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[Plates 12, 13]

with Plate 6)Reducing the light intensity under which plants were grown in summer to one-third increased their susceptibility to infection with tobacco necrosis, tomato bushy stunt, tobacco mosaic and tomato aucuba mosaic viruses. With the first two viruses shading increased the average number of local lesions per leaf by more than ten times and by more than five times with the second two.Reducing the light intensity increased the virus content of sap from leaves inoculated with Rothamsted tobacco necrosis virus by as much as twenty times. As it also reduced the total solid content of sap by about one-half, purification was greatly facilitated; crystalline preparations of the virus were readily made from shaded plants but not from unshaded controls.Reducing the light intensity also increased the virus content of systemically infected leaves; the greatest effect was with tomato bushy stunt virus with which increases of up to ten times were obtained, but with tobacco mosaic and aucuba mosaic viruses there were also significant increases.The importance of controlled illumination in raising plants for virus work and the possible mechanisms responsible for the variations in susceptibility are discussed.The symptoms produced by certain Viruses depend greatly on the environmental conditions in which the host plants are grown. With some infections, variations in temperature and illumination are sufficiently important to determine whether infected plants behave as carriers or react severely. Except for their effects on symptom expression, however, environmental factors have received little attention in work on virus diseases, despite the fact that there are several diseases that show seasonal differences in their prevalence and in the ease with which they can be tranSmitted.

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