Young sour orange seedlings are more easily infected with the tristeza virus by approach-grafts than by means of the aphid vector. Under natural conditions adult plants in the field are highly resistant to infection by the aphid vector. Recovery of the tristeza virus from infected sour orange plants is also more easily accomplished by tissue union than by means of the aphid vector. Sour orange seedlings show yellowing of young leaves followed by stunting and shedding of leaves when infected with the ordinary strain of the tristeza virus in Brazil. These symptoms do not differ from those described for seedling yellows. The symptoms of leaf yellowing are less evident in plants infected with mild strains of the tristeza virus and there is subsequent recovery. New flushes of young leaves may show occasional vein dashes. Sour orange tissues below the bud union of the sweet orange onsour orange rootstock show phloem degeneration when the plant is affected with tristeza. These degeneration symptoms have been used to distinguish tristeza disease from other decline diseases that affect the same stock-scion combination. Over 45 varieties of sour oranges have been tried as rootstocks for tristeza infected sweet orange tops and they all behaved as non-tolerant rootstocks. So far no tristeza tolerant sour orange rootstock is known, but it seems entirely possible that crosses between sour orange and tristeza-tolerant citrus varieties may yield F1 hydrids that combine the desirable characteristics of the sour orange rootstock with tristeza tolerant tissues. In case such an F1 hybrid produced large number of nucellar seedlings these could be used as rootstocks. Sour orange varieties can be used as rootstocks for commercial acid lemons or sour orange tops even in areas where tristeza is present. There is as slight possibility that if a somewhat tolerant sour orange is found, it might be used a3 rootstock for grapefruits, sweet orange, mandarin, and other tristeza susceptible tops if complementary controlling measures are employed such as protective inoculation of the scion with mild strains of the virus, use of multiple bud union, etc. Tests have shown that buds from a single budstick taken from a tristeza infected sour orange plant may produce healthy and diseased tops when budded on a sour orange rootstock, thus indicating that tristeza virus was not present in all buds of the infected plant. It has been observed that commercial acid lemon and sour orange buds make satisfactory growth when top worked on tristeza-diseased sweet orange tops on sour orange rootstocks. Attempts have been made to control tristeza disease in sweet orange on sour orange rootstock by inducing the rootstock to develop sprouts and thus produce a sour orange top that would supply sinthates for the roots. The presence of this composite sweet orange and sour orange top may be of benefit to the plant when it is infected by tristeza, but this effect is not lasting and the method has been unsuccessful when tried in commercial orchards in Brazil. Sour orange seedlings in nursery rows have been used in Brazil to determine the presence of the ordinary strains of the tristeza virus in budwood from many citrus varieties. In this "sour test" the growth of buds to be tested has been compared with the growth of healthy buds of the same variety. Intergrafts of sour orange stem between a sweet orange top and a sweet orange rootstock were sufficient to induce the plants to show tristeza symptoms when infected. Three, 10, and 80-100 cm intergrafts were tried and even the 3 cm intergraft was enough to cause injury in the infected plants. When sweet orange was budded with a multiple union, i. e. on three or four upper branches of sour orange plants there was less apparent injury from tristeza virus infection than in case of a single bud union.