Abstract/Summary

In agricultural systems plant diseases caused by soil borne fungi are regarded as the most devastating. Wheat is recognized as an important crop worldwide, but it is highly susceptible to Take-all disease caused by the soil ascomycete fungus Gaeumannomyces graminis var. tritici (Ggt). Interest in biological control of Take-all has increased due to a lack of resistant wheat cultivars and chemical pesticides. Although this disease has been extensively studied it is still regarded as an excellent model for biological control of plant root diseases. Pseudomonas bacteria in the P. fluorescens complex are well recognized for their plant growth promoting and disease suppressive properties and they can often be found to be prevalent in controlling Take all. In this work Pseudomonas isolates from the rhizosphere and endosphere of two wheat varieties, Hereward High Take-all Build up (H-TAB) and Cadenza Low Take-all Build up (L-TAB) were investigated. These isolates were screened for the presence or absence of two rhizosphere fitness loci, wsm and fecB, involved in host recognition and iron acquisition, respectively. It was found that these loci were significantly differentially associated with the two wheat cultivars where wsm was more abundant within the Hereward isolates while the fecB was found more within the Cadenza isolates. In addition, these isolates were tested for their in vitro inhibition of Ggt, which led to the identification of six strong Ggt growth inhibition isolates. Furthermore, testing these antagonistic isolates in the presence of the plant revealed that isolate 25R-7 was able to reduce the number of infected roots on Cadenza, while isolate 30R-11 reduced the number of infected roots on Hereward. Overall, the mixture of the six strong isolates reduced the number of infected roots in both cultivars more than that of individual strains. In addition the structure of bacterial communities associated with five wheat varieties (two L-TAB and three H-TAB) along with one barley (Unknown iii TAB) grown continually at the same site in three fields for a period of 5 years were investigated. The main difference in the structure of bacterial communities was based on field type and the rhizosphere samples separated well from those of bulk soil. Overall, no significant differences were found between the cultivars over time. The soil DNA concentrations of Ggt along with the soil DNA concentrations of bacteria, Pseudomonas and fungi were also studied from the same treatments. Year-to-year variation was the major factor in determining the amount of bacterial, Pseudomonas, fungal and Ggt DNA in the three fields.