Root cause

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Interactions between root and crown disease are being investigated. Varieties with reduced tolerance to P. thornei can suffer an increased level of yield loss from crown rot infection.

The fungal disease crown rot is more severe when crops experience moisture stress during the period of grain fill. The feeding activity of root lesion nematodes, which have been found to be widespread in the cropping soils of northern New South Wales and Queensland, affect the ability of crop roots to extract soil water.

To establish if an interaction is occurring between the root lesion nematode Pratylenchus thornei, the dominant root lesion species, and crown rot, an experiment was established in 2009. This field trial aimed to quantify the interaction in terms of both yield (tolerance) and build up of P. thornei populations within the soil (resistance). The trial also provided additional information on varietal resistance and/or tolerance to P. thornei.

The trial was established at Coonamble, NSW. Bread wheat, durum and barley varieties (Table 1) were sown with and without crown rot inoculum into a site with a known high P. thornei population.

The seasonal finish at Coonamble in 2009 was only moderately conducive to the expression of crown rot and yield loss from this disease. However, crown rot significantly reduced yield in all four durum varieties, in three of the four barley varieties, and two of the eight bread wheat varieties.

Yield loss to crown rot was highest in the durum EGA BellaroiA (23 per cent) and bread wheat StrzeleckiA (27 per cent).

The average yield of moderately tolerant P. thornei bread wheats (EGA GregoryA, LivingstonA and EGA WylieA) in the absence of crown rot was 3.14 tonnes a hectare compared with less P. thornei-tolerant varieties (EllisonA, SuncoA, SunvexA and StrzeleckiA) with an average of 2.23t/ha. It therefore appears that P. thornei reduced yield by about 29 per cent in 2009 in varieties moderately intolerant to intolerant to P. thornei.

The only two bread wheat varieties to suffer significant yield loss to crown rot were the most P. thornei-susceptible varieties (StrzeleckiA and SunvexA). This trial identified that wheat variety choice can have a huge impact on P. thornei build up within the soil. Following susceptible bread wheats, P. thornei populations were up to 5.6 times greater than following resistant varieties.

Generally, barley and durum developed lower populations of P. thornei when populations were measured using the PreDicta B® DNA-based soil test. However, at sowing in 2010 all varieties had P. thornei populations above the Queensland-based threshold for yield loss of 2000 P. thornei per kilogram of soil.

Bread wheat varieties had a large impact on the reproduction of P. thornei, with a small difference in resistance rating appearing to result in big differences in the development of P. thornei populations under field conditions. For example, two varieties each rated susceptible to P. thornei had 19,400 Pt/kg soil (SuncoA) and 8049 Pt/kg soil (EGA WylieA).

In plots heavily infected with crown rot, P. thornei populations tended to be lower. This may be due to crown rot reducing the size of the root system and hence reproductive opportunity for the root lesion nematode.

Varieties with reduced tolerance to P. thornei can suffer an increased level of yield loss from crown rot infection.

Three similar winter cereal variety trials are being conducted in 2011 to further explore the interaction of either P. thornei or P. neglectus with crown rot and the influence of sowing time on disease expression, yield loss and resulting nematode populations.

These trials will ideally provide growers with better information on resistance/tolerance of newer bread wheat, barley and durum varieties to both nematodes and to crown rot and associated disease interactions under field conditions.

This information can then be used as a guide when planning rotation sequences to minimise the build up and yield loss from these pathogens.

More information: Dr Steven Simpfendorfer, senior plant pathologist, NSW Department of Primary Industries, 02 6763 1261,;;;

GRDC Project Code DAN00109, DAN00143,

Region National