Crown rot remains a serious constraint to wheat production in the drier, more marginal western area of the north-eastern grain belt. It also occurs in other wheat-growing regions and has caused significant losses in some crops in South Australia and Victoria. Increased sowings of highly-susceptible durum wheat have favoured the disease.
What causes this rot?
This insidious disease is caused by a fungus, Fusarium pseudograminearum (formerly known as F. graminearum Group 1), which infects all winter cereals and many grasses and can survive in cereal stubble residues for two years or more. The fungus infects the cereal plant through the crown region and grows through the bases of the stems, usually being confined to the lower three to four internodes.
The fungus causes mild or no symptoms in plants growing with adequate soil moisture at moderate temperatures. However, when infected plants are subjected to hot, dry, stressful conditions, the fungus causes obvious crown rot, which appears as stem browning. The crown rot cuts off water supply to the upper parts of the plant. This may be followed by plant death and the most conspicuous symptom, whitehead formation.
Insidious — why?
The fungus can cause symptomless infection, building up in a paddock in successive crops in wet seasons without causing whiteheads, its presence going unnoticed. If the next year is marked by a dry finish, significant whitehead formation and yield loss occur.
Similarly, it can maintain or increase in grass weeds in a resistant rotational crop such as chickpeas or canola. Some grassy weeds such as wild phalaris (paradoxa grass) and barley grass show obvious symptoms but black oats is a symptomless host. The very wet seasonal conditions in 1998 northern NSW, for example, made weed control difficult and grassy weeds infested many rotational crops. The crown rot fungus persisted through the rotation by infecting such weeds.
Control grasses young
It is preferable to kill susceptible grass hosts in the juvenile stage as infected seedlings decompose quickly leaving little of the crown rot fungus to carry over. More mature grasses are slow to decompose and can carry higher levels of the fungus if infected.
The fungus is found in grasslands and consequently crown rot is commonly present in the first wheat crop on new country. The dominance of barley grass in some of the grazing country being brought into cropping is cause for serious concern.
Growers need to adopt a strategy for minimising carryover of the fungus from the barley grass to the first wheat crop. The use of a resistant 'cleaning' crop as the first crop is one option. Other options involve eliminating the barley grass from the grazing country, one to two years before the first wheat crop.
Not all grasslands carry high levels of the crown rot fungus. Local experience in the Walgett area indicates that coolah grass country along the river carries lower levels than adjacent Mitchell grass/barley grass country.
A sequence of drought years and missed crops can lead to a significant decline in the crown rot fungus to levels from which it may take three to four successive crops before the disease again becomes a serious problem. Thus new growers entering such areas may not experience the disease for some years.
All cereals are equally infected by the crown rot fungus and it causes obvious symptoms in wheat and barley under warm dry conditions. However, the fungus does not cause symptoms in cereal oats. Grazing oats are a useful rotational species if totally grazed out.
Wheat cultivars differ significantly in their tolerance to the crown rot fungus. Tolerant cultivars such as Sunco (regarded as the most tolerant), Sunvale and Baxter show significantly fewer whiteheads than intolerant cultivars such as Sunlin, Sunstate, Hartog, and the durums. The durums as a group are the most intolerant.
Intolerant cultivars should be grown only in country with low levels of crown rot. Note that tolerant varieties such as Sunco may have lower yields than many intolerant varieties when crown rot is absent.
High nitrogen levels may favour crown rot by increasing host susceptibility and the risk of late stress. Growers should consult their agronomist about appropriate nitrogen rates for country affected by crown rot. There is also some experimental data indicating that inadequate levels of zinc favour the disease.
Strategies to reduce crown rot
The most important first step is to monitor the levels of disease in each paddock remembering that symptoms are suppressed in wet years. Seek advice and note that we can assist agronomists with laboratory assays to determine the level of infected plants, a particularly valuable test at the end of wet seasons.
When the percentage of infected plants reaches 20-30 per cent, there is a risk of significant loss from whiteheads in the following season if stubble is retained and the next season is characterised by a dry finish.
Further management strategies to reduce the disease include:
- rotation to resistant crops such as sorghum, sunflower, canola, chickpeas, faba beans
- control grassy weeds in fallow and in the break crops
- use tolerant wheat cultivars
- minimise the risk of moisture stress by attention to nitrogen rates and conservation of fallow moisture
- overall nutrition including zinc should be appropriate, and
- if necessary, use strategic autumn burns. The most appropriate timing of a strategic burn is before the final wheat (cereal) crop preceding the rotational crops. This approach minimises the amount of infested stubble being carried into the rotational cropping sequence.
Further information may be obtained from the authors, from your agronomist, or from the Winter Crop Variety Sowing Guide published by NSW Agriculture. This work is supported by growers and the Federal Government through the GRDC.
Program 2.6.1 Contact: Professor Lester Burgess 02 9351 2935, Dr David Backhouse 02 6773 2341; Ms Linda Swan 02 6799 2203
(above) Crown rot appears as stem browning.
(below) Crown rot's most conspicuous symptom: whiteheads.
(right) Crown rot in the paddock.