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Richard P. Marini, John A. Barden, John A. Cline, Ronald L. Perry, and Terence Robinson

The influence of rootstock on average fruit weight was evaluated for a subset of data from a multilocation NC-140 apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] rootstock trial. Data for eight dwarf rootstocks were collected at four locations for 2 years. Analysis of covariance was used to evaluate the effect of rootstock on average fruit weight when crop density or number of fruit per tree was included in the linear model as a covariate. When number of fruit harvested per tree was used as a covariate, average fruit weight was not affected by rootstock in either year in Ontario. In Michigan and Virginia, rootstock and number of fruit per tree, but not the rootstock × number of fruit interaction, were significant, so common slopes models were used to estimate least squares means for average fruit weight. In general, trees on M.27 and P.1 produced the smallest fruit, and trees on B.9, M.9 EMLA, and Mac.39 produced the largest fruit. In New York the interaction of rootstock × number of fruit was significant, so least squares means were estimated at three levels of number of fruit per tree. Both years, at all levels of number of fruit, trees on M.26 EMLA produced the smallest fruit and trees on M.27 EMLA produced the largest fruit. Average fruit weight was most affected by number of fruit per tree when Mark was the rootstock. In general, results were similar when crop density was used as the covariate, except that trees on M.27 EMLA did not produce small fruit in Michigan and Ontario.

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Jens N. Wünsche, Alan N. Lakso, Terence L Robinson, Fritz Lenz, and Steven S. Denning

Although apple (Malus domestica Borkh.) system yield differences are generally related to whole-canopy light interception, this study tested the hypothesis that these orchard yields are related primarily to total light intercepted by the spur canopy. Seasonal leaf area development of different shoot types, exposed bourse shoot leaf net photosynthesis, fruit growth, whole canopy light interception (by image analysis of fisheye photographs) and relative light interception by different shoot types (by a laser assisted canopy scanning device) were estimated within four 14-year-old `Empire' apple production systems (slender spindle/M.9, central leader/M.7, central leader/M.9/MM.111 and Y-trellis/M.26). The final LAI values were CL/M.7 = 1.8, CL/9/111 = 2.3, SS/M.9 = 2.6 and Y/M.26 = 3.6. Exposed leaf net photosynthesis showed few differences and was not dependent upon the production system. Yields of the pyramidal shaped tree forms were 40 to 42 t·ha-1 while Y-trellis produced 59 t·ha-1, with similar fruit sizes. Again, yields were primarily related to the percentage of light intercepted by the whole canopy, 48% to 53% for conic forms versus 62% for the Y-trellis system. Laser analyses showed that the Y-trellis system intercepted about 20% to 30% more light with the spur canopy than the conic tree forms, supporting the hypothesis. Yields were better correlated with spur canopy LAI and spur canopy light interception than with extension shoot canopy LAI and light interception.

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William C. Johnson, Phil L. Forsline, Herb S. Aldwinckle, William C. Johnson, Phil L. Forsline, H. Todd Holleran, Terence L. Robinson, and John J. Norelli

In 1998, the USDA-ARS and Cornell Univ. instituted a cooperative agreement that mobilized the resources for a jointly managed apple rootstock breeding and evaluation program. The program is a successor to the Cornell rootstock breeding program, formerly managed by Emeritus Professor of Horticultural Sciences James N. Cummins. The agreement broadens the scope of the program from a focus on regional concerns to address the constraints of all the U.S. apple production areas. In the future, the breeding program will continue to develop precocious and productive disease-resistant rootstock varieties with a range of vigor from fully dwarfing to near standard size, but there will be a renewed emphasis on nursery propagability, lodging resistance, tolerance to extreme temperatures, resistance to the soil pathogens of the sub-temperate regions of the U.S., and tolerance to apple replant disorder. The program draws on the expertise available at the Geneva campus through cooperation with plant pathologists, horticulturists, geneticists, biotechnologists, and the curator of the national apple germplasm repository. More than 1000 genotypes of apple rootstocks are currently under evaluation, and four fire blight- (Erwinia amylovora) resistant cultivars have been recently released from the program. As a service to U.S. apple producers, rootstock cultivars from other breeding programs will also be evaluated for productivity, size control, and tolerance to a range of biotic and abiotic stress events. The project will serve as an information source on all commercially available apple rootstock genotypes for nurseries and growers.

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Jaume Lordan, Terence L. Robinson, Mario Miranda Sazo, Win Cowgill, Brent L. Black, Leslie Huffman, Kristy Grigg-McGuffin, Poliana Francescatto, and Steve McArtney

The use of highly feathered trees can make high-density apple plantings more profitable through enhanced precocity and increased early yield. Currently, nurseries are asked to provide highly feathered trees with wide branch crotch angles. The use of plant growth regulators (PGRs) can play a key role when it comes to branch induction; however, dose and timing both need to be tested to enhance branching without compromising other tree quality attributes. Over the last 4 years, we have conducted studies of the use of MaxCel® (6-benzyladenine) and Promalin® (a mixture of 1.8% 6-benzyladenine and 1.8% GA4+7) in comparison with Tiberon™ SC (cyclanilide) at several nurseries in NY, WA, DE, Ontario (Canada), and Chile. The best results were obtained with four applications of MaxCel® or Promalin® (400 mg·L−1) beginning when leader growth reached 70 cm above the soil line and reapplied at 10–14 days intervals. Promalin® was a slightly less effective branching agent than MaxCel®. On the other hand, Promalin® stimulated leader growth resulting in improved final tree height, whereas MaxCel® induced the widest branch angles. Overall, we observed good response and quality ratings with ‘Cameo’, ‘Cripps Pink’, ‘Enterprise’, ‘Fuji’, ‘Ambrosia’, ‘Crimson Crisp’, ‘Gingergold’, and ‘Granny Smith’, whereas less quality ratings were observed on ‘Ambrosia’, ‘Cortland’, ‘Goldrush’, ‘Honeycrisp’, and ‘Suncrisp’. Response with ‘Gala’ varied depending on the temperature range. Multiple sprays of Gibberellins (GA4+7, or GA3) at 250 mg·L−1 applied to nursery trees in the late summer inhibited flower bud development and flowering in the orchard the next year. This reduces the risk of fire blight infection in newly planted trees.

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William C. Johnson, William C. Johnson, Martin Goffinet, Mary J. Welser, Terence L. Robinson, H. Todd Holleran, Karl J. Niklas, and Steve A. Hoying

An increased incidence of graft union failure of apple trees during high wind events has been noted by researchers participating in the NC-140 regional rootstock testing project for certain rootstock-scion combinations. By measuring the strength of graft unions in a survey of mature apple trees in multiple stock-scion combinations, we have determined that there are significant differences. These differences may be attributable to genotype specific characteristics of rootstocks, scions, and/or rootstock-scion interactions. We are presently exploring potential biophysical and anatomical differences related to weak graft unions of apple rootstock and scion varieties. As traits correlated with weak graft unions are identified, they will be useful to help growers avoid the rootstock-scion combinations that are particularly susceptible to tree failure.

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Alan N. Lakso, Terence L. Robinson, Eddie W. Stover, Warren C. Stiles, Stephen Hoying, Kevin Iungerman, Craig Telgheder, Chris Watkins, and Kenneth Silsby

Many chemical, environmental, and physiological factors have been reported to be important to apple chemical thinning, so we have been developing a multi-site and multi-year database of chemical thinning results and potentially important factors. For 3 years, we have conducted replicated thinning trials in `Empire' and `McIntosh' apple orchards at six or seven sites around New York state in different climatic regions. Different concentrations of NAA and Accel (primarily benzyladenine), NAA/carbaryl and Accel/carbaryl combinations and unthinned controls were tested with treatments applied at the 10-mm king fruit stage by airblast sprayers. Flower cluster counts, set counts, yields, fruit sizes, and other factors thought important to thinning response (orchard condition/history, weather, application conditions, etc.) were measured or estimated in each trial. Analysis of factor importance is continuing, but some general results have come from the thinning trials so far. Thinning effectiveness varied among years from poor to adequate. There have not been consistent thinner concentration responses. Commercial NAA and Accel concentrations have not thinned adequately. NAA/carbaryl and Accel/carbaryl have thinned the most. For the same crop load, trees thinned with Accel or the carbaryl combination have had better fruit size than when thinned with NAA.

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Emily E. Hoover, Richard P. Marini, Emily Tepe, Wesley R. Autio, Alan R. Biggs, Jon M. Clements, Robert M. Crassweller, Daniel D. Foster, Melanie J. Foster, Peter M. Hirst, Diane Doud Miller, Michael L. Parker, Gregory M. Peck, Jozsef Racsko, Terence L. Robinson, and Michele R. Warmund

Researchers have collected a considerable amount of data relating to apple (Malus ×domestica) cultivars and rootstocks over the past 30 years, but much of this information is not easily accessible. The long-term goal of our working group is to increase access to this information using online technology available through eXtension. In eXtension, researchers and extension personnel are developing a community of practice (CoP) to increase the quality and amount of online information for individuals interested in our work [referred to as a community of interest (CoI)]. For this project, our CoI is broadly defined as commercial apple producers, nursery professionals, county extension educators, Extension Master Gardeners, home gardeners, and consumers. Our CoP is developing diverse educational tools, with the goals of increasing productivity, profitability, and sustainability for commercial apple production. Additionally, we will provide other members of our CoI access to research-based, reliable information on the culture of apples. We chose to begin our focus on cultivars and rootstocks adapted to the eastern United States and will add other U.S. regions as our resources and interest in our project grows.