Stephen L. Krebs, PhD

Scientist and Director, David G. Leach Research Station


  • Ph.D. | Michigan State University, Plant Breeding and Genetics/Horticulture

Adjunct Appointments

  • Adjunct Associate Professor
    Ohio State University
    Department of Horticulture and Crop Sciences
  • Adjunct Associate Professor
    Iowa State University
    Department of Horticulture

Research Interests

My job is to manage The Holden Arboretum’s David G. Leach Research Station, a satellite facility maintained for horticultural research on a 30-acre property in Madison, Ohio. The Leach Station is staffed and equipped to support extensive and diverse display gardens, in addition to large breeding populations and research plots. The station continues its founder’s commitment to breeding superior rhododendrons and azaleas for continental climates (i.e. cold winters and hot summers). Other adaptive traits, such as resistance to Phytophthora root rot disease and high pH tolerance, are more recent program goals. In practical terms, these traits can improve consumer success with rhododendrons (which have a narrow comfort zone) and expand their geographical market. For basic research purposes, the interactions between biotic and abiotic stress adaptations are topical because the environment is poised to change significantly. My current research has focused on the predisposing effects of environmental stress (high temperature and flooding) on root rot disease development in rhododendrons, and the identification of a species from Taiwan – R. hyperythrum  – that is able to maintain Phytophthora disease resistance under warm, wet (predisposing) conditions. This species has been used to breed and field test rhododendron hybrids with novel adaptations to the Gulf South climate.

Select Publications

  • Krebs SL (2018) Rhododendron. In: Van Huylenbroeck J (ed) Handbook of Plant Breeding: Ornamental Crops, Springer. (in press)

Disease Resistance

  • Krebs, SL (2018) Heat-induced predisposition to Phytophthora root rot in Rhododendron.   Acta Horticulturae  DOI 10.17660/ActaHortic.2018.1191.9
  • Krebs, S (2013). Resistance to Phytophthora root rot varies among rhododendrons subjected to repeated flooding in the field. Acta Horticulturae 990: 243-252.
  • Long, M, S Krebs, S Hokanson (2010) Field and growth chamber evaluation of powdery mildew disease on deciduous azaleas. HortScience 45:784-789.
  • Krebs, SL and M Wilson (2002) Resistance to Phytophthora root rot among contemporary rhododendron cultivars. HortScience 37: 790-792.

Cold Hardiness

  • Krebs, S.L. (2017) Chillin’ up north with the Florida azalea, Rhododendron austrinumThe Azalean 39: 66-68.
  • Lim, C.-C., Krebs, S. L., & Arora, R. (2014). Cold hardiness increases with age in juvenile Rhododendron populations. Frontiers in Plant Science, 5, 542.
  • Peng, Y, W Lin, H Wei, SL Krebs, R Arora (2008) Phylogenetic analysis and seasonal cold acclimation-associated expression of early light-induced protein genes of Rhododendron catawbiensePhysiologia Plantarum 132: 44-52.
  • Kalberer, SR., R Aurora, N Leyva-Estrada, and SL  Krebs (2007) Cold hardiness of floral buds of deciduous azaleas: dehardening, rehardening, and endodormancy in late winter. Journal of the American Society for Horticultural Science 132: 73-79.
  • Marian, CO, SL Krebs and R Arora (2004).  Dehydrin variability among Rhododendron species: a 25- kDa  dehydrin is highly conserved and associated with cold acclimation across diverse species. New Phytologist 161: 773-780.


  • Jones, JR, TG Ranney, NP Lynch, and SL Krebs (2007) Ploidy levels and relative genome sizes of diverse species, hybrids, and cultivars of rhododendron. Journal of the American Rhododendron Society 61: 220-227.
  • Krebs, SL (2005). Loss of winter hardiness in R. ‘Supernova’, an artificial polyploid. J. American Rhododendron Society 59: 74-75. (non-refereed)

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