Evaluation of rootstock and crop load effects on the performance of `Gibson Golden Delicious\u27 and three scab-resistant apple cultivars

At the beginning of the 20th century, Iowa ranked sixth in national apple production. Iowa currently provides about 15 percent of the 1.3 million bushels of fresh apples eaten by Iowans annually (Pirog, 1999) and is ranked at 28th in the nation in production. The reduction in rank was mostly due to the Armistice Day freeze that killed many apple trees in 1940. Given the time needed to establish an orchard and mechanization of grain crops, many farmers changed to corn and soybeans that are less vulnerable to low temperature injury. However, current changing economic conditions and food sourcing policies are prompting farmers to consider crop diversification including apple production (Pirog, 1999). Iowa\u27s apple industry is affected by production challenges such as selection of good quality rootstocks and crop load management for high quality fruit and consistent apple production across growing seasons. The primary objective of this investigation was to evaluate the effect of rootstocks and cropping densities on fruit quality attributes and return bloom of `Gibson Golden Delicious\u27 apples\u27 and three apple scab-resistant cultivars (Redfree, Liberty, and GoldRush). Two field research studies were conducted at the Iowa State Horticulture Research station near Ames, Iowa. The first experiment was conducted in an established `Gibson Golden Delicious\u27 apple (Malus ×domestica Borkh.) orchard to evaluate the effects of dwarfing rootstocks under different crop load levels on tree growth, yield, fruit quality, and return bloom. Treatments included five dwarfing rootstocks and trees that were hand thinned to crop loads ranging from 3 to 13 fruits per cm2 trunk cross-sectional area. In the second experiment, all the three apple scab-resistant cultivars (Redfree, Liberty, and GoldRush) were grown on `Malling 9 T337\u27 (M.9 T337) rootstock and were hand thinned to crop loads ranging from 3 to 13 fruits per cm2 trunk cross-sectional area. Fruit quality tests (size distribution, starch content, fruit skin color, flesh firmness, and soluble solids) were measured at harvest and after 60 d of refrigerated storage for experiment one and for `GoldRush\u27 apples in experiment two. Fruit quality attributes for Redfree and Liberty were measured only at harvest in experiment two because these cultivars are stored for a short duration only. From the first experiment, `Cornell-Geneva 3041\u27 and `Budagovsky 62-396\u27 are promising rootstocks based on the higher percentage of marketable fruit and soluble sugar content. Results from both studies indicate that high crop densities increased yield but reduced the fruit quality attributes. The optimum crop load for a high yield with the highest percentage of marketable fruit was 6 - 8 fruits per cm2 of trunk cross-sectional area