Take-all decline in New Zealand wheat soils and the microorganisms associated with the potential mechanisms of disease suppression

Take-all, caused by the soilborne pathogen Gaeumannomyces graminis var. tritici, (Ggt), is an important root disease of wheat. Continuous wheat cropping has been shown to induce take-all decline (TAD). This research investigated the mechanisms of TAD in 13 New Zealand soils in two experiments and identified the associated microorganisms using denaturing gradient gel electrophoresis (DGGE). In Experiment 1, a sterile sand/maize-meal mixture inoculated or not inoculated with Ggt, was added at 4 % (w/w) to sterilised and non-sterilised soils to determine their ability to suppress take-all, and to help identify the nature of suppression. Experiment 2 investigated the transferability of suppressive properties in five of the soils from Experiment 1. The microbial communities of these five soils were analysed using PCR-DGGE. Ten of the soils were able to suppress take-all but the suppression was biological in nature in only four of these soils. The suppressive properties of two of the soils were transferred to a gamma-irradiated base soil amended with Ggt, indicating that suppression could be specific in nature (i.e., attributed to a specific microorganism or group of microorganisms). The suppressive properties in one soil were not transferrable, suggesting a general form of suppression, most probably because the conditions in the soil were suitable for other microorganisms to compete with Ggt. DGGE analyses of the microbial communities for five of the soils showed similar banding patterns for those with similar forms of suppression (specific, general and non-suppressive) and identified the potential microorganisms that distinguished them. These distinguishing microorganisms are likely to independently or interactively have a function in suppressing take-all.