4.5 In search of a recipe for disease suppressive soil

by Stephen Neate, CSIRO Land and Water

Take home messages:

• Disease suppressive soils are found throughout the world

• We have shown that suppression develops due to changes in soil biology

• Suppression identified in South Australia is against several disease organisms (pathogens) and exists in rotations

• Intensive cropping and increased carbon inputs are associated with the development of suppression.

For many years, farmers and scientists have focused on the soil organisms that cause disease, the pathogens.  However, more recently, soil organisms which prevent disease have been put under the microscope and the search is on for farming practices which encourage the multiplication of disease suppressive organisms.

Soil organisms carry out a range of functions (see Figure 1).  One of these functions is disease transmission and suppression.  Suppression is restoring the balance between the soil organisms in the soil so the pathogens do not dominate.

A suppressive soil is one that, despite having conditions that favour the establishment of a disease forming organism, results in the pathogen:

• either not establishing,

• establishing, but not producing disease,

• establishing and then declining.

Suppression is not the elimination, but the control of disease forming organisms.  In agriculture, we are trying to achieve a level of suppressiveness which prevents disease levels that cause economic loss.  But suppression is relative - a soil may be suppressive, but disease at a lower level is still experienced.

Table 6: Advantages and disadvantages of controlling disease with suppressive soils

ADVANTAGES  

• reduces the legal, environmental and public

• safety hazards of chemicals

• should achieve control equal to the best

• currently available by other methods

• can be used in organic or reduced pesticide

• systems, thereby adding value to the product

DISADVANTAGES

• takes more intensive management and planning

• may restrict flexibility in early years

• requires new skills

• disease control is not achieved immediately

What makes a soil suppressive?

Naturally, all soils have the capacity to suppress disease.

Microbial activity, which depends on soil moisture, temperature and the ratio of carbon to nitrogen, is the precursor to suppression.  Table 7 illustrates the increase in soil organisms in suppressive soils compared to organism levels in non-suppressive soil at trial sites in Avon, South Australia. 

Conditions that change biological activity or relationships between organisms can effect suppression.

Warm, moist soils with high levels of carbon to nitrogen will have higher levels of microbial activity and a relatively higher level of suppression.

Table 7: 
Different levels of soil biota in suppressive and non-suppressive soil at Avon SA

The degree of suppression will also relate to the balance between disease-causing organisms and those organisms which feed on these pathogens.  Figure 3 illustrates that the level of disease inoculum will vary from season to season.  Consequently, a soil that is able to suppress moderate levels of disease inoculum may not be able to suppress disease in a year when large amounts of effective disease organisms have survived.

Figure 3: Survival and effectiveness of disease inoculum from one season to the next

(V Gupta)

Table 8: Disease capacity and stubble decomposition rates for the crowns/stubble recovered from field incubation which commenced in May

Where soils were suppressive, the disease level fell dramatically between July and August.  At the same time rate of stubble breakdown, which is carried out by soil organisms, was found to be greater under suppressive soils.

Which diseases can be suppressed?

In Europe, suppressive soils have been found to suppress a single disease, for example Take-all decline.  However, this suppression requires a monoculture to be maintained.  Researchers in South Australia have found soils which are able to suppress a range of diseases under a range of crop types and which remain suppressive over different seasons.

Table 9: 

Disease in broad acre cropping which may or may not be suppressed

What management practices improve suppression?

Maximising the return of crop residues to the soil has been found to increase microbial activity and turn a soil into a disease suppressive soil.

Stubble retention and minimising grazing have both been found to help maximise residue return.

Case Histories-Management systems under which disease suppression has been found to develop.

Case History 1: Booborowie, South Australia

Soil type:  Red Brown Earth

Annual rainfall:  425mm

Production System:

Prior to the development of suppression

• ley farming rotation system of wheat and barley crops with grassy pastures

• sheep - heavily grazed pastures

In the early 80’s moved into

• more intensive cropping and stubble retention

• narrow sowing points to minimise soil disturbance

• 70kg DAP + Zn and 60kg Urea

• pastures sprayed year before for weed control, cultivated after the break with narrow sowing points and use of knockdown herbicides

Mid 90’s - suppression recorded

Case History 2: Lock, South Australia

Soil type:  Grey Calcareous Sandy Loan

Annual rainfall:  400mm

Production System:

Non suppressive soil

• pasture/wheat/barley rotation

• one cultivation with narrow sowing points after the break

• Roundup three weeks before seeding

• Trifluralin before seeding

95kg 26:11 + Zn applied with the seed

Suppressive soil

• continuous crop, wheat/barley/peas rotation

• one cultivation with narrow sowing points after the break

• Roundup three weeks before seeding

• Trifluralin before seeding

95kg 26:11 + Zn applied with the seed

How long does it take for a soil to become suppressive?

Suppression develops over a period of time.  The duration will depend on the conditions and the return of organic residues.  At Avon, disease suppression developed over ten years irrespective of rotation or tillage.

4.5 In search of a recipe for disease suppressive soil

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