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United States Department of Agriculture

Agricultural Research Service

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Thomas Tworkoski
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B.S. 1973  Rutgers University, New Brunswick, NJ

M.S. 1978 ClemsonUniversity, Clemson, SC

Ph.D. 1982 Virginia Polytechnic Institute and StateUniversity, Blacksburg, VA





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. 


Peach tree architecture is influenced by auxin and its associated gene expression.


Apple trees possess a higher abscisic acid concentration in xylem exudates and leaves when grown on dwarfing than on invigorating rootstocks.





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)




 1.  Tworkoski, T. and Miller, S.  Method for Thinning Fruit Blossoms Using Eugenol.  Serial No.:  11,801,622.  Patent approved by The United States Patent and Trademark Office8/24/2009.


 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.


 3.  Tworkoski, T. J. and Fazio, G.  Physiological and Morphological Effects of Size-controlling Rootstocks on 'Fuji' Scion.  Acta Horticulturae.  In press accepted 8 Aug 2008.


 4.  Tworkoski, T. J. and Glenn, D. M.  Orchard Floor Management Systems.  In.  The Peach: Botany, Production and Uses (D.R. Layne and D. Bassi, ed.), CABI, Cambridge, MA.  332-351.  2008. 


 5.  Tworkoski, T. J. and Glenn, D. M.  Response of young apple trees to grass and irrigation.  International J. of Fruit Sci.  8:89-108.  2008.


  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.


  7.  Tworkoski, T. and Takeda, F.  Rooting response of shoot cuttings from three peach growth habits.  Scientia Horticulturae.  115:98-100.  2007.


  8.  Tworkoski, T. J. and Miller, S. S.  Rootstock effect on growth of apple scions with different growth habits.  Scientia Horticulturae.  111:335-343.  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.


10.  Takeda, F., Glenn, D. M. and Tworkoski, T.  Weed control with hydrophobic and hydrous clay particle mulches.  HortScience.  40:714-719.  2005.


11.  Brown, M. W. and Tworkoski, T.  Pest management benefits of compost mulch in apple orchards.  Agriculture Ecosystems & Environment.  103:465-472.  2004. 


12.  Preusch, P. L. and Tworkoski, T. J.  Nitrogen and phosphorus availability and weed suppression from composted poultry litter applied as mulch in a peach orchard.  HortScience.  38:1108-1111.  2003.


13.  Miller, S. S. and Tworkoski, T.  A Review:  Regulating vegetative growth in deciduous fruit trees.  Plant Growth Regul. Soc. Amer. Quarterly Reports.  31:8-46.  2003. 


14.  Preusch, P. L., Adler, P. R., Sikora, L. J. and Tworkoski, T. J.  Nitrogen and phosphorus availability in composted and uncomposted poultry litter.  J. Environ. Qual.  31:2051-2057.  2002. 


15.  Tworkoski, T.  Herbicide effects of essential oils.  Weed Sci.  50:425-431. 2002.


16.  Tworkoski, T. J. and Glenn, D. M.  Yield, shoot and root growth, and physiological responses of mature peach trees to grass competition.  HortScience.  36:1214-1218.  2001. 


17.  Tworkoski, T. and Miller, S.  Apple and peach orchard establishment following multi-year use of diuron, simazine, and terbacil.  HortScience.  36:1211-1213.  2001.


18.  Tworkoski, T. and Scorza, R.  Root and shoot characteristics of peach trees with different growth habits.  J. Amer. Soc. Hort. Sci.  126:785-790.  2001. 


19.  Tworkoski, T.  Response of potted peach trees to pruning and grass competition.  HortScience.   35:1209-1212.  2000. 


20.  Tworkoski, T. J., Welker, W. V. and Vass, G. D.  Soil residues following repeat applications of diuron, simazine, and terbacil.  Weed Technology.  14:191-196.  2000.


21.  Tworkoski, T. J., Welker, W. V. and Vass, G. D.  Weed community changes following diuron, simazine, or terbacil applications.  Weed Technology.  14:197-203.  2000.


22.  Tworkoski, T. J., Engle, M. E. and Kujawski, P. T.  Effect of moisture stress and glyphosate on adventitious shoot growth of Canada thistle.  Weed Science.  46:59-64.  1998.


23.  Tworkoski, T. J., Glenn, D. M. and Welker, W. V.  Carbohydrate and nitrogen partitioning within one-year shoots of young peach trees grown with grass competition.  HortScience.  32:1174-1177.  1997.

24.  Thomas, R. F., Tworkoski, T. J., French, R. C. and Leather, G. R.  Puccinia punctiformis affects growth and reproduction of Canada thistle (Cirsium arvense).  Weed Technology.  8:488-493.  1994.


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.


27.  Tworkoski, T. J.  Developmental and environmental effects on assimilate partitioning in Canada thistle (Cirsium arvense).  Weed Science.  40:79?85.  1992. 


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. 


29.  Tworkoski, T. J. and Young, R. S.  Effects of rate and time of triclopyr application on Virginia creeper management in a West Virginia peach orchard.  HortScience.  25:443?445.  1990.


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.  

Last Modified: 8/11/2016
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