Crown rot in winter cereals

Dry seasonal conditions across much of Australia in 2017 favoured the expression of whiteheads in wheat crops associated with infection by the fungal disease crown rot.

Crown rot infection of seedlings is favoured by a wet start to the season. If the season stays wet and/or mild there may be no obvious symptoms (whiteheads) in the crop even though crown and stem symptoms will be present. However in 2017 many regions (Victoria and SA the main exceptions) had below average growing season rainfall and the infection of stems and crowns have resulted in the crop struggling to fill the developing heads and leading to the expression of obvious whiteheads.

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White/clear tillers on left uninfected by crown rot. Tillers on right showing characteristic basal browning associated with crown rot infection.

Figure 1: White/clear tillers on left uninfected by crown rot. Tillers on right showing characteristic basal browning associated with crown rot infection. Source: Steven Simpfendorfer, NSW DPI

What is crown rot?

Crown rot is a significant disease of winter cereals in Australia. Crown rot is caused by the fungi Fusarium pseudograminearum or F. culmorum and symptoms include discolouration (honey brown) of the crown, lower leaf sheaths and tiller bases and whiteheads (Figure 1).

How do I recognise crown rot in my crop?

Separating crown rot from other causes of whiteheads and poor growth in wheat requires pulling some plants with crowns out of the soil – any time from flowering through to physiological maturity. The leaves that encircle the stem need to be peeled from the tiller, removing them all to the base of the tiller to inspect the lower stem for discoloration.

Indicators of crown rot are a brown or tan colour instead of the green of the immature plant or the off white of a mature plant. Sometimes a pink fungal mass can be seen growing around the lower nodes in rainfall occurs late in the season after crown rot has already expressed as whiteheads. The greater the incidence of tillers with brown discolouration and the greater the number of nodes up the stem that are discoloured, the greater the disease severity and eventual yield loss in seasons with a dry finish.

Also pay attention to crops around trees within a paddock or along tree lines. Even in good years whiteheads associated with crown rot infection are likely to be seen around trees (Figure 2). This is due to the extra competition for water.

Scattered whiteheads around trees at Merriwa, NSW.

Figure 2 Scattered whiteheads around trees at Merriwa, NSW. Source: Steven Simpfendorfer, NSW DPI

Whiteheads associated with crown rot are generally scattered in amongst green heads and are most evident from flowering through grain-filling.

This is quite different from moisture induced drought that often occurs around trees and where all heads ripen prematurely. It is important to validate the actual cause as other factors e.g. mice or insect chewing damage, take-all and frost can also cause the appearance of whiteheads in a crop.  All of these are easily distinguished from crown rot in that they lack browning at the base of affected tillers.

In the medium rainfall areas of Western Australia and parts of southern NSW, high levels of whiteheads are caused by take-all (Gaeumannomyces graminis var. tritici) and not crown rot. Take-all causes blackening of roots and stem bases, but this is a distinct black colour and should not be confused with the browning caused by crown rot. As a general rule, wheat plants infected with take-all are easily pulled out of the soil as it is a root pathogen while plants infected with crown rot are harder to pull from the soil.

Phases of crown rot

There are three distinct and separate phases of crown rot – survival, infection and expression.

Survival of the fungus

The crown rot fungus survives as mycelium (cottony growth) inside winter cereal stubble (wheat, barley, durum, triticale and oats) as well as in infected grass weed residues. The fungus survives in the stubble for as long as the stubble remains intact. Decomposition of stubble is a very slow process and varies with soil and weather conditions.

Infection of cereals

The crown rot fungus grows out of stubble residues and infects new winter cereal plants below the soil surface. The fungus can also infect plants above ground at the soil surface through the outer leaf sheaths. Direct contact with previously infected residues is required and infections can occur throughout the whole season in the presence of moisture. Wet seasons favour increased infection events and when combined with the production of greater stubble loads, significant levels of inoculum can build–up in seasons with good growing conditions.

Expression of the disease

Yield loss is related to moisture/temperature stress around flowering and through grain-fill. This stress triggers the crown rot fungus to proliferate in the base of infected tillers causing a restriction of water movement from the roots through the stems, and producing whiteheads that contain either no grain or lightweight shrivelled grain. The expression of whiteheads in plants infected with crown rot is less evident in wet seasons yet increases greatly with increasing moisture/temperature stress during grain-fill.

Reducing crown rot inoculum levels within a paddock decreases the potential for yield loss but the actual extent of yield loss is very sensitive to the level of moisture stress during grain-fill. Up to 25% yield loss can still occur with a ‘low’ starting inoculum level if the crop becomes severely stressed during grain-fill.

How can I manage crown rot?

The lead up to harvest is an important time to monitor crops, identify diseases and where these occur across a farm. This will assist in planning for next year’s crop. Crown rot infection levels can also be determined in stubble post-harvest but needs to be conducted within 1-2 months to prevent weathering from complicating observations.

Reduce inoculum

1. Crop rotation

The most effective way to reduce crown rot inoculum is to include non-host crops in the rotation sequence. The crown rot fungus can survive for two to three years in stubble. Growing a non-host crop for at least two seasons is recommended to reduce high inoculum levels. This allows time for decomposition of winter cereal residues that host the crown rot fungus. Stubble decomposition varies with the type of break crop grown – their canopy density and rate of the canopy closure as well as row spacing, the amount of soil water they use and seasonal rainfall. Trials in the northern region have indicated that faba beans and canola are better break crops for crown rot than chickpeas.

2. Cultivation

Cultivation can reduce trash load prior to sowing and in theory should increase the rate of decomposition by reducing the particle size of stubble. However, cultivation dries out the soil which reduces decomposition of incorporated stubble. Decomposition requires adequate moisture for an extended period and one summer fallow (even if extremely wet and stubble has been cultivated) is not long enough!

Another problem with cultivation is that infected stubble is now chopped up into smaller pieces and spread throughout the paddock. With the majority of infection sites below ground, there is now an increased chance of physical contact between infected residue and the plant parts required to initiate infection.

3. Stubble burning

Burning removes the above ground portion of crown rot inoculum yet the fungus will survive in infected crown tissue below ground so this is not a quick fix for high inoculum situations. Removal of stubble through burning increases evaporation from the soil surface and impacts on fallow efficiency. A ‘cooler’ autumn burn is preferable to an earlier ‘hotter’ burn as it minimises the impacts on soil moisture storage whilst still reducing above ground inoculum levels.

Reduce water loss

Inoculum level is important in limiting the potential for yield loss from crown rot but the overriding factor dictating the extent of yield loss is moisture/temperature stress during grain-fill. Any management strategy that limits storage of soil water or creates constraints that reduce the ability of roots to access this water will increase the probability and/or severity of moisture stress during grain-fill and exacerbate the impact of crown rot.

Grass weed management

Grass weeds should be controlled in fallow periods and in-crop, especially in break crops, as they host the crown rot fungus and can also significantly reduce soil moisture storage. In pasture situations grasses need to be cleaned out well in advance of a following cereal crop as they serve as a host for the crown rot fungus.

Row placement

In a no-till system the crown rot fungus becomes confined to the previous cereal rows and is more reliant on infection through the outer leaf sheathes at the soil surface. This is why inter-row sowing with GPS guidance has been shown to provide around a 50% reduction in the number of plants infected with crown rot when used in a no-till cropping system.

Soil type

Soil type does not affect the survival or infection phases of crown rot. However, the inherent water holding capacity of each soil type interacts with expression by potentially buffering against moisture stress late in the season. Any sub-soil constraints e.g. sodicity, salinity or shallow soil depth can reduce the level of plant available water and increase the expression of crown rot.

Interaction with other pathogens

The expression of crown rot has been shown in the northern region to be exacerbated in the presence of the root lesion nematode Pratylenchus thornei or the fungal disease common root rot (Bipolaris sorokiniana). These two biotic constraints are commonly found in soils in the northern grains region often in association with crown rot. Interaction of crown rot with other common soil-borne pathogens such as Rhizoctonia root rot, take-all and/or other RLN species (e.g. P. neglectus) are likely to be important in other regions.

Cereal crop and variety choice

All winter cereal crops host the crown rot fungus. Yield loss varies between crops and the approximate order of increasing loss is oats, barley, triticale, bread wheat and durum.

Barley is very susceptible to crown rot infection and will build-up inoculum levels but tends to suffer reduced yield loss through its earlier maturity relative to wheat. Late planted barley can still suffer significant yield loss especially when early stress occurs within the growing season.

Bread wheat varieties appear to differ significantly in their level of yield loss to crown rot with newer varieties in the northern region (Sunguard, Suntop, LRPB Spitfire, LRPB Lancer) appearing to suffer less yield impacts compared to the widely grown variety EGA Gregory.

However, variety choice is NOT a solution to crown rot with even the best variety still suffering up to 40% yield loss from crown rot under high infection levels and a dry/hot seasonal finish. All current durum varieties are very susceptible to crown rot and should be avoided in medium and high risk situations.

Sowing time

Earlier sowing within the recommended window of a given variety for a region can bring the grain-fill period forward and reduce the probability of moisture and temperature stress during grain-fill. Earlier sowing can increase root length/depth and provide greater access to deeper soil water later in the season, which buffers against crown rot expression. This has been shown in previous NSW DPI research across seasons to reduce yield loss from crown rot. However, earlier sowing needs to be balanced against any increased risk of frost damage for the selected variety and region.

How do I know my level of risk for crown rot?

SARDI’s PREDICTA® B is a commercial DNA based soil test for cereal growers across Australia. It measures the levels of a range of cereal pathogens. Because the crown rot fungus is stubble-borne, normal soil samples are unreliable and disease detection is highly sensitive to the sampling technique used. We recommend carefully reading the specific protocols for how to collect samples for crown rot testing which includes adding pieces of cereal or grass weed residue.

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Acknowledgements

This article was originally compiled by Kate Charleston (DAFQ).

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