Nematodes and crown rot a costly union

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Figure 1   Impact of crown rot and Pt on yield (tolerance). Figure 2   Effect of winter cereal variety on Pt build-up in soil (resistance)

North-west NSW farmer Andrew Yates, of ‘Delvin’ Garah, checking his outstanding crop of LivingstonA wheat before harvest. LivingstonA is one of few varieties that has moderate tolerance to root lesion nematodes (RLN), along with other desirable featur
North-west NSW farmer Andrew Yates, of ‘Delvin’ Garah, checking his outstanding crop of Livingston wheat before harvest. Livingston is one of few varieties that has moderate tolerance to root lesion nematodes (RLN), along with other desirable features. Useful tolerance to RLN is increasingly showing up in research as a critical factor in minimising yield and quality losses from crown rot as well as RLN. Photo: Bob Freebairn

By Bob Freebairn
Agricultural consultant, Coonabarabran, NSW

Root lesion nematodes, especially Pratylenchus thornei (Pt), can cause wheat yield losses of more than 50 per cent, research over the past several seasons has shown.

An extensive NSW farm survey conducted by Industry & Investment (I&I) NSW, involving district agronomists and farmers from Wellington in central-west NSW to the Queensland border and west to Warren, Walgett and Mungindi, also highlighted the extensive level of nematodes, especially Pt, throughout the cropping belt.

Some 69 per cent of paddocks randomly surveyed had Pt infestations. A second known crop-damaging nematode, Pratylenchus neglectus (Pn), was found in 23 per cent of paddocks. Pt tended to be more common in the north and north-west and Pn more common in southern areas of the survey.

Soil populations of Pt were generally higher, with 23 per cent of paddocks having infestation levels sufficient to cause major yield loss, compared with only two per cent of paddocks having Pn levels sufficient to be serious.

One of the research studies conducted by I&I NSW at Coonamble emphasised the destructive nature of Pt when found at high levels (‘Management of fusarium and other winter cereal diseases in the northern cropping zone’, DAN00109). Wheat varieties with moderate tolerance to Pt yielded 29 per cent higher (0.9 tonnes a hectare more) than varieties rated moderately intolerant or intolerant.

While crown rot was also a focus of the Coonamble research, the only two bread wheats to lose significant yield to crown rot were the most Pt-susceptible varieties.

Six other tolerant or moderately Pt-tolerant bread wheats were little damaged by crown rot. Crown rot significantly reduced yield in all four durum varieties tested (noted as being highly susceptible) and three of the four barley varieties.

The researchers concluded that where Pt combines with high levels of crown rot (a common scenario), yield losses can be exacerbated if varieties are susceptible to Pt. Instead of a 10 per cent yield loss from Pt in a susceptible variety it could be 30 to 50 per cent if crown rot is combined with a Pt-intolerant variety.

The research has also shown that not only does Pt cause high yield loss in susceptible varieties, but Pt numbers can increase much faster than in an area in which tolerant varieties are growing. These increased Pt numbers can lead to even greater damage in future crops.

Surprisingly, barley and durum varieties generally led to lower build-up in Pt populations than the less tolerant bread wheat varieties.

However, all varieties in the study had Pt populations above the Queensland-based threshold for yield loss (2000 Pt per kilogram soil at sowing time).

In Northern Grower Alliance (NGA) trials, choosing varieties with good Pt tolerance, especially when combined with some crown rot tolerance, dramatically reduced yield loss. In 22 trials in collaboration with I&I NSW, wheat varieties rated Pt tolerant to moderately tolerant suffered far less yield loss where crown rot and Pt infestation combined than did varieties with low Pt tolerance.

A cautionary note from NGA’s Richard Daniel is that crown rot does not necessarily express itself in an obvious or yield-reducing way in a year like 2010, with moist finishing conditions. But crown rot inoculum levels will continue to increase and create an even bigger threat for future crops.

Mr Daniel stresses that variety choice is the key management option when it comes to managing Pt risk. However, when it comes to crown rot management, although varieties have some impact, rotation and stubble management are by far our most important management tools.

Other research by Dr John Thompson, of the Queensland Department of Employment, Economic Development and Innovation (DEEDI), found that yield losses from Pt can be as high as 70 per cent for intolerant wheat cultivars.

Mr Daniels and Steven Simpfendorfer, of I&I NSW, believe that root lesion nematodes, especially Pt, need to be taken far more seriously and better factored into crop rotation considerations as well as variety choice.

Every season throws up a different challenge and re-emphasises the need to choose wheat varieties on a multitude of criteria. Root lesion nematodes are one of the least appreciated problems, but these studies show they need to be given at least equal importance to other variety features such as stripe, stem and leaf rust, yellow leaf spot and crown rot when choosing varieties.

They advocate soil tests such as PreDicta B®, a DNA-based test developed by the South Australian Research and Development Institute (SARDI), or manual counts such as by DEEDI.

The annual NSW I&I and DEEDI Winter Crop Variety Sowing Guides provide updated tables listing individual variety resistance and tolerance levels to most of the important problems such as Pt and crown rot, as well as other diseases and agronomic attributes. No variety rates high in all aspects a wheat grower requires in a variety, so it is worth spreading the risk by growing a number of varieties to ensure at least some of the crop is protected.

Researchers involved in these studies included Richard Daniel, Dr Steven Simpfendorfer, Dr Guy McMullen and Elizabeth Farrell (I&I NSW Tamworth), Coonamble district agronomist Rohan Brill (I&I NSW), Dr John Thompson (DEEDI) and Alan McKay (SARDI). The GRDC is a major funder of the research.

GRDC Research Code DAN00109
More information: Steven Simpfendorfer, 02 6763 1261, email;
Richard Daniel, 07 4639 5344, email;
John Thompson, 07 4639 8806, email; www.grdc.com.au/DAN00109

GRDC Project Code DAN00109

Region North, South