The growth and disease mitigating activity of fluorescent pseudomonad isolates under modified soil atmospheres.

Abstract

The growth of selected isolates of fluorescent pseudomonads in the rhizosphere of tomato (Lycopersicon esculentum cv. Earlypak) and cucumber (Cucumis sativus cv. Straight 8) seedlings was determined under soil atmospheres containing O₂:CO₂ concentrations of 21:0.03 (ambient), 18:3, 15:6, and 12:9%. While growth varied depending on the isolate, plant, and the composition of the soil atmosphere, it was generally greater under atmospheres containing lower-than-ambient levels of O₂ and higher-than-ambient levels of CO₂. The average percent growth increases, relative to control (the ambient atmosphere), for the most sensitive isolate were 16.5, 51.9, and 116.6, under O₂:CO₂ concentrations of 18:3, 15:6, and 12:9%, respectively. The effect of soil atmospheric composition on disease mitigating activity of two fluorescent pseudomonad isolates was also studied in the tomato-Pseudomonas solanacearum combination. Significant (p = 0.05) decreases in disease mitigating activity were observed when roots were placed under modified atmospheres containing lower-than-ambient levels of O₂ and higher-than-ambient levels of CO₂, compared to those placed under an ambient atmosphere. Modification of soil gaseous composition did not cause significant (p = 0.05) changes in the incidence of disease in plants inoculated with P. solanacearum alone.