Source DocumentTable Of ContentsNext Page

Identifying a non-destructive technique to assess nematode tolerance in wheat variety trials.

Jeremy Whish1, John Thompson2, Tim Clewett2 and John Lawrence1

1 CSIRO Ecosystem Sciences/APSRU, 203 Tor St Toowoomba 4350, Email jeremy.whish@csiro.au
2
Queensland Department of Employment, Economic Development and Innovation, Leslie Research Centre PO Box 2282, Toowoomba Qld 4350.

Abstract

Wheat cultivars and breeding lines are rigorously tested to identify their level of tolerance to nematodes under field and controlled environment conditions. Unfortunately field-testing can often be compromised by poor seasons or inclement weather resulting in incomplete or lost data.

Root-lesion nematodes (Pratylenchus thornei) decrease root function resulting in reduced uptake of water and nitrogen from the soil and in turn results in poor canopy development. This plant damage as a result of the nematode population can be measured by the use of electro magnetic induction (EM38) for soil water estimations and ceptometry for canopy cover analysis. These tools offer the potential to non-destructively compare different wheat plots growing on soils with known high population of nematodes, thus providing an in-season comparison between varieties.

This technique was used to assess four wheat varieties that ranged in nematode tolerance from intolerant to tolerant. The EM 38 identified the four varieties as having significantly different water use, ten weeks after sowing. The ceptometer identified the most intolerant varieties before symptoms were easily visible to an observer. The combined use of ceptometry and EM38 found differences between the tolerant and intolerant varieties that normally would require final yield comparison.

In the coming years this early screening method will be tested across the cohort of varieties used in the national variety testing program (NVT). This simple non-destructive method of screening varieties will provide additional quantifiable information and reduce the reliance on final yield data.

Introduction

Root-lesion nematode (Pratylenchus thornei) is a common pathogen found in the Australian wheatbelt and causes significant damage to wheat crops grown in northern NSW and southern Queensland. The lost production as a result of P. thornei damage is estimated to be $38 million per annum (Murray & Brennan, 2009). Wheat plants infected by P. thornei suffer reduced root function, limiting the supply of water and nutrients to the plant. This restriction on supply results in poor plant growth with wheat plants appearing stunted, with chlorotic leaves, reduced tillering and poor yield (Thompson et al. 1995). This poor growth has been reflected in water uptake measurements with intolerant wheat using less water than barley in long term rotation trials (Thompson et al., 1995).

These symptoms of nematode damage, combined with final yields are used to rank wheat varieties for nematode tolerance in breeding trials. This qualitative method of in crop ranking is dependent on the skill and knowledge of the observer and the quantitative final yield measurement. Measuring plant water use and leaf area is one way to quantitativly identify differences in plant growth (demand) and plant available water (supply) during the season. Historically, the monitoring of leaf area and soil moisture have been destructive, time consuming or required permanent installation of sensors; however, the development of portable lightweight photosyntheticaly active radiation (PAR) sensors and recent work on the use of electro magnetic induction (EM38) for soil moisture monitoring (Huth & Poulton, 2007) has simplified these measurements.

This paper describes an experiment that tested if EM38 measurements combined with leaf area measurements using a ceptometer, have sufficient precision to identify differences in water use and leaf growth, of wheat varieties with a range of tolerances to nematodes.

Methods

This work examined a sub set of varieties that were part of the national wheat variety-testing program. Wheat varieties were sown (25/6/2011) on a site with a managed high population of nematodes in a completely randomised block configuration. The sub-set of varieties monitored were selected for their varying susceptibility to nematodes and ranged from susceptible (Strzelecki) to the tolerant and partially resistant breeding line (QT8447). The final two varieties (EGA Wylie, and Kennedy) are positioned on the tolerant side of the scale. Kennedy was sown on both high and low initial-nematode populations (Kennedy H and Kennedy L).

Soil water was measured non-destructively during the season using a hand held EM38. Measurements were collected from the vertical and horizontal dipoles at two places within the inter-row space, of the centre rows in each plot. The two readings were temperature corrected and converted to a single Ectot reading following the method described by Huth & Poulton (2007). The Ectot values were converted to mm of water from the 0 to 120mm depths via a calibration curve developed from destructive sampling of the buffer strips. To ensure a range of moisture contents the buffers were strategically sampled during the season.

Leaf area index was calculated non-destructively with an AccuPAR LP-80 Ceptometer (Decagon Devices, Inc). The crop rows were sown in a north south direction and the ceptmeter was inserted below the canopy, across the centre 3 rows, at a slight angle to ensure both ends of the sensor rod were in the centre of the first and third crop row. Eight below-canopy and four above-canopy readings were collected in each plot and integrated to calculate a leaf area index (LAI) using the default values for wheat. All readings were collected during the middle of the day between the hours of 10 am and 3 pm.

Statistical analysis was undertaken using the statistical software package R (Team, 2010).

Results and Discussion

EM38 and LP-80 ceptometer readings recorded on the 4/10/2011 showed significant differences (P ≤ 5%) between the susceptible variety Strzelecki and the tolerant varieties. The reduced water use and corresponding limited leaf area measured in the Strzelecki plots contrasts with the higher water use and larger leaf area observed in the partially resistant line (QT8447; Figure 1). The tolerant varieties had similar water use to QT8447 but did not produce the same quantities of biomass. On going measurements of LAI maintained this pattern with the tolerant species extracting similar amounts of water as QT8447, but producing less leaf area. We hypothesise that this is an indication of the tolerance mechanisms in these species. Such mechanisms would include continually producing new roots to depth to replace those damaged by nematodes instead of growing more leaves.

xplots

Figure 1. Boxplots showing available soil water, LAI and final grain yield for each wheat variety. Soil water and LAI were measured by the EM38, and LP-80 ceptometer on 4/10/2011, the average wheat developmental stage at this time was Z47, flag leaf sheath opening, (Zadoks et al., 1974). The nematode susceptible variety Strzelecki had used the least water, produced the smallest leaf area and by harvest (23/11/11) had the lowest grain yield. Dotted lines on the soil water figure indicate drained upper limit (DUL) and crop lower limit (LLwheat).

Soil water measurements and leaf area measurements recorded in early October correlated well with the final grain yields recorded 7 weeks later (Figure 2). This correlation highlights the value of these non- destructive measurements in providing quantitative data on the tolerance of different wheat cultivars to high populations of nematodes and providing an indication of the mechanisms with which these pathogens reduce wheat grain yields.

Figure 2. Regressions of LAI (a) and soil water (b) at Z47, (flag leaf sheath opening; (Zadoks et al., 1974) against final yield showing a strong predictive relationship (R2 = 0.76 for LAI and 0.62 for soil water) between the non-destructive measurements and final grain yield.

Conclusions

The results from this preliminary study show the non-destructive measurement of soil water and leaf area could distinguish differences between the varieties before flowering. The varieties selected for this study covered a range of tolerances to P.thornei future work will examine the whole national variety testing suite to see where each variety lies on this continuum, and how the soil water within each plot and leaf area readings relate to final yields.

The use of these non-destructive techniques on existing breeding plots has significantly enriched the data collected beyond current practice. Current practices rely on qualitative assessments and grain yields to rank each cultivars’ level of tolerance. These methods are well tested and reliable, however, their quantitative nature provides little insight into the mechanisms or timings of the damage caused by the nematodes. The use of EM38 to estimate soil water combined with a ceptometer to measure leaf area, allows monitoring of both the supply and demand terms for plant growth. Regular use of these tools will identify the individual tolerance point for each variety. That is, the point where growth by intolerant and tolerant varieties differs. Over time the collection of this data combined with meteorological data, and pathogen population numbers will improve our knowledge of how pathogens, crops and the environment interact to set final yield.

The strong relationship between leaf area and water use measured before flowering and grain yield, offers some protection if inclement weather conditions prevent final harvest or damage before harvest can be completed. The relationship observed in this study was strong, despite the season being particularly wet with above average in-crop rainfall. Below average rainfall or a more variable season would increase stress and may accentuate the differences between the intolerant, tolerant and resistant varieties.

The work presented in this paper is preliminary, over the coming season the techniques described will be applied to NVT nematode trials at different sites and at different sowing dates to further assess the technique.

References

Huth, N. I., & Poulton, P. L. (2007). An electromagnetic induction method for monitoring variation in soil moisture in agroforestry systems. Australian Journal of Soil Research, 45(1), 63.

Murray, G. M., & Brennan, J. P. (2009). Estimating disease losses to the Australian wheat industry. Australasian Plant Pathology, 38(6), 558–570.

Team, R. (2010). R: A language and environment for statistical computing. R Foundation for Statistical Computing Vienna Austria.

Thompson, J., Mackenzie, J., & Amos, R. (1995). Root-lesion nematode (Pratylenchus thornei) limits response of wheat but not barley to stored soil moisture in the Hermitage long-term tillage experiment. Australian Journal of Experimental Agriculture, 35(7), 1049–1055.

Zadoks, J., Chang, T., & Konzak, C. (1974). A Decimal Code for Growth Stages of Cereals. Weed Research, 14(6), 415–421.

Top Of PageNext Page