Ph.D. 1982 Virginia Polytechnic Institute and
The overall goal of our group research project has been to develop knowledge and technology to optimize yield and fruit quality through environmentally and economically sustainable modifications in orchard design and management. Our group integrates horticultural, entomological, and engineering technology with a broad base of expertise to solve major problems affecting temperate tree fruit production, the sustainability and environmental impact of tree fruit production, and consumer acceptance of tree fruits. My individual research contributes to the project goals through improved development of light, water, and pest management with new vegetation manipulation of the orchard floor and through improved understanding of how hormones and tree growth habit affect carbon partitioning and tree development.
Objective is to elucidate environmental and physiological factors that regulate tree architecture and control carbon partitioning. Two research programs focus on this objective.
Research integrated field with laboratory approaches to understand tree and weed physiology to improve management of orchard floors and fruit tree development.
Characterizing growth and hormone physiology of apple and peach. Genetically-based morphological relationship in shoots and roots of peach trees were determined and hormonal differences in genetically-distinct peach growth habits were identified. Such hormonal differences may be focal points for genetic or management manipulation. For example, we recently demonstrated that size controlling apple rootstocks export different quantities of a hormone which may have reduced the water-conducting capacity of the shoots, leading to reduced shoot growth. Reduced shoot growth can be highly desirable for efficient and productive fruit trees. (Publication Nos. 2, 3, 6, 7, 8, 9, 18)
Long-term use of herbicides and alternative weed management tools. Several soil residual herbicides had been used for orchard weed management for decades and we investigated the effects of such long-term use on carryover and buildup dynamics and their impact on soil properties, weed populations, and replanted fruit trees. A small pool of herbicide carried over from year-to-year in the soil but the size of the pool did not increase, which indicated that high toxic levels of the residual herbicides did not occur. However, in some cases soil organic matter was reduced and growth of young peach trees was adversely affected. In addition, long-term residual herbicide applications also caused distinct shifts in weed populations that contributed to new pest problems. New weed control technologies were developed that included ground cover grasses combined with mowing, organic mulch, and natural product herbicides (clove or cinnamon oil) to suppress broadleaved weeds in orchards. (Publication Nos. 4, 10, 11, 12, 14, 15, 17, 20, 21)
Crop load management. In addition to its herbicide activity it was discovered that a component of natural product herbicides, eugenol, could be used safely as a flower-thinner to manage crop load of peach and apple trees to increase crop value. A patent was granted in 2009 and cooperative research with this technology is being conducted. (Publication No. 1)
Managing tree growth with controlled grass competition. In addition to managing weeds, grass ground cover was used to regulate the size of peach trees which may facilitate high density plantings of peach. (Publication Nos. 5, 13, 16, 19, 23)
Biology and management of perennial weeds. Reduced soil tillage can be an important conservation practice but a consequence of this practice is an increase in perennial weeds. Improved management of perennial weeds relies on understanding the biological basis for their growth and reproduction. Over several years we focused on understanding the regulation and seasonal patterns of carbohydrate movement and storage in perennial weeds. This knowledge enabled prescriptions for weed management based on times of weed susceptibility and for improved control by biological or chemical technologies. (Publication Nos. 22, 24, 25, 26, 27, 28, 29, 30)
2. Tworkoski, T., Scorza, R. and Glenn, D. M. Leaf N and P in different growth habits of peach: Effects of root system morphology and transpiration. Journal of Applied Horticulture. In press accepted 14 Sep 2009.
6. Tworkoski, T. J. and Miller, S. S. Endogenous hormone concentrations and bud-break response to exogenous benzyl adenine in shoots of apple trees with two growth habits grown on three rootstocks. The Journal of Horticultural Science & Biotechnology. 82:960-966. 2007.
9. Tworkoski, T., Miller, S. S. and Scorza, R. Relationship of pruning and growth morphology with hormone ratios in shoots of pillar and standard peach trees. Journal of Plant Growth Regulation. 25:145-155. 2006.
25. Frank, J. R. and Tworkoski, T. J. Response of Canada thistle (Cirsium arvense) and leafy spurge (Euphorbia esula) clones to chlorsulfuron, clopyralid, and glyphosate. Weed Technology. 8:565-571. 1994.
26. Nickerson, R. G., Tworkoski, T. J. and Luster, D. G. Colletotrichum coccodes and thidiazuron alter specific peroxidase activities in velvetleaf (Abutilon theophrasti). Physiological and Molecular Plant Pathology. 43:47-56. 1993.
28. Tworkoski, T. J. and Sterrett, J. P. Phytotoxic effects, regrowth and 14C‑sucrose translocation in Canada thistle treated with mefluidide, flurprimidol and systemic herbicides. Journal of Plant Growth Regulation. 11:105‑111. 1992.
30. Tworkoski, T. J. and Sterrett, J. P. Modification of root bud growth in Canada thistle with selected plant growth regulators: Effect on translocation of glyphosate. Journal of Plant Growth Regulation. 6:221‑232. 1987.