As a result of its excellent agronomic qualities, sour orange continues to be used for citrus in the regions where citrus tristeza virus (CTV) is rare or absent. In Italy, sour orange was the most widely used rootstock until recently. However, the recent discovery of CTV infections in some citrus groves made it necessary to exchange sour orange for a rootstock tolerant to the virus (Caruso et al., 2003; Zurru et al., 2004). Soil sickness and insufficient adaptability to particular environments and/or cultivars reinforced the need for alternative rootstocks to replace sour orange.
Using controlled hybrids as candidate rootstocks is possible if the hybrids meet the following conditions: 1) selection propagation through nucellar embryony; 2) tolerance or resistance to primary biotic and abiotic stresses; 3) grafting compatibility with most citrus cultivars; and 4) adaptability to different pedoclimatic conditions.
Long periods of research are required to verify the presence of these characteristics. Rootstock alternatives to sour orange include ‘Troyer’ and ‘Carrizo’ citranges yielded by a cross performed in 1909 (Webber, 1948); the rootstock C 35 selected in 1951 (Cameron and Soost, 1986); and the citrumelo ‘Swingle’ that resulted from a cross performed in 1907 (Hutchison, 1974; Wutsher, 1974). The widespread use of seed propagation for citrus rootstock led to neglected consideration of the monoembryonic species, particularly those of the subgenus Papeda, as well as rootstocks as parents in breeding.
In 1968, the CRA-Research Center for Citriculture and Mediterranean Crops (CRA-ACM) started a research program aimed at breeding citrus rootstocks. The monoembryonic species C. latipes (Swing.) Tan. was used as the female parent; trifoliate orange [Poncirus trifoliata (L.) Raf.], sour orange, and volkamer lemon (C. volkameriana Pasq.) were used as male parents. In previous observations, C. latipes showed high vigor and tolerance to “mal secco.” Progenies were planted in the CRA-ACM experimental fields located at Fonti Ciane in Siracusa (Sicily, Italy). Seedling selection was performed for nucellar embryony, plant vigor, tolerance to “mal secco,” and tolerance to Phytophthorae spp. (De Simone et al., 1998; Reforgiato Recupero et al., 1997; Reforgiato Recupero and Russo, 1992).
In this article, we report on the behavior of some of these hybrids tested with other citrus rootstocks in Sicily and Sardinia. The cultivars grafted on the rootstock under comparison included ‘Washington’ navel orange and ‘SRA 92’ clementine in Sardinia and ‘Tarocco’ orange in Sicily.
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