The symptoms of crown rot (caused by Fusarium pseudograminearum or Fusarium culmorum ) are most obvious on plants close to maturity. However, the fungus may cause a seedling blight which can result in either pre or post emergent death. Plants affected by crown rot are frequently stunted and produce fewer tillers. Disease symptoms at the base of plants include honey brown discolouration of the crown, lower leaf sheaths and tillers, (Figure 3.15). Symptoms can extend up the stem where the fungus may form pink spore masses at the nodes, (Figure 3.16). By comparison, plant roots and crowns infected with take-all are distinctly black in colour (See Figure 3.8).
When the disease is severe, plants die prematurely and deadheads or whiteheads are produced, (Figure 3.17). The deadheads are either empty or partially filled with shrivelled grain. Affected plants may be scattered across a paddock or occur in patches. Yield losses are greatest in dry years, especially when moisture stress occurs after flowering.
Figure 3.15 Honey discolouration on stem caused by the crown rot fungus
Figure 3.16 Honey discolouration stem and pink fungal growth caused by the crown rot fungus
Figure 3.17 Crown rot causes deadhead that are usually found on scattered plants or single tillers in the crop
The fungus can survive for up to two years on infected cereal stubble from previous cereal plants, volunteer plants or grass weeds. For this reason, stubble management is an important aspect in the control of crown rot. Survival is enhanced by dry summer conditions, but disease development is favoured by moist humid conditions with temperatures between 15°C and 20°C, (Figure 3.18).
Figure 3.18 Disease cycle of crown rot in cereals.
Illustration by Kylie Fowler
Reducing crown rot inoculum is the key to controlling this disease and rotations are the most important component in the integrated management of crown rot.
Rotations with non-susceptible crops will reduce the severity of crown rot. A two year break with a pulse, oilseed crop or fallow will reduce crown rot following wheat or barley crops. Grass-free lucerne and medics will also reduce disease severity. Some varieties have partial resistance to crown rot and can reduce the severity of crown rot. However, most bread wheat, durum wheat and barley varieties are either susceptible or highly susceptible to crown rot.
Stubble management is an important factor determining the survival of the fungus. Agronomic practices that encourage greater retention or slower decomposition of stubble can increase the severity of crown rot. Tillage helps break down infected stubble by accelerating its decomposition thereby reducing inoculum levels.
Burning stubble can reduce, but not control crown rot. However, burning stubble is undesirable as the retention of stubble helps reduce erosion and aids the storage of water over summer. As a compromise solution, burn stubble as close as possible to sowing.
Infection rates can be reduced by sowing between rows of standing stubble. In the southern region inter- row sowing using accurate GPS autosteer has shown a decrease in the number of infected plants by 50 percent – resulting in a 5 to 10 percent yield increase.
There are no fungicides currently available to control crown rot.
Crown rot can be a serious problem in durum wheat, as well as bread wheat, barley and oats. Other host plants include wild oats, canary grass (phalaris), wheat grass, brome grass, barley grass, winter grass and ryegrass.
Crown rot is more likely when:
- susceptible varieties are grown sequentially
- stubble retention and reduced tillage are practiced.
RHIZOCTONIA ROOT ROT (aka BARE PATCH)
ROOT LESION NEMATODE (RLN)