Thompson Seedless is a white seedless grape of ancient origin and suitable for a variety of uses, including table, raisin, wine, concentrate, and canning. Its leaves are also edible and used as a wrap around fillings, in types of dolma known as sarma. In California, most Thompson Seedless grapes are grown for raisins, dried on trays placed on the vineyard floor, or in dehydrators; the grapes of this variety ripen too late to dry on vine (DOV). Tray-dried raisins develop a dark brown color and are known as ‘naturals’. Some Thompson Seedless grapes are picked fresh and taken to a dehydrating facility for pretreatments, drying, and treatment with sulfur dioxide, to preserve color. Such raisins are known as “goldens,” or “golden raisins.”. Other types of raisins, including “Sultanas” and “dipped” raisins, are also made from Thompson Seedless grapes dried in a dehydrator.
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Selma Pete is a white seedless “Thompson-type” raisin grape from the USDA-ARS, selected by Ramming and Tarailo (Okie, 2002) and named after former UC Davis viticulture specialist L. Peter Christensen. Selma Pete was selected for its early maturity, ripening approximately 14 to 21 days earlier than ‘Thompson Seedless’, making it suitable for dry-on-vine (DOV) (Fidelibus, 2021; Petrucci, 2002). Selma Pete is currently the earliest-ripening Thompson-type raisin grape grown in California. It is less productive than Fiesta, but ripens sooner, dries faster, and usually produces a higher proportion of ‘B and better’ raisins (Fidelibus et al., 2008; Fidelibus, 2021). Selma Pete berries have the highest titratable acidity of Thompson-type raisin grape varieties, and the raisins are tarter as a result (Parpinello et al., 2012).
Fiesta is a white seedless “Thompson-type” raisin grape from the USDA-ARS, selected by Weinberger and Loomis (1974). Selected for its early maturity, Fiesta ripens approximately 10 to 14 days earlier than ‘Thompson Seedless’, making it less susceptible to rain damage and more suitable for dry-on-vine (DOV) raisins (Fidelibus, 2021). Raisin varieties that ripen earlier than Fiesta have since been released, but Fiesta remains popular because it is among the most productive raisin grapes available, and ripens early enough to DOV. However, Fiesta’s large crops tend to limit soluble solids accumulation, and thus result in lower raisin quality compared to other Thompson-type DOV raisin grapes (Fidelibus et al., 2008; Fidelibus, 2021). Therefore, the need to balance acceptable soluble solids accumulation, which is critical for raisin yield and quality, against timely cane severance, which is critical to ensure adequate drying, is more difficult for Fiesta than it is for varieties such as Selma Pete or DOVine, which ripen sooner than Fiesta (Fidelibus, 2021). Fiesta is also particularly susceptible to powdery mildew, and the fruit may have fewer but larger seed traces than Thompson Seedless (Christensen et al., 1983).