If you ain't with us ... Sorting out soil inhabitants in the fight against disease by Erica McKay
GroundCover™ Issue: 45
"THE SOIL is home to a rich diversity of organisms from all major taxonomic groups," says John Thompson. "The ones we are interested in are those organisms that affect crop performance."
Figuring ont which microbes are good or bad for our farming system and how to manage them is under the microscope of Queensland Department of Primary Industries (QDPI) researcher Dr Thompson and colleague Mike Bell.
Dr Bell is looking at the soil's ability to naturally regulate disease outbreaks, so that the incidence of a particular disease is minimised and the level of yield loss reduced. "We are not talking about the total elimination of a particular disease. Rather we are looking at a suite of soil properties (chemical, physical, biological) that either do not suit the disease or that promote the activity of natural competitors of those disease organisms. Soils that act to reduce a disease are called 'disease suppressive soils'.
"When a particular soil-borne disease (e.g. root lesion nematode) is introduced into a suppressive soil, it will build up more slowly and will have a relatively minor impact on the growth and yield of the crop," said Dr Bell.
"The key to a disease-suppressive soil in most farming systems revolves around organic matter management in interaction with the tillage system," he said.
"Many modern agricultural practices actually favour the development of plant diseases. Inadequate crop rotation, excessive tillage, inappropriate use of chemicals and inorganic fertilisers, lack of organic inputs and lengthy periods of weedy or bare fallow all contribute to an imbalance.
"This results in low microbial activity and reduced diversity of soil microbes. Outbreaks of soil-borne plant diseases tend to be more severe than would be expected in natural systems."
More challenges in northern region
"In the northern grains region, limitations on soil water supply throughout the growing season are a key issue likely to spoil the chances of developing soils that suppress disease. Long bare fallows and relatively infrequent crops don't encourage the· build-up of soil organic atter and microbial activity," Dr Bell said.
He indicated they will have a better understanding of northern disease-suppressive soils in a few years' time. "The current project will determine the extent to which current farming systems on cracking clay soils affect suppression of diseases. We will also look for management practices that enhance disease-suppressive characteristics of the soil, so that future management decisions can be made from a better knowledge base."
What happens under 'friendly fire'?
Meanwhile, soil fumigation trials conducted by Dr Thompson show how important the 'good guys' are to our system. Trials have shown yield decreases through killing beneficial vesiculararbuscular mycorrhizae (YAM) fungi. Yield decreases typically occur when crops that need YAM for efficient uptake of nutrients are sown into soils where YAM levels are depleted and made worse when levels of phosphorus and/or zinc are low. YAM fungi (mycorrhizae) grow in a mutually beneficial relationship with crop cereals.
"The extent of this effect shows how dependent various crops are on YAM to acquire phosphorus and zinc from the soil. In general, crop species with poorer root hair development are more dependent on YAM for nutrition, growth and yield," Dr Thompson said.
'In previous trial work, the phosphate benefit of YAM was in the range of $200-$500/ha for highly dependent crops and $90/ha for wheat." (This was measured as the cost of P fertiliser required to bring the soil to a P level where non-mycorrhizal plants grew and yielded equal to mycorrhizal plants.)
Soil was fumigated in 2002 following chickpeas in 2001 and resulted in wheat yield increases of up to 71 per cent for an intolerant variety. This response was largely due to the control of root-lesion nematodes (Pratylenchus thorne;) in the soil profile.
"The fumigation is uneconomic, but it is a useful tool to identify yield constraints that can be overcome through more practical methods such as the use of tolerant and resistant crop varieties and resistant rotation crops," Dr Thompson said.
These responses were obtained where the farmer had adopted best practices of educed tillage, stubble retention, controlled traffic and crop rotation. "Root-lesion nematode is a particularly serious problem because of its host range that includes wheat, chickpea and mungbean," he said.
Region North, South, West