Curing the 'common cold' of cropping systems

Figure 3: Germinating spore of Pythium species. Pythium produces large numbers of spores that enable the pathogen to reinfect growing roots.

Pythium root disease has been described as the "common cold" of cropping systems and as a result its effect on the productivity of crop rotations has often been underestimated or completely overlooked. Pythium rapidly infects germinating seeds and seedlings of all major grain crops and pastures (Figures 1 and 2).

Once the crop is infected, the disease has a continuous impact throughout the season and across all phases of the crop rotation (Figure 3).

Figure 3: Germinating spore of Pythium species. Pythium produces large numbers of spores that enable the pathogen to reinfect growing roots.

Recent reports suggest that Pythium root rot is occurring with increasing frequency and severity in southern Australia. This appears to be related to increasing adoption of reduced tillage systems and the switch to less diverse, potentially higher-risk rotations (for example repetitive canola-wheat rotations).

As both these trends are here to stay, there is an urgent need to develop pre-emptive strategies to manage this increasingly important disease. Collaborative research funded by the GRDC involving CSIRO Land and Water and agribusiness company Landmark, aims to develop an integrated management package for Pythium through targeted use of fungicides and crop rotation strategies.

The research confirmed that Pythium was widespread in southern Australia, infecting all major grain crop and pasture species, and was widely distributed throughout cropping soils in the mid-to-higher rainfall (>350 mm) cropping zone.

Field trials with a Pythiumselective fungicide seed dressing significantly reduced soil-borne inoculum levels and frequencies of root infections, resulting in yield increases in canola, cereals and pulses (Table 1).

Table 1. Effects of Pythium-selective fungicide on 2002 and 2003 Pythium inoculum levels, root infection frequencies and grain yields. Data were averaged across all crops at the four trial sites.

1 Fifteen crops were tested across four sites. NA - not available, NH - not harvested.
2 Measured at time of plant sampling ie 8 - 12 weeks after sowing.

Yield benefits generally occurred with no significant improvements in crop emergence. Only limited disease control was achieved by using the fungicide (Table 1), implying that sustained control will lead to further improvements in yields.

As all crop and pasture plants are hosts of Pythium, crop rotation has generally been considered to be of little use in managing Pythium root rots. However the research revealed that certain crops showed significant differences in their susceptibility to Pythium infection and capacity to "carryover" disease inoculum.

Lupins were ranked as supporting the highest levels of disease, followed by canola, peas, wheat and barley.

Greater levels of Pythium root infections were also found in crops grown for their second successive year compared with crops grown in annual rotation (Table 1).

This implies that Pythium strains are adapting to different crop species and that crop rotation is preventing or at least slowing this process.

Sustainable disease control will require an integrated program of targeted fungicide applications during highly susceptible phases of the rotation (for example grain legumes or canola), and timing the rotation sequence to match less susceptible crop species with the soil-borne Pythium population. This strategy will limit the pathogenic impacts of Pythium, thereby avoiding severe disease incidence in the current cropping phase and high levels of inoculum carryover in subsequent years.

For more information:
Paul Harvey, 08 8303 8589, Paul.Harvey@csiro.au

GRDC Research Code: CSO 00016, program 4