By Tony Pattison, Department of Agriculture & Fisheries, South Johnstone
Having soils that can overcome soil borne diseases, like Panama disease of bananas, is a major goal for banana growers.
But can soil management, through changes in fertiliser, pesticide and ground cover practices have a big enough impact to reduce
the disease? The answer is yes, but it is not that simple.
The ability for soil to suppress Panama disease was tested on a field site over a two-year period.
When bananas were planted, and the soil tested for suppression, there was none.
When Fusarium race 1 was added to the soil, all plants were equally showing symptoms of the disease.
Two years later, when soil from the field site was tested again in the same way, some soils showed high levels of disease while others did not.
The soils that showed high levels of disease had been kept bare, with high rates of nitrogen fertiliser applied; 350 kgN/ha/crop cycle.
The soils with the lowest disease had vegetated ground covers and low rates on nitrogen; 220 kgN/ha/crop cycle.
In the two-year period, with the different soil management practices, there were changes in the soil micro-organisms.
Under low nitrogen and with continual ground cover, soil organisms that decompose and recycle organic matter become more dominant.
For soil micro-organisms to decompose organic matter they must produce enzymes.
Enzymes are substances produced by a living organism, which increase the rate of a biochemical reactions.
In particular, the amount of beta-glucosidase measured in the soil related to the amount of disease in bananas.
Beta-glucosidase is an enzyme that is used in the breakdown of cellulose to sugars.
Soil microorganisms produce the beta-glucosidase, which is secreted into the soil.
In the soil the betaglucosidase degrades the cellulose found in organic matter into simple sugars that the organisms uses for energy.
Fusarium living in the soil can produce betaglucosidase when it is living as a saprophyte, getting its energy from dead organic matter.
However, most soil micro-organisms that produce large amounts of beta-glucosidase are more efficient at decomposing the organic matter and getting the sugar than Fusarium.
This results in competition with Fusarium in the soil and suppression of Panama disease.
A downside in this situation is that soil microorganisms that produce large amounts betaglucosidase are most efficient in soils with low nitrogen.
Low nitrogen for soil micro-organisms, means low nitrogen available for plant growth, which can result in reduced banana production.
The upside is better nitrogen management practices can maintain production while encouraging the soil organisms that produce beta-glucosidase to compete with Fusarium.
Better nutrient management will also reduce off-farm movement of nitrogen, reducing farming impacts on the environment.
Fertiliser practices like fertigation, where small amounts of nutrients are applied often, or slow release fertilisers that trickle nitrogen into the soil system are more likely to maintain the suppressive soil organisms than when fertiliser is applied in large amounts monthly.
Nitrogen management is only one part of building a suppressive soil. Soil pH, organic matter, clay content are all important characteristics that affect suppression of Panama disease.
The physical and chemical properties of the soil can dictate the types of soil micro-organisms that control soil functions.
Beneficial micro-organism typically decline rapidly and are slow to build up, whereas the unfavourable organisms are usually quick to build up, but slow to decline.
Using the soil beta-glucosidase test and quick assessments of banana productivity, such as finger number assessments, it is possible to view soils in four ways:
- Low production, disease conducive
- Low production, disease suppressive
- High production, disease conducive
- High production, disease suppressive.
As our knowledge on the impact of soil management on crop production and diseases increases, we can use the four different scenarios of production and suppression to identify better banana farming practices. This may be done at a farm level or an industry level.
We now know that soil management practices have an important role in the suppression of Panama disease.
The beta-glucosidase soil test gives us a tool to quickly identify situations where disease can be suppressed.
The challenge is to manage the soil so that productivity can be maintained.
* This work was funded by the Queensland Government and the Australian Centre for International Agricultural Research (ACIAR)