Balancing crown rot and nematodes in wheat

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Crown rot is a priority wheat disease in the northern grains region. Crown rot infection requires moist soil, and the past few years have caused problems for growers by increasing the disease’s prevalence.

Speaking at the GRDC Research Update in Goondiwindi, Queensland, Dr Steven Simpfendorfer from the New South Wales Department of Primary Industries said that as well as crown rot infection growers have to contend with root lesion nematodes, producing a situation that may need careful balancing if they are to manage and minimise yield losses.

Crown rot

“We spend most of our time trying to minimise crown rot inoculum levels, yet soil water is by far the biggest factor in the impact of this disease on profitability,” Dr Simpfendorfer says. “The effect of moisture on crown rot yield losses is huge.”

The impact of a bad crown rot season can make or break a crop, with bread wheat yield losses of up to 55 per cent possible at high inoculum levels, and losses in durum up to 90 per cent.

Cultivation can also have a huge impact. “Crown rot is a stubble-borne disease and for a plant to become infected it must come into contact with inoculum from previous winter cereal crops,” Dr Simpfendorfer says.

“By cultivating soil, you’re actually helping to spread the crown rot inoculum to next year’s crop. The best thing a grower can do with infected stubble is leave it alone,” he says.

FIGURE 1 Wheat yield in trials with low and high pressure from the root lesion nematode Pratylenchus thornei (Pt).

A bar chart

Note: High and Low Pt refers to a species of root-lesion nematode, Pratylenchus thornei (Pt)

* denotes significant yield loss for variety when grown under “High” Pt pressure.

T = tolerant, MT = moderately tolerant, MI = moderately intolerant, I = intolerant, VI = very intolerant

SOURCE: Dr Steven Simpfendorfer, NSW DPI

Root lesion nematodes

Five years ago in northern NSW, crown rot was the major disease focus for growers, according to Richard Daniel, CEO of the Northern Grower Alliance. It is still a key disease, but growers also need to deal with the widespread prevalence of root lesion nematodes (RLN), predominantly Pratylenchus thornei (Pt), which exacerbates the effects of crown rot, he says.

“For four out of the past five years, this has been a bigger issue and caused more yield loss than crown rot alone.”

Unlike crown rot, where losses are driven by a combination of inoculum quantity and external stressors such as soil moisture levels, Pt appears to reduce yield of intolerant varieties in nearly all years and conditions when present in the soil.

“The only way to be confident is to test your soil for presence of RLN and ‘know before you sow’,” Mr Daniel says.

The commonly accepted threshold for Pt in the soil is less than 2000 Pt per kilogram of soil – above that is a threat to production. “Growers should not be misled by this figure,” Mr Daniel says. “At the 2000/kg level, yields can still be down by 20 per cent in less-tolerant varieties, which is why knowing the levels in the soil is critical.”

Many trials concentrate on crown rot, but in the northern region, it is becoming more important to build a picture of the interaction of crown rot with other factors, especially in combination with Pt levels, according to both Dr Simpfendorfer and Mr Daniel. As well as reducing yield, Pt reduces grain quality and nitrogen use efficiency, and increases the severity of crown rot infections.

Crop rotation and variety choice are the important factors in protection against both diseases. Choosing a variety solely on crown rot resistance is not critical, especially if appropriate management techniques have been carried out, but choice of variety is crucial when it comes to Pt tolerance.

In a recent series of GRDC-funded trials near Yallaroi, in northern NSW, a range of wheat varieties were evaluated when grown at two different levels of Pt population. The low level was close to the standard threshold, the high level had approximately 10 times as much Pt. Yield losses of 35 to 60 per cent were recorded for Pt-intolerant bread wheat varieties such as SunvexPBR logo, KennedyPBR logo and StrzeleckiPBR logo due to the increased Pt populations. Under the conditions of high Pt populations, these varieties were only yielding 33 to 55 per cent of the most tolerant commercial bread wheat variety, EGA WyliePBR logo (see Figure 1).

Soil testing for RLN is a crucial part of farm management – if you find out you have Pt in the soil you need to avoid intolerant wheat varieties and consider your long-term rotations and varietal selection to reduce build-up of the disease.

More information:

Richard Daniel
Northern Grower Alliance
07 4639 5344,

Dr Steven Simpfendorfer
0439 581 672

The GRDC Update paper giving full details of the crown rot and root lesion nematode trials is available on the GRDC website:
Root-lesion nematode management: the cost of getting it wrong

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GRDC Project Code NGA00003

Region North