SUMMARY: Submerging leaves in water soon after they are inoculated with viruses can prevent infection, and for studying factors that affect the rate of virus multiplication leaves should not be placed in solutions until a day after inoculation.

The rate at which viruses multiply in tobacco leaves is decreased by spraying with solutions of thiouracil, but less so than by floating leaves in the solutions. The physiological state of floated leaves affects the extent to which thiouracil impedes virus multiplication; least virus is produced in the presence of thiouracil when the condition of leaves otherwise most favours virus formation. Multiplication of virus can be checked at any time by thiouracil, but is most affected when leaves contain little virus; multiplication is resumed when thiouracil is removed. Thiouracil impedes the multiplication in tobacco of all viruses tested, but not of a tobacco necrosis virus in French bean or broad bean mottle virus in .

When mixed with inocula, thiouracil can prevent infection from occurring, both in tobacco and French bean. It also affects the growth and appearance of both plants. These effects, unlike the impedance of virus multiplication in tobacco, are not counteracted by an excess of uracil. Tobacco leaves in which tobacco mosaic virus is multiplying develop necrotic spots and rings when treated with thiouracil, and local lesions can be made evident by spraying inoculated leaves. Necrotic lesions also occur on infected with broad-bean mottle virus and treated with thiouracil.


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