Crown Rot of Cereals

Fusarium graminearum Group 2

Fusarium pseudograminearum (Fp)

Description:

Light brown colouring of the base of tillers and a pink discolouration around the crown and under the leaf sheaths. A brown stem base from mid grain fill through to harvest is the most reliable indicator of crown rot and this may be more easily seen if the outer leaf sheaths are pulled back. It may only appear on some stems of an infected plant and is often difficult to see in oats.
Seedlings occasionally show yellowing and death. Odd tillers or whole plants may become bleached over winter and develop white heads with no grain at maturity or may have normal grain. A pink discolouration caused by the salmon coloured spores may develop under the leaf sheath. General decay of the base of the plant and other symptoms may be seen at any stage but is usually more obvious after flowering. Part or all of the heads of wheat is white or yellow and often without grain or with shrivelled and pinkish grain. White heads may be seen as the crop hays off and these tend to be scattered through the crop rather than being in patches which is more typical of take-all. White heads are often more prevalent on wheel tracks that are most noticeable as the crop ripens or at harvest. They are often associated with depressions or wetter areas of the paddock or in weedy patches. In humid conditions the white heads may turn grey of black. Barley generally doesn't produce white heads and oats only very rarely have white heads.
Nodes of infected plants are often pinkish. The straw may be full of a white to pink fungus when split.
The spores are multi celled and banana shaped when viewed under a microscope.
Brown to tan lesions on primary and secondary roots and sub crown internode.

Species Affected:

Cereals, grasses.
Of the cereals, durum wheat is most sensitive followed by wheat and triticale, then barley then oats.
Barley grass and phalaris are very susceptible.
Oats are partially tolerant.
Most broad-leaved species are resistant.

Biology:

Requires warm moist conditions.
Tends to build up after a series of dry springs.
Usually survives on cereal and grass residues for 2 years. If decomposition rates of stubble are slow then it may last longer.
Builds up rapidly after Durum wheat.
Zinc deficient plants are more susceptible to crown rot. Applying zinc to healthy plants has no protective effect.
76% of the crown rot inoculum is in the 7 cm of plant stubble above the original seed. 24% is in the upper stubble.
Soil type appears to have little effect on the occurrence of crown rot. However, crops on shallow soils with poorer moisture holding capacities tend to have larger yield losses when infected by crown rot.

Life Cycle:

Spores from infested stubble germinate in autumn and infect Wheat seedlings that are in close proximity to the infested pieces of stubble. Infection occurs in the sub crown internode, crown or leaf sheaths at the base of tillers. Infection is favoured by moderate soil moisture conditions. The fungus proliferates in stems over winter reducing the supply of water and nutrients to the head. Most of the inoculum is near the base of the plant. Spores are produced in spring and over summer in the stubble. Spores may remain viable in stubble for about 18 months or longer if the stubble doesn't break down.

Origin and History:

Distribution:

Occurs on most soil types but are often more severe on poorly drained areas and on heavy black or grey soils.
More common in the northern wheatbelts of WA and NSW.

Significance:

May cause significant yield losses in wheat, barley, triticale and most grasses especially in stubble retention systems.
Yield losses of 90% have been recorded from durum wheat and 50% for bread wheats.
In wheat it may increase screenings. In barley, grain quality is not usually affected.
The disease is usually worst in seasons with a wet start and a dry finish.

Management and Control:

Keeping inoculum levels low by having a grass free break between cereals is the best strategy.
Check wheat or barley for browning of the stem base from grain fill to harvest stage. Collect 100 plants across the paddock. If 10 plants have basal browning the risk to following cereals is low; If 10-25 the risk is medium and if more than 25 have basal browning the risk to following cereals is high.
Test soil samples including residues using the DNA based soil test PreDicta B® in late summer.
Burn or bury infected cereal stubble to increase the rate of breakdown.
Adjust crop rotations to reduce cereals and grasses.
Control grasses in pastures, broad-leaved break crops and fallows.
Plant resistant varieties. Oats are somewhat tolerant and provide a partial disease break. Crown rot may still proliferate in resistant varieties after grain fill to produce disease in following crops.
Avoid sowing cereals after cereal crops or grassy pastures if the disease is present on more than 25% of plants. Only sow durum wheat into low risk situations.
12-18 months fallow usually provides reasonable control.
Barley Grass and Phalaris are severely affected and should be controlled in break crops.
Reduce stubble or encourage breakdown by grazing, burning, trampling, burying baling or raking.
In dry conditions where stubble breakdown is limited, spreading stubble may increase the disease.
Cultivation at seeding provides partial control by enhancing stubble breakdown but may also spread the disease.
Planting tolerant cereal cultivars reduces yield losses but rarely reduces the carry over of disease into the following season.
Apply adequate fertiliser and especially Zinc.
Control weeds, adjust agronomy and balance nitrogen fertiliser to soil moisture and yield potential to reduce the risk of water stress in spring. Limit nitrogen applications around planting to reduce excessive early crop growth.
Most broadleaved plants are resistant and are useful for reducing the disease.
Brassica break crops are more effective than legume break crops. A break crop of Canola or Mustard led to a 50% reduction in severity of disease in a following susceptible Wheat whereas Chickpeas resulted in a 30% reduction (Simpfendorfer, 2004). Trials in SA and Victoria found fallow and peas reduced the levels of crown rot in soils most followed by medic, canola and vetch. Oats cause a slight increase, bread wheat a moderate increase, triticale and barley a greater increase and durum wheat the greatest increase in crown rot in soil (GRDC, 2009).
Planting cereals between the rows of standing stubble from last season usually decreases the disease by 50% and results in a 5% yield increase and is a useful technique for low disease situations. In high disease situations the loss in yield due to the disease is not compensated for by the benefits of inter row planting (Simpfendorfer, 2007).
If more than 30% of durum wheat plants are infected at anthesis consider cutting the crop for hay.
Seed dressings containing triadimenol, triticonazole, difenoconazole + metalaxyl, difenoconazole + mefenoxam, tebuconazole, tebuconazole + metalaxyl, tebuconazole + metalaxyl + imazalil, carboxin + imazalil + thiabendazole and imazalil are used overseas to suppress crown rot of cereals.
Triticonazole + thiram, fludioxonil, maneb, carbathiin(carboxin) + thiram are used as seed dressings in other crops.

Biological Control:

A naturally occurring fungus (Trichoderma spp.) appears to limit Crown Rot effects and is encouraged by Brassica (Canola) crops.

Related and Similar Species:

Copper deficiency has similar symptoms.
Take-all (Gaeumannomyces graminis) produces similar white heads but they tend to be in patches rather than scattered through the crop.
Head Scab of Wheat (Fusarium graminearum Group II)
Common root rot (Cochliobolus sativus) has chocolate coloured sub crown internode rather than the brown to tan lesions on primary and secondary roots and sub crown internode.
Yellows of Brassicas (Fusarium oxysporum f.sp. conglutinans)

References:

1295

GRDC (2009) Crown Rot in Cereals Fact Sheet.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.HerbiGuide.com.au for more information.