Sunpreme is a white seedless raisin grape bred by David Ramming, USDA-ARS. It is the first raisin variety to spontaneously dry-on-vine, without cutting canes or otherwise separating the fruit from the vine. It has fruitful basal nodes and may be spur-pruned. The combination of spontaneous drying and fruitful basal nodes should make it possible to harvest and prune Sunpreme with machines, a degree of mechanization that is not possible with other raisin varieties. However, Sunpreme is also prone to preharvest fruit drop caused by the abscission of cluster branches. Compared to other Thompson-type raisin grapes, Sunpreme has larger, rounder, berries, and heavier raisins that are lighter in color. Sunpreme grapes and raisins also have a pleasant, mild, Muscat flavor.
Origins and History
Sunpreme was selected in the mid-1990s, and released by the USDA-ARS in 2017.
Sunpreme is a moderate to vigorous vine with fruitful basal nodes; its leaves are shown in Figure 1. Spur-pruned vines typically have two clusters per shoot. The clusters are conical in shape, with relatively large, round berries that are heavier than those of Thompson Seedless. Basic berry characteristics are shown in table 1. In the San Joaquin Valley of California, Sunpreme blooms a few days before Thompson Seedless (see Figure 2). Veraison occurs in early to mid June, and the onset of drying is evident by late July or early August. Fruit dry into raisins in September. Sunpreme is susceptible to delayed spring growth, so care should be taken to reduce the risk of this disorder. Magnesium and potassium deficiency symptoms have been observed on young, heavily cropped vines.
Sunpreme has high yield potential due (Table 2) to its high fruitfulness (high number of clusters per shoot), and large berries. However, this variety is prone to a type of preharvest shatter in which prominent cluster arms abscise (are shed) between veraison and harvest (Table 3). This can lead to relatively high crop loss, though yields may still be good.
Sunpreme’s fruitful basal nodes and spontaneous drying enable Sunpreme to be grown on different types of trellises than have historically been used for dry-on-vine (DOV) raisins. Optimal trellises for Sunpreme are an area of active research, but the current emphasis is on trellises designed for ease of mechanization, probably similar to modern wine grape trellises. Fruitful basal nodes should make mechanical pruning possible but Sunpreme’s high fruitfulness may make it prone to overcropping, so more experience is needed to develop mechanical pruning practices that optimize raisin yield and quality.
Sunpreme raisins are larger than those of Thompson Seedless and have excellent quality grades (Table 2). They have a mild muscat-flavor and tend to be lighter in color than Thompson Seedless or Thompson-type raisin grapes. Disuniform drying on a vineyard scale may make it difficult to determine harvest timing to ensure adequate drying (<16% moisture). Regulated deficit irrigation may improve drying.