Microbial inoculation for improving the growth and health of micropropagated strawberry

Multimicrobial inoculation has been proposed as a way of protecting plants against environmental stress and increasing the sustainability of plant production. To study these possibilities in a micropropagation system, microplants of strawberry, Fragaria x ananssa, were inoculated or left uninoculated with five microorganisms (Glomus mosseae BEG29, Bacillus subtilis M3, Trichoderma harzianum DB11, Pseudomonas fluorescens C7rl2 and Gliocladium catenulatum Gliomix(R)), used either singly or in dual mixtures in the presence or absence of the strawberry diseases crown rot (Phytophthora cactorum) and red stele (P. fragariae). Finnish light Sphagnum peat was used as the growth substrate in the experiments. Seven experiments were performed as two to three months pot experiments in greenhouses of research laboratories in Finland and Belgium and in a nursery in Finland. In most experiments, the inoculated microorganims were detected at sufficient densities four weeks after inoculation. Exceptions were T harzianum and G. mosseae which were detected at insufficient densities in several experiments. This might have been due to the biological and/or nutritional properties of the peat. None of the microorganisms or their mixtures caused significant growth-promoting effects in more than two experiments. Dual inoculation did not increase growth more than inoculation with single organisms. B. subtilis was the most promising growth promoting microorganism. Most of the microbial treatments decreased crown rot shoot symptoms as well as the numbers of oospores in the roots when the experiment was performed in autumn. In the summer experiment with conditions more favourable for strawberry growth, no disease control was obtained, but some of the microorganisms increased the severity of crown rot. No microbial treatment decreased shoot symptoms of red stele, but the degree of root necrosis was slightly decreased by B. subtilis and G. mosseae + G. catenulatum. The numbers of oospores of P. fragariae in strawberry roots were not decreased by any treatment, but several treatments increased them. Both growth promotion and disease control considered, the single microorganisms T harzianum, G. catenulatum and B. subtilis as well as the mixture T harzianum + G. catenulatum were the most promising treatments in this study. However, the great variation between experiments indicates that more studies are needed for optimization of the whole plant-substrate-microorganism system. The importance of microbial inoculation for ensuring subsequent growth in the field also needs to be studied. (C) 2004 Elsevier B.V. All rights reserved.