testing of spawn for virus infection by growing subcultures at 20°C and 30°C for about three weeks. After this time, transfers from each culture were taken and growth rates were compared. He drew the conclusion, that if the high temperature incubation produced an increase in growth rate then the original culture was virus-infected. On the other hand, no increase showed that the culture was not sufficiently virus-infected to have any effect on its growth rate. In 1970 Dieleman-van Zaayen reported treatment of virus-infected cultures at 33°C for 2 weeks. Some of the cultures showed increased growth rate as did also some of the healthy controls. When spawn made from the heat-treated cultures was grown it produced mushrooms showing virus infection as determined by the electron microscope. It was concluded that the temperature treatment is not very promising and that improved growth rate of a treated colony does not mean that is has become virus-free or that it carried virus before-hand. Yields, however, from these spawns were not reported. Dieleman-van Zaayen went on to say that such a method, even if it was found to be effective, would not be of direct interest to mushroom growers. In the opinion of the present authors this could be of vital importance to the grower, who depends upon the spawn maker to provide spawn having the greatest possible freedom from disease, and producing the highest yield possible. The object of the present work was to investigate more systematically and thoroughly the effect of heat-treatment of cultures on the productivity of the spawn made from them.
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