Population dynamics of Pseudomonas corrugata 2140R lux8 in earthworm food and in earthworm casts

Abstract

Earthworm food was tested as a carrier and inoculum source for Pseudomonas corrugata 2140R, a biocontrol agent for Take-all disease of wheat, in order to use the feeding, burrowing and casting activity of earthworms to disperse the biocontrol agent through soil. Three experiments are reported here using a bioluminescent derivative, P. corrugata 2140R lux8 (hereinafter called P. corrugata). In the first experiment, P. corrugata was inoculated into autoclaved sheep dung pellets at 10 6 colony forming units [cfu] g −1 wet weight and maintained in the laboratory either in moist soil or in a soil-free environment. In both situations, bacterial numbers increased to ca. 10 9 cfu g −1 of moist dung in 2 days and declined only slowly to about 10 8 cfu g −1 over the following 29 days at 18°C. The second experiment tested the effects of simple, carrier pre-treatments (which are likely to reduce indigenous microbial competition and increase nutrient availability) on the population dynamics of freshly inoculated P. corrugata in small pellets of two different organic plus mineral soil mixtures (cow dung and decomposed leaf litter mixed with soil). Autoclaving of cow dung led to significantly ( P < 0.05) higher P. corrugata populations over a period of 21 days after inoculation compared to untreated dung. Similarly, a freshly prepared mixture of oven-dried, milled leaf litter and soil supported larger populations of P. corrugata than the same mixture that had been conditioned for 5 days ( P < 0.05). In the third experiment, four species of earthworm ( Aporrectodea caliginosa, A. longa, Lumbricus rubellus, L. terrestris) were fed either soil or a mixture of soil and leaf litter containing P. corrugata. High numbers (10 5–10 7 cfu g −1 fresh wt) of P. corrugata were recovered from fresh casts of all species tested.