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The characteristics of l-year-old vegetative spurs growing on 2-year-old branches were measured on 28 ‘Delicious’ apple (Malus domestica Borkh.) strains growing on M.7 rootstocks at Clarksville, Mich., and on 23 strains of ‘Delicious’ on M.7a rootstocks at Kearneysville, W.Va. Spur-type strains typically had densities >20 to 21 spurs/m, and high spur leaf numbers, leaf areas per spur, leaf areas per leaf, and terminal bud diameters, whereas values for standard strains were generally lower. However, for most spur quality characteristics, there was a continuous range of values between the extremes rather than any distinct grouping into either spur or standard type. At both sites, spur density was significantly and positively correlated with yield efficiency. In a related study, the spur characteristics of ‘Starkspur Supreme’ were measured on nine rootstocks: M.7 EMLA, M.9 EMLA, M.26 EMLA, M.27 EMLA, M.9, MAC 9, MAC 24, OAR 1, and Ottawa 3. Spur leaf number and spur leaf area were both high with vigorous rootstocks, whereas spur density was low. The rootstocks MAC 9, M.9, and M.9 EMLA had the highest yield efficiencies. A feature of red apple cultivars in general, and of ‘Delicious’ in particular, is the availability of many different strains from commercial nurseries (Tukey and Ballard, 1969a). Most ‘Delicious’ strains have been selected primarily either for some specific fruit characteristic (mainly color and shape) or for a particular type of growth and related spur-bearing habit that is predominantly either a spur type or standard form (Fisher and Ketchie, 1981; Tukey and Ballard, 1969b). However, while fruit characteristics are well known and definable (Crassweller et al., 1985; Fisher and Ketchie, 1981; Ingle, 1972; Lord et al., 1980), few data are available on the yield performance of the different strains or on the relative performance of strains within and among apple-producing regions (Dozier et al., 1984; Ferree et al., 1975; Ketchie, 1984, 1987; Lord et al., 1980; Rom and Ferree, 1984). Therefore, the selection of the most appropriate and highest-yielding strains for a particular region is not possible unless either comparative regional trials are carried out (Ketchie, 1987) or indices can be identified that would allow a prediction to be made of relative performance. An obvious difference in the growth characteristics of various ‘Delicious’ strains is the degree of spur-bearing habit. Spur leaves are critically important for fruit development (Ferree and Palmer, 1982), and differences in spur habit and form may, therefore, influence fruit yield and the distribution of fruit on a tree. Before bloom, spur leaves constitute the major leaf area on a Received for publication 8 June 1989. Salaries and research support provided nd federal funds appropriated to the Ohio Agricultural Research and ent Center, The Ohio State Univ. Journal Article no. 18-89. The cost ing this paper was defrayed in part by the payment of page charges. stal regulations, this paper therefore must be hereby marked adverolely to indicate this fact. address: Plant Physiology Division, DSIR, Private Baa, Palmerston ew Zealand. ddress: Highmoor Farm, Univ. of Maine, PO Box 178, Monmouth, 9. tree and provide the majority of photosynthate available for early fruit growth and development (Hansen, 1971). During fruit set and the cell division phase of fruit growth, spur leaves play a major role in determining fruit size, shape, and fruit calcium concentration (Ferree and Palmer, 1982). The removal of spur leaves before and during bloom reduces fruit set (Dalbro, 1966), while a certain minimum spur leaf area is necessary for flower bud formation (Harley et al., 1942). Differences among apple cultivars for spur diameter, flower number, spur leaf number, area, and individual spur leaf size have been reported, and total spur leaf area or average spur leaf size has been related to tree productivity, yield efficiency, and yield variation (Rom and Ferree, 1984). The objective of this study was to evaluate these relationships on various commercially designated spur and standard ‘Delicious’ strains growing in two locations. In addition, the influence of rootstock on spur quality, tree size, and yield of ‘Starkspur Supreme Delicious’ was investigated on trees planted at the Ohio Agricultural Research and Development Center (OARDC), Wooster. (Throughout the manuscript, the word ‘Delicious’ has been deleted from each strain name to avoid unnecessary repetition.) Materials and Methods Twenty-eight ‘Delicious’ strains were evaluated at the Michigan State Univ. Clarksville Horticultural Research Station (Table 1). The trees were established in 1980 in a planting 18 trees long and 21 rows wide, with each strain represented by one tree in each of six blocks. All of the strains studied were planted on M.7 EMLA rootstock. An equal number of trees on MM.111 EMLA rootstock were randomly distributed among the study trees, and four individual trees of ‘Idared’ were included in each block as pollenizers. Trees of similar age and size surrounded the planting. The within/between row spacing was 3.65 × 5.5 m. The trees were part of a cooperative research program to J. Amer. Soc. Hort. Sci. 115(3):348-356. 1990. Table 1. ‘Delicious’ strains studied, name of the supplying nursery, plant patent number, growth type, and location of study site. Strain Nursery U.S. plant Growth Study company patent type site

Strain and Rootstock Effects on Spur Characteristics and Yield of `Delicious' Apple Strains