Population dynamics of Pythium aphanidermatum and response of tomato plants as affected by root-zone temperature

Abstract

Tomato plants were cultivated in a climate chamber in 12-L-containers with aerated nutrient solution at root-zone temperatures of 20, 25 and 30 °C. Half of the containers were inoculated with oospores of Pythium aphanidermatum. During cultivation, the density of oospores in a sample of the nutrient solution was estimated one, three and five weeks after inoculation, using a haemocytometer, and the numbers of propagules in the nutrient solution and in the roots were also measured, by incubating serial dilutions of samples in a selective liquid medium. Six weeks after inoculation, plants were harvested, and their root and shoot characteristics recorded. With increasing root-zone temperature, the population densities of P. aphanidermatum in the roots and in the nutrient solution increased, while the growth of inoculated plants was reduced. Five weeks after inoculation, the pathogen density at 30 °C was 3.0 · 107 propagules per g dry mass in the roots and 6.6 · 104 propagules l−1 in the nutrient solution. At 20 °C, these quantities were 1.1 · 106 and 1.9 · 104, respectively. A high pathogen density at 30 °C resulted in significantly reduced photosynthesis, transpiration and dry mass of all plant components. Nitrogen and potassium concentration in the leaves decreased, which indicated limitations in the nutrient uptake of severely infected roots. At 20 °C, however, plant characteristics other than transpiration were not affected. It was concluded that the tomato tolerates infections of P. aphani- dermatum at low root-zone temperatures.