Characterization of Pythium and Phytopythium species frequently found in irrigation water

Open Access
Lanze, Carla Elizabeth
Graduate Program:
Plant Pathology
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 29, 2015
Committee Members:
  • Gary William Moorman, Thesis Advisor
  • pythium
  • phytopythium
  • recycled irrigation water
  • greenhouse
  • plant pathogen
  • mefenoxam
  • new species
  • pythium apanidermatum
Some Pythium and Phytopythium species are problematic greenhouse crop pathogens. This project aimed to determine if pathogenic Pythium species are harbored in greenhouse recycled irrigation water tanks and to determine the ecology of the Pythium species found in these tanks. In previous research, an extensive water survey was performed on the recycled irrigation water tanks of two commercial greenhouses in Pennsylvania that experience frequent poinsettia crop loss due to Pythium aphanidermatum. In that work, only a preliminary identification of the baited species was made. Here, detailed analyses of the isolates were conducted. The Pythium and Phytopythium species recovered during the survey by baiting the water were identified and assessed for pathogenicity in lab and greenhouse experiments. The Pythium species found during the tank surveys were: a species genetically very similar to P. sp. nov. OOMYA1702-08 in Clade B2, two distinct species of unknown identity in Clade E2, P. coloratum or one of the very closely related species such as P. diclinum, P. middletonii, an unknown species in Clade B2, an isolate somewhat similar to P. sp. nov. OOMYA1646-08 (E2), P. rostratifingens, and an unknown species in Clade A. In addition, three Phytopythium species were found: Phytopythium litorale, Ph. helicoides, and Ph. chamaehyphon. Many of these species are considered weak pathogens and some display resistance to the Oomycete fungicide, mefenoxam. Of the baited isolates, seven expressed resistance (Ph. helicoides, Clade E2-2 unknown, P. middletonii, P. sp. nov. OOMYA1646-08 (E2),) with three displaying high resistance (P. coloratum, P. rostratifingens, Clade A unknown). Seven expressed sensitivity (Ph. helicoides, Clade B2 unknown, P. sp. nov. OOMYA1646-08 (E2), Ph. chamaehyphon) with three displaying high sensitivity (Clade E2-1 unknown, P. coloratum, Clade E2-2 unknown). In a lab experiment, using Pelargonium X hortorum seeds germinated on moistened filter paper, some of the baited isolates were pathogenic. However in another test using small pots containing pasteurized, peat-based soilless potting mix, none of the baited isolates were pathogenic on geranium seedlings. It was assessed whether or not these isolates that were frequently obtained by baiting interfere with known pathogenic Pythium species, P. aphanidermatum, P. irregulare, and P. cryptoirregulare, in disease development. Some of the isolates slowed or promoted plant disease in the lab test using geranium seedlings on moistened filter paper, but these results were unable to be reproduced in the greenhouse experiments under more natural production conditions. At the end of the greenhouse experiments, root sections were plated in order to recover isolates. It was found that in the co-inoculated plants, P. irregulare and P. cryptoirregulare were almost always the only species recovered from the roots. The baited isolates were still recovered from the roots in the control plants. Lastly, a simulation of the greenhouse ebb and flood irrigation system was set up to determine if P. aphanidermatum can coexist with representatives of the frequently baited isolates in recycled irrigation water tanks. P. aphanidermatum was not recovered from any of the tanks or on the roots of plants the tanks watered. We conclude that there is an array of Pythium and Phytopythium species that reside in greenhouse irrigation systems, and that P. aphanidermatum is not one of those species. Thus, treating irrigation water with chlorine or other chemicals to remove Pythium spp. may not be necessary in greenhouses where potted plants are irrigated with recycled water. We also conclude that the highly virulent species Pythium irregulare and Pythium cryptoirregulare have attributes that allow them to dominate the niche of plant roots over those species frequently found in the irrigation water.