Crown rot in wheat: identification, management, and economic thresholds

Crown rot, caused by the fungus Fusarium pseudograminearum, is a persistent soilborne disease that colonises the base of cereal tillers and disrupts water and nutrient flow during grain fill. It is most damaging in seasons with a dry finish, when the grain fill period is stress-limited and whiteheads appear across the paddock.

How infection occurs and spreads

The pathogen survives in infected crop residues for two to three years. Infection of the crown and lower internodes occurs during seedling establishment from soil inoculum or infected seed. Early infection is symptomless, making the disease invisible through most of the growing season.

Symptoms appear at grain fill, when infected tillers fail to fill grain and produce bleached or partially bleached heads (whiteheads). By this point, chemical intervention is too late. The disease cycle is complete when residues are returned to the soil, rebuilding inoculum for the following season.

Risk factors and paddock assessment

Several factors increase crown rot pressure:

• Previous crown rot-affected crops. Inoculum builds over consecutive cereal seasons.
• Minimum tillage. Retention of infected surface residues increases seedling exposure.
• Dry spring conditions. Moisture stress during grain fill amplifies yield loss from infected plants.
• Wheat-on-wheat rotations. Continuous cereals allow inoculum to accumulate beyond manageable levels.

Assess paddock risk by pulling tillers at the base and looking for brown or pink discolouration of the crown and lower nodes. Scoring tillers (crown rot index) before sowing, using infected residues from the previous crop, gives a rough indication of inoculum load.

Management options and their limits

Rotation. Breaking the cereal sequence with at least one non-host break crop (canola, chickpeas, field peas) substantially reduces inoculum. A single-year break reduces inoculum by roughly 50 to 70%. Two consecutive break crops achieves greater reduction. This is the most cost-effective management tool available.

Variety tolerance. GRDC-funded research has identified varietal differences in crown rot tolerance, though no fully resistant commercial varieties exist. Varieties with higher stem mass at the crown tend to tolerate infection better. Check the National Variety Trial (NVT) disease ratings for your region; some varieties carry ratings for crown rot tolerance.

Seed treatments. Triazole-based fungicide seed treatments (e.g., triadimenol, tebuconazole, prothioconazole) provide partial protection at the seedling stage but do not eliminate the disease. They reduce early infection pressure and are worth including in high-risk paddocks but should not be relied on as the primary management tool.

Seeding timing and rates. Delayed sowing into warmer soils reduces the seedling establishment window when infection is heaviest. Higher seeding rates partially compensate for tiller losses by increasing the number of uninfected tillers per square metre.

Economic thresholds and forward planning

There is no in-season economic threshold for crown rot; management is entirely preventive. The economic question is whether the cost of a break crop (foregone cereal margin) is offset by the reduction in crown rot yield loss over the rotation.

In paddocks with a crown rot index above 20 to 30%, modelling by GRDC projects yield losses of 10 to 30% in dry-finish seasons. A canola break crop at equivalent margin to wheat will typically recover that loss within two seasons while also providing herbicide resistance benefits.