Title page for ETD etd-03222011-000425

Type of Document Dissertation
Author Harrison, Leigh Ann
Author's Email Address hleigh8@vt.edu
URN etd-03222011-000425
Title Characterization, development of a field inoculation method, and fungicide sensitivity screening of the Pythium blight pathogen of snap bean (Phaseolus vulgaris L.)
Degree PhD
Department Plant Pathology, Physiology, and Weed Science
Advisory Committee
Advisor Name Title
Rideout, Steven L. Committee Chair
Bush, Elizabeth A. Committee Member
Freeman, Joshua H. Committee Member
Hong, Chuanxue Committee Member
Langston, David B. Committee Member
Stromberg, Erik L. Committee Member
  • field evaluation procedures
  • azoxystrobin
  • cyazofamid
  • Pythium aphanidermatum
  • Pythium ultimum
  • Pythium deliense
  • Pythium myriotylum
  • Pythium catenulatum
  • ITS region
  • morphological characterization
  • mefenoxam
  • potassium phosphite
  • copper hydroxide
Date of Defense 2011-03-08
Availability unrestricted
New Jersey, Georgia, and the Eastern Shore of Virginia (ESV) are important snap bean (Phaseolus vulgaris L.) growing regions, but profitability is threatened by Pythium blight. Causal agents of Pythium blight on snap bean were identified using morphological characterization and sequence analysis of the rDNA-internal transcribed spacer (ITS) regions of 100 isolates. Most isolates were Pythium aphanidermatum (Edson) Fitzp. (53%), and also included Pythium deliense Meurs (31%; all from Georgia), Pythium ultimum Trow (12%), Pythium myriotylum Drechsler (2%), Pythium catenulatum Matthews (1%), and unknown Pythium sp. (1%). To our knowledge, this is the first report of P. deliense in Georgia and on common bean and squash (Cucurbita pepo L.); as well as the first report of P. catenulatum on lima bean (Phaseolus lunatus L.) and in New Jersey. Fungicide labeling and cultivar selection for Pythium blight management is hindered by difficulties associated with conducting successful trials, because the disease occurs sporadically and clustered in the field. Three P. aphanidermatum-infested inoculum substrates were evaluated at three concentrations. The vermiculite/V8 juice (5:3 weight to volume) inoculum (10,000 ppg/0.3 m) consistently caused at least 50% disease in 3 field trials. Sensitivity of the Pythium blight pathogens was determined in vitro against five fungicides. Twenty-two Pythium isolates representing P. aphanidermatum, P. deliense, P. ultimum, and P. myriotylum were inoculated to media amended with each active ingredient at 0, 100 μg/ml, the concentration equivalent to the field labeled rate if applied on succulent beans at 187 L/ha, and the equivalent if applied at 374 L/ha. All isolates were completely sensitive (100% growth reduction, or GR) to all active ingredients at the labeled rates, except azoxystrobin. At 100 μg/ml azoxystrobin, one P. deliense isolate had 8.9% GR. All isolates had 100% GR to copper hydroxide at 100 μg/ml, and the lowest GR on mefenoxam-amended medium was 91.9%. At 100 μg/ml cyazofamid, all P. deliense isolates were completely sensitive and variation was observed in P. aphanidermatum isolates. At 100 μg/ml potassium phosphite, significant GR similarities were recorded within isolates of the same species, and less than 50% GR was observed in all P. deliense isolates.
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