Fusarium crown rot

What is fusarium crown rot?

Crown rot is caused by the fungal pathogens Fusarium pseudograminearum and/or F. culmorum. It is a widespread stubble-borne disease in grain-growing regions of the south-western agricultural region of Western Australia. The fungus host range is confined to cereals and grasses. Therefore, crown rot is worse in paddocks with frequent cereal rotations (particularly of susceptible varieties) or where grass weeds are not controlled. Crown rot inoculum survives from one season to the next in stem bases and crowns. The inoculum can survive within the stubble for several seasons, until the stubble is completely broken down. This means that stubble retention and reduced tillage practices support the carry-over of crown rot inoculum.

When growing cereal plants come in contact with infected plant residues, crown rot infection can occur. Even tiny pieces of residue can infect plants and paddocks with little visible stubble may still present a crown rot risk.

Infection is favoured by moderate soil moisture and can occur at any time during the season. Infection occurs through the coleoptile, sub-crown internode, crown and/or outer leaf sheaths at the tiller bases. The fungus spreads up the stem during the season, with most inoculum being concentrated near the base of the plant. Fungal growth at the base of the cereal plant, and extending into the vascular system, slows or blocks water movement up the stem to the head, causing the formation of whiteheads. Yield losses can be severe in seasons with reduced rainfall and hotter temperatures during grain filling. However, in seasons with adequate rainfall and mild temperatures during grain filling, yield loss from crown rot in the south-western agricultural region is minimised.

Symptoms and management of fusarium crown rot

A visible honey-coloured/dark brown stem base and crown is the most reliable indicator of crown rot infection. This symptom becomes more pronounced from mid to late grain filling through to harvest. To inspect for the brown discolouration, pull back the leaf sheaths at the stem base.

Symptoms may not appear on all stems of an infected plant and browning is often more difficult to see in oat. This browning may also be seen in combination with white/pink fungal growth that can be on either the outside of the lower stems underneath the leaf sheaths or visible if the stem is cut open. The white/pink fungal growth is evident only when moisture is still available late in the season and should not be used as the definitive symptom of crown rot infection.

An easily visible symptom of crown rot in wheat is the presence of whiteheads in the crop during early grain fill. These heads mature early, associated with the vascular restriction of water and nutrients, and, as a result, contain shrivelled or no grain. Whiteheads caused by crown rot may not appear on all stems of an infected plant and are usually scattered through the crop, in contrast with whiteheads that are caused by the soilborne disease take-all, which appear in distinct patches. Whiteheads may first appear in wheel tracks, old weed patches or around trees where crop-available soil moisture is more limited. Barley generally does not produce whiteheads because it matures earlier than wheat, which helps it escape late-season moisture or heat stress and ‘haying off’. But yield losses associated with lower water and nutrient acquisition can still be significant and equal those in wheat. Oat rarely exhibits whiteheads and has been shown to have limited yield loss in Western Australian trials. In seasons with good spring rain, whiteheads in wheat and barley may not occur, even in infected crops. The expression of whiteheads is closely related to the extent of moisture and/or heat stress increases.

Managing crown rot in a cropping operation relies on: identification of paddocks at risk before planting employment of crop rotations that include non-cereal crops, such as, lupin and canola, to reduce inoculum levels stubble management through cultivation and baling or burning to reduce inoculum levels strategies that minimise yield losses when cereals are grown. An integrated disease management strategy can minimise yield losses where a cereal must be sown in a paddock with an identified crown rot risk and will include a combination of: choosing the best cereal crop or most resistant variety sowing early sowing on the inter-row ensuring nutrition is adequate for the season using seed/in-furrow fungicides registered for suppression of crown rot.