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SUMMARY The differing responses of a wide range of tobacco mosaic virus (TMV) isolates to various genes controlling resistance in tomato were not correlated with the amino acid composition of their coat proteins. Nor were they necessarily linked with ability to cause necrotic local lesions in White Burley tobacco, which is usually associated with the presence of methionine in the coat protein.

SUMMARY Tomato mosaic virus derived from susceptible tomato plants (the standard virus) was cultured in resistant plants. Sap from non-inoculated leaves of resistant tomato plants infected with virus from the resistant host was more infective and contained more virus particles than leaf sap of resistant plants infected with the standard virus. Leaves of resistant tomatoes infected with virus from the resistant host also showed more obvious symptoms. Susceptible plants infected with virus from resistant plants not only showed fewer symptoms than when infected with standard virus, but samples were less infective and contained less virus up to 26 weeks, when values for infectivity were similar. This modification in activity was not reversible and was obtained with two lines of tomato having different types of resistance. Passage of virus from resistant plants through susceptible plants did not impair its ability to infect resistant plants.

SUMMARY Multiplication of tomato mosaic virus (strain of tobacco mosaic virus) in susceptible and in resistant tomato plants was studied by assaying the infectivity and virus content of sap from non-inoculated leaves on infected plants. In susceptible plants, infectivity and virus particle number increased rapidly to a maximum at about 14 days after inoculation; leaves produced subsequently contained less virus. In resistant plants, no virus was detected in non-inoculated leaves until 5 weeks after inoculation. Infectivity then gradually increased at a somewhat higher rate than did numbers of virus particles. The concentration of virus in resistant plants remained lower than in susceptible ones. At first the ratio of infectivity to particle number was larger for samples from the susceptible than the resistant line, but the ratio from the resistant line increased slowly until it eventually equalled that from the susceptible one. Virus cultured in either of two lines of tomato with different types of resistance invaded healthy plants of either line more readily than virus cultured in the susceptible variety. Resistant lines seem to select self-invasive new forms of tomato mosaic virus.