There are two types of fusarium disease that affect NSW crops, Fusarium head blight (FHB) and crown rot (CR). Durum wheats are all highly susceptible to both diseases.
FHB is usually caused by the fungus Fusarium graminearum but the crown rot fungus Fusarium pseudograminearum may cause the disease in wet years as rainsplash distributes the fungus from lower stem nodes into grain heads.
Both FHB and CR become apparent after flowering, however head blight requires prolonged wet weather during flowering and grain fill whilst crown rot expresses as whiteheads following periods of moisture and/or heat stress. Crown rot can sometimes be first seen in patches or in wheel tracks, but is often not obvious until after heading. Dead heads containing shrivelled or no grain, called ‘whiteheads’ appear, although it is important to note that yield loss can occur even without the formation of whiteheads.
Figure 1: (a) Durum wheat head infected with FHB. (b) Barley head infected with FHB. Photos: Steven Simpfendorfer.
Fusarium survives from year to year in previous season’s stubble, root residues and on host plants such as grass weeds, maize, wheat, barley, sorghum, oats and triticale. Fusarium graminearum spores (perithecia) are produced on host plants (especially maize), which are released in wet weather and can be carried into heads by the wind. The overriding factor assisting FHB is wet conditions during flowering and/or grain fill so overhead irrigation can favour infection.
Fusarium pseudograminearum, the main species that causes crown rot, survives as mycelium (‘cottony growth’) with the residues of host plants both above (stubble) and within the soil (crowns). Infection occurs in moist soil when there is physical contact between infected residue and the host plant. In no-till systems inoculum becomes concentrated in the previous winter’s cereal rows. All current barley varieties are very susceptible to CR and encourage considerable build-up of inoculum but tend to suffer lower levels of yield loss relative to wheat. Its earlier maturity limits the impact of moisture stress interactions with CR infection, which lead to the production of whiteheads.
FHB is an infection of the head rather than root or crown as with CR. In wheat, FHB appears as premature bleaching of spikelets within a head (figure 1a). Frequently only part of the head (usually the upper half) is affected. Salmon pink to orange spore masses (sporodochia) at the bases of infected spikelets can also be apparent during prolonged warm, humid weather. Infected wheat grains have a chalky white appearance and are usually shrivelled and lightweight; they may sometimes have pink staining too (figure 2). In barley, infected spikelets have a brown or a water-soaked appearance, rather than bleaching (figure 1b). The grains have an orange or black encrustation on their surfaces rather than being chalky white.
Figure 2: Wheat grains infected with FHB. Photo: North Dakota State University.
Crown rot affects the base of the stems rather than the roots. Tiller (stem) bases are always brown. This discolouration often extends up 2–4 nodes, however there can be uninfected tillers present on diseased plants. Plants infected with crown rot are difficult to pull up with roots attached, and often break off near ground level.
Cottony fungal growth may be found inside tillers. Pinkish fungal growth may form on lower nodes especially during moist weather. Whilst infection is encouraged by moist conditions, the expression of whiteheads is exacerbated by hot/dry conditions during grain-fill, which results in pinched grain at harvest. Yield and grain quality losses from CR are most severe in seasons with a wet start and dry finish.
Reducing the level of Fusarium inoculum in your paddock is the aim of most control strategies for both these diseases. Rotations and controlling grass weeds remain the key management strategies. Burning stubble does not guarantee freedom from crown rot. Burning removes only above-ground inoculum; the Fusarium fungus still survives in crown tissue within the soil.
Adapted with permission, from:
Moore K, Manning B, Simpendorfer S and Verrell A (2005) ‘Root and crown diseases of wheat and barley in Northern NSW’, New South Wales Department of Primary Industries, Tamworth.
Compiled from the original document by Abigail Jenkins (Department of Primary Industries), 2013
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