Succession of indigenous Pseudomonas spp. and actinomycetes on barley roots affected by the antagonistic strain Pseudomonas fluorescens DR54 and the fungicide imazalil

In recent years, the interest in the use of bacteria for biological control of plant-pathogenic fungi has increased. We studied the possible side effe cts of coating barley seeds with the antagonistic strain Pseudomonas fluore scens DR54 or a commercial fungicide, imazalil, This was done by monitoring the number of indigenous Pseudomonas organisms and actinomycetes on barley roots during growth in soil, harvest after 50 days, and subsequent decompo sition. Bacteria were enumerated by traditional plate spreading on Gould's S1 agar (Pseudomonas) and as filamentous colonies on Winogradsky agar (acti nomycetes) and by two quantitative competitive PCR assays. For this we deve loped an assay targeting Streptomyces and closely related genera, DR54 cons tituted more than 75% of the Pseudomonas population at the root base during the first 21 days but decreased to less than 10% at day 50, DR54 was not s uccessful in colonizing root tips, Initially, DR54 affected the number of i ndigenous Pseudomonas organisms negatively, whereas imazalil affected Pseud omonas numbers positively, but the effects were transient, Although plate c ounts were considerably lower than the number of DNA copies, the two method s correlated well for Pseudomonas during plant growth, but after plant harv est Pseudomonas-specific DNA copy numbers decreased while plate counts were in the same magnitude as before. Hence, Pseudomonas was 10-fold more cultu rable in a decomposition environment than in the rhizosphere, The abundance of actinomycetes was unaffected by DR54 or imazalil amendments, and CFU an d quantitative PCR results correlated throughout the experiment. The abunda nce of actinomycetes increased gradually, mostly in numbers of DNA copies, confirming their role in colonizing old roots.