1Cooperative Research Centre for Soil and Land Management and 2CSIRO, Division of Soils, Private Bag No 2, Glen Osmond, SA 5064, Australia

Background and objectives
Gaeumannomyces graminis var. tritici (Ggt) is the causal agent of take-all disease and is the major soil-borne fungus of wheat and barley world-wide. Development of suppression to this disease in rotational cropping (as distinct from take-all decline in cereal monoculture) has been identified at Avon, South Australia (DK Roget, pers. comm.). The mechanism regulating the development of suppression and the location of the suppressive activity in relation to the bulk soil or root zone (rhizosphere/rhizoplane/ endophytic growth) is not known. The ability to manage natural suppression in soils would be of major benefit in control of this disease. A DNA-based assay has been used routinely to quantify the amount of Ggt inoculum in soil [1, 2] and has the potential to be a valuable tool in the study of suppression of Ggt. The aims of this study were to determine the location of suppression activity as an aid to develop improved assays for the quantification of suppression to Ggt


Materials and methods
Suppression in the bulk soil was determined by assessing the rate of decline in added inoculum in seven soils selected for an expected range of Ggt suppression levels. The soils (100 g) were inoculated with 50 ;mg Ggt isolate BB26 on ryegrass seed and placed in plastic tubes. Sterilized water (15 ;ml) was added and the soils incubated at 15C for 6 or 12 ;days. The change in inoculum was determined by DNA slot-blot hybridization assay using the pG158 probe. Suppression in the root zone was assessed in three soils by growing wheat plants in either gamma irradiated or non-irradiated soils. Wheat was grown in specifically designed pots where one side could be removed to add inoculum (1 ;cm2 agar containing the fungus) on the surface of one of the roots after 2 ;weeks. The length of the take-all lesion and the level of Ggt DNA were assessed after a further 3 ;weeks.

Results and conclusions
In the bulk soil experiments the inoculum level declined, on average, by 85% at 6 ;days and by 96% at 12 ;days. The rate of inoculum decline was very rapid and was a function of the inoculum source (ground ryegrass seed). The only significant difference between the soils occurred at day 6, with the Avon soil containing less Ggt DNA than any other soil. In the root zone experiment, significant differences occurred in the length of Ggt lesions with two soils, including the Avon soil showing reductions in lesion length of approximately 80% compared with the third test soil. Assessment of Ggt DNA showed results similar to the lesion length measurements, as there was a strong relationship between these two measurements [2]. These initial studies have provided evidence for suppression activity occurring in both the bulk soil and root zone, however, the effect in the bulk soil appears quite transitory and is less likely to be of practical importance.

1. Herdina, Harvey P, Ophel-Keller K, 1996. Mycological Research 100, 962-70.
2. Herdina, Yang HA, Ophel-Keller K, 1997. Mycological Research 101, 1311-17.