Pythium's role in making root systems vulnerable to attack by other root pathogens has been uncovered by research aimed at helping growers get the insidious fungal disease under control.

Pythium root disease has been described as the 'common cold' of cropping systems but now the research, led by the CSIRO in partnership with Landmark AWB, could help to both cure and prevent it. In common with a human cold, pythium is prevalent, hard to beat and causes significant setbacks to productivity.

The research team has shown that only partial disease control can lead to a yield increase in cereals of 5-20%. Canola showed an improvement of 5-30% and pulse yields increased by 5-50%. The scale of the yield response depended on the season and previous cropping history.

The project is supported by growers and the Australian Government through the Grains Research and Development Corporation (GRDC). It has found that pythium is the first cab off the root disease rank because it is fast growing and can infect as soon as the plant seed starts to germinate. Chemicals released by germinating seeds and growing roots, known as exudates, act as growth stimulants and attractants for the pythium spores and allow them to grow quickly and infect the roots. This weakens the plant and makes it vulnerable to infection by other diseases, such as take-all and rhizoctonia.

Project leader and CSIRO Entomology scientist Dr Paul Harvey said pythium was an important disease in its own right, but the fact that it can form a disease complex with other root pathogens makes it a greater threat.

Previously, risk ratings have only considered pythium as a seedling 'damping-off' disease that reduces crop emergence and this rating has lead to the disease being underestimated. However the research showed that the major disease impact results when pythium root rot develops during the first few months of the growing season.

Once plants are infected, pythium strips off the fine lateral feeder roots and root hairs, reducing nutrient and water uptake. Unlike rhizoctonia and take-all, pythium produces large numbers of spores that enable the pathogen to rapidly and continuously re-infect growing root s as the plant struggles to grow. This leads to the development of pythium root rot and root disease complexes with other fungi later in the season.

The good news is that there are some options for controlling pythium. The project found that different crop types had varying degrees of susceptibility to the disease itself and varying capacity to carry over the disease to the next season. Dr Harvey said this information could be used to plan rotations to thwart the disease. For example, legumes and oilseeds are most vulnerable to pythium and cereals are the least vulnerable. In a pasture phase ryegrasses are a better choice than medics, for which pythium can be a severe problem.

Pythium can also be controlled using fungicides with the active ingredient metalaxyl. This is usually applied as a seed dressing, so it only offers a short window of protection against the pathogen. Dr Harvey said the trials found that although seed dressing was only giving about 20% control of the disease in the first 2-3 months of crop growth, is was still enough to produce good yield responses. The team is now experimenting with different modes of application, including adding the fungicide to fluid fertiliser and applying it with drench nozzles.

The team is also working on developing a biological control agent in the form of a naturally occurring soil fungus that will both out-compete pythium and prey upon it. Initial field results with the beneficial fungus have been encouraging and it is being developed as an inoculant for commercial release as an environmentally friendly disease control option.

The Crop Doctor is GRDC Managing Director, Peter Reading