Acquired Immunity from Curly Top in Tobacco and Tomato'

Since Wingard (^4) ^ reported that tobacco plants that had recovered from severe stages of the ring spot disease failed to show the usual disease symptoms when reinoculated with the ring spot virus, similar reactions have been observed in several other plant viruses, including curly top (4, i^, ^i). Price (la) has presented an extensive and detailed review of this subject. The term ' ^recovery'^ is used in this paper to designate a permanent change in the plants from a severely diseased to a mildly diseased condition without total loss of the causative virus; and the recovery dealt with in these investigations is distinct from a simple masking of symptoms, which results from such factors as environment and natural resistance. With some viruses and under certain conditions, recovery is of such a high degree that it is difficult to distinguish between the recovered plants and the disease-free plants. In other instances the recovered plants continue to show mild symptoms but do not at any time revert permanently to a diseased condition as severe as that before recovery. In all instances where it has been demonstrated that such recovered plants and their vegetative progeny are resistant to injury from reinoculation with the virus of the disease from which they recovered, it also has been shown that the plants continually harbor virus. Although the concentration of virus in the recovered plants may be lower than that in nonrecovered plants, there is obviously some multiplication of virus since it is still present in vegetatively propagated plants many generations removed from the plant in which recovery originally occurred (18^