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4.7 Soil Disease and Suppressive Soils

Root disease can be a major restriction to plant production in all sectors. When root disease is observed in crops, we are actually seeing an imbalance in the soil biota food web, coinciding with appropriate environmental conditions. These changes have permitted a pathogenic organism to become dominant.


Throughout the world examples have been found of soils that are able to suppress disease. One of the most familiar examples is 'take-all decline' following 4-6 consecutive wheat or barley crops. Disease levels increase initially and yield declines in the first 3-4 years, but after this the level of disease falls and yield increases. This particular decline phenomenon is only seen in higher rainfall areas, especially in Europe and North America.


The following information is based on research relating to cereal root disease. However, the principles relating to increasing the suppressive characteristics of a soil are the same across all production sectors.

Factors Contributing to Suppressive Soils

An important point to consider is that all soils have a natural level of disease suppressive activities. In most soils long term management can either reduce or increase this level of suppression.


A number of management factors have been associated with increases in the level of soil suppression to cereal root disease. These include intensification in cropping, moderate to high levels of nutrients, management of weeds, pests and other diseases, stubble retention and limited grazing.


All these factors have a common end result - an increased return of residues to the soil, providing a large food supply to fuel microbial activity.

Crops infected with root disease will return less stubble and, consequently, organic matter to the soil, than a healthy crop. Less stubble means less food and lower microbial activity.


In broadacre cropping, rotations have been an important part of the root disease control strategy and, hence, a major influence on yields. In the long term trials at Avon, South Australia, it was found that the influence of rotation on the control of root fungal disease was greatly reduced once the level of soil suppression had increased. This is clearly illustrated in Figure 1.


In these experiments prior to 1989, wheat following a range of different crops showed considerable yield variability. For example, in 1979 a wheat following peas produced 3t/ha compared to wheat following wheat at 1.75t/ha, a difference of 1.25t/ha, due to high level of take-all root disease in the wheat following wheat. In 1994, the difference in yield was reduced to 0.6t/ha. Over the life of the trial a very similar result was observed for the direct drilled and conventional cultivation treatments.


Rotations that include a break crop such as grain legume or canola greatly reduce root disease in cereals because these crops do not host the cereal root disease fungi. Canola has a second beneficial effect, the release of chemicals into the soil which kill root disease causing fungi and other soil organisms. This process is known as 'biofumigation'.


Rotations will continue to play an important role in root disease control, but an increase in the level of root disease suppressive activity in the soil will allow far greater flexibility in the choice of rotations.

Results from the long term trials in South Australia indicate that increased root disease does occur when conservation farming is first introduced, but this can be significantly reduced over time without the re-introduction of burning and tillage.


The adoption of conservation farming practices results in the formation of a whole new soil environment and, consequently, the balance in the food web is adjusted. Different elements of the conservation farming system impact on the soil biota in different ways.


Soil organisms are concentrated into the top 10cm of soil. The use of minimum tillage reduces soil mixing, maintaining biota concentrations near the surface rather than diluting them through a greater depth. For some Australian soils, the greater the number of tillage passes, the greater the risk of soil erosion. Erosion results in the removal of topsoil, the home of the soil biota.


When soil is lost from a paddock it will take soil organisms along with it.


Stubble retention has a significant effect on the level of organic material (carbon) returned to the soil. Plant residues are a vital energy source for many soil biota and readily available carbon energy sources will result in rapid multiplication of the soil population. Stubble retention can also reduce moisture evaporation that may be beneficial to some organisms. Conversely, stubble burning not only allows greater moisture loss, but also physically heats the soil surface layer. This will be detrimental to some organisms.


Figure 1:
Decline in root damage by Rhizoctonia under different rotations and cultivation methods.

4.7 Soil Disease and Suppressive Soils

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