Use of mild strains of citrus tristeza virus (CTV) to reestablish commercial production of 'Pera' sweet orange in Sao Paulo, Brazil.

'Pera' sweet orange (Citrus sinensis (L.) Osb. ) is susceptible to CTV-induced decline when budded on sour orange (C. aurantium L) and is also highly susceptible to CTV-induced stem pitting. Many 'Pera' groves were severely affected by stem pitting even though propagated on the decline-resistant Rangpur lime (C. reticulata var. austera hybrid), and by the late 1950's, planting of this important variety was discouraged in the State of Sao Paulo, Brazil. Sub sequently, we found that trees experimentally infected with mild strains were protected against the severe, naturallyoccurring strains. The stem-pitting effects of tristeza are now being satisfactorily controlled under commercial conditions 62 by the use of mild strains. Apf woximately 10,000,000 trees of mild strain-protected 'Pera' sweet orange, the most important variety for Sao Paulo, are in nurseries, young groves, and producing orchards. Good protection also has been observed against the very severe Capao Bonito strain of CTV which occurs in the south of Sao Paulo. The protection of other citrus types with mild CTV strains, as well as the reutilization of the sour orange as a rootstock, is under investigation. Tera' sweet orange {Citrus sinensis (L.) Osbeck), a late variety that presently represents 45% of the commercial plantings of the State of Sao Paulo, Brazil (6), always has been one of its most important round oranges (16). The fruit holds well on the tree without loss of quality, and also has good storing and shipping qualities. Along with the other citrus budded on sour orange (C. aurantium L.), Tera' "was nearly wiped out when the tristeza virus was introduced into Brazil. The observation that scions budded on sweet orange, Rangpur lime (C. Proc. Fla. State Hort. Soc. 93: 1980. reticulata var. austera hybrid) and mandarin (C. reticulata Blanco) rootstock's were unaffected by CTV decline led to the use of tristeza-tolerant rootstocks for control (2). Initially, the shift from sour orange to tristeza-tolerant rootstocks was satisfactory. Within a few years, however, it was noted that many Tera' groves were severely affected by CTV-induced stem pitting, even though propagated on the decline-resistant Rangpur lime. Stunting, mineral-like de ficiency patterns on the leaves and small fruit of no com mercial value were additional symptoms of CTV infection on the affected plants (5). Growers claimed that Tera' trees stopped growing, or grew at a slower rate than trees of other varieties (17). CTV decline can cause dramatic nearly complete temporary loss of production. This loss, however, is restored by use of CTV-tolerant stocks. Stem pitting effects of CTV cause a less dramatic drop in produc tion but the effect is continuous (Fig. 1). By the late 1950's, planting of this important variety was discouraged in the State of Sao Paulo and other late varieties, such as Valencia and Natal (a type of Valencia) were propagated to replace it in the market. However, because of its high fruit quality, this variety was preferred for both local and export markets, as well as for the then new processing industry; a gap diffi cult to be filled was left.