Interactions among Pythium aphanidermatum, roots, root mucilage, and microbial agents in hydroponic cucumbers

Abstract

Epidemics and biological control of Pythium root rot were investigated in greenhouse cucumbers grown in small-scale hydroponic systems with troughs and recirculating nutrient solution. The systems were inoculated with Pythium aphanidermatum only or with the pathogen plus an isolate of Pseudomonas fluorescens or Pseudomonas chlororaphis. In systems with P. aphanidermatum only, growth rate of roots, amount of root mucilage in the nutrient solution, and root discoloration (browning) oscillated in patterns that were generally similar and synchronous. These variables abruptly increased for 6-14 days, starting at about 21 days after transplanting, subsequently declined to low levels at 41-56 days, and then increased again starting at 53-61 days. Logarithm to the base 10 of propagule density of P aphanidermatum usually was several orders higher in the mucilage (2.09-4.90/g) than in the nutrient solution (0.25-2.67/mL). The pathogen was also more frequent in roots with associated mucilage (14-60 colonies/60 cm of root) compared to those lacking mucilage (2-26 colonies/60 cm of root). These findings, combined with positive correlation between mucilage and discolored roots, and microscopic observations of Pythium hyphae in mucilage and discolored roots, indicated that the mucilage supported prolific growth of P. aphanidermatum and served as a food base from which the pathogen invaded roots. The bacterial agents each reduced root mucilage until about 68 days after transplanting, and root discoloration chiefly at 47-61 days. Mucilage breakdown by the agents was an apparent mode of biological control of P. aphanidermatum. Pseudomonas fluorescens reduced mucilage and root discoloration more effectively than P. chlororaphis, but did not significantly promote marketable yield (fresh mass basis). Pseudomonas chlororaphis increased cumulative marketable fruit yield at 47-78 days, but not at 78-127 days. Both agents increased numbers of marketable fruit. The agents did not affect incidence of the pathogen in young roots, which are normally infected by zoospores. Densities of incidental microbes in nutrient solution were high after 69-74 days. It is hypothesized that shifts in assimilate partitioning to roots successively affected root growth, mucilage production, saprotrophic growth of P. aphanidermatum, and root invasion by hyphae of the pathogen. The microbial agents appear to have value for mucilage management and root rot control.

Keywords:

• epidemiology
• root rot
• biological control
• Pseudomonas chlororaphis
• Pseudomonas fluorescens