ASSESSMENT OF HOST-SPECIFICITY AMONG DIFFERENT SPECIES OF GLYPHOSATE SYNERGISTIC PYTHIUM

A total of 39 Pythium isolates representing 14 species of Pythium was assessed for host specificity as glyphosate synergists. This was done using three groups of Pythium isolates from roots of glyphosate-treate d bean (PBI), wheat (PWI), and isolates from various glyphosate-untrea ted hosts. PBI consisted of 15 isolates and included P. ultimum, P. sy lvaticum, P. coloratum, P. irregulare and P. group 'HS'. PWI consisted of 14 isolates representing the first four of these species. Pythium from glyphosate-untreated hosts included single isolates representing P. aphanidermatum, P. spinosum, P. paroecandrum, P. hypogynum, P. sple ndens, P, sulcatum, P. vanterpooli, P. acanthicum, P. arrhenomanes and P. coloratum. The glyphosate synergistic potential of the Pythium iso lates was determined by treating a-week-old seedlings growing in soil infested with individual isolates of Pythium with different doses of g lyphosate. LD(50) values associated with each isolate were estimated b y logistic regression analysis of plant mortalities recorded 4 wk afte r treatment with glyphosate, and compared with LD(50) values for plant s grown in the absence of Pythium. Host specificity was assessed by co mparing the glyphosate synergistic potential of PBI on bean and PWI on wheat seedlings, with the potential of these same isolates of PBI on wheat and PWI on bean seedlings. Glyphosate synergistic potential of P BI was also estimated on sunflower and pepper, to test whether PBI wer e capable of glyphosate synergistic interaction (GSI) on other unrelat ed dicot yledonous species. Pythium isolates from glyphosate-untreated hosts were tested on bean to determine if Pythium species not represe nted in the PBI and PWI groups were capable of GSI. The glyphosate syn ergistic potentials of the PBI and PWI on wheat seedlings were low and inconsistent compared to those observed on dicot plants. AU PWI and 1 2 of the 15 PBI were glyphosate synergists on beans, and all the PBI w ere glyphosate synergistic on sunflower and pepper seedlings. All Pyth ium isolates from glyphosate-untreated sources tested were also glypho sate synergists on bean seedlings. These various tests of glyphosate s ynergistic potential of Pythium isolates from diverse sources on vario us plant species revealed no evidence of host specificity among the is olates and species tested.