Use of Normalised Difference Vegetation Index (NDVI) to assess tolerance of wheat cultivars to root-lesion nematodes (Pratylenchus thornei)

Use of Normalised Difference Vegetation Index (NDVI) to assess tolerance of wheat cultivars to root-lesion nematodes (Pratylenchus thornei)

Increasing food production is crucial to adequately nourish the global population. Wheat is a staple cereal as a major food source in many countries worldwide, and growing it is attractive and lucrative to growers, traders, and to whole countries’ economies. Growers in Australia have become heavily reliant on wheat as their primary winter season crop and economic income. Continuous cropping of wheat cultivars that are susceptible to the root-lesion nematode, Pratylenchus thornei has caused increased population densities in the soil leading to significant yield loss of intolerant crops. Yield losses of up to 70% have been demonstrated. Therefore, growers are reliant on wheat cultivars that are tolerant to P. thornei to minimise yield losses. Wheat breeders need to develop regionally adapted cultivars with improved tolerance to P. thornei. The use of new technologies, such as normalised difference vegetation index (NDVI), could assist with selecting cultivars with improved tolerance. This forms the focus of the research questions posed in Chapter one, and the review of literature is addressed in Chapter two of this thesis.

The aim of this study was to investigate NDVI, measured by GreenSeeker™, as a tool to improve the selection of P. thornei tolerance of wheat genotypes in research and wheat breeding programs. To do this, three two-stage field experiments tested 36 wheat cultivars. In the first stage, two wheat cultivars, one moderately resistant and one susceptible to P. thornei were grown as plots in replicated experimental designs to establish low and high nematode population densities respectively. In the second stage, these plots were sown with 36 wheat cultivars across low and high nematode population densities. NDVI measurements were taken regularly during the growing season and grain was harvested at crop maturity from each plot. The NDVI values for intolerant wheat cultivars were inversely related to P. thornei population densities. The NDVI values for tolerant cultivars were independent of P. thornei population densities. Cultivars could be classified into groups by their response to P. thornei as determined by the predicted NDVI values. Higher P. thornei compared to lower population densities improved the correlation between the NDVI predicted tolerance and grain yield for the wheat cultivars. These correlations were observed when comparing the area under disease progress curve (AUDPC) with respect to NDVI and by single critical point sensing (CPS). An advantage of AUDPC with respect to NDVI compared to CPS was that even on population densities, as low as 1245 P. thornei/kg soil, AUDPC-NDVI is predictive of tolerance (R2 = 0.35, P>0.001). It was found that CPS can be used to predict the tolerance of wheat cultivars at approximately 1000°Cd after sowing on land with initial population densities greater than 2500 P. thornei/kg soil. These results are presented in Chapter three as a accepted article in the journal, Annals of Applied Biology.

This study demonstrated that NDVI can be used to predict tolerance of wheat cultivars to P. thornei. More research is required to determine the suitability of NDVI on small plots, such as three row plots that are used in breeding programs, and this is described in the concluding chapter (Chapter four). Briefly, there is additional scope to determine whether NDVI can be used to predict the tolerance of other important crops, such as chickpea and barley that are known to suffer yield losses due to P. thornei. Furthermore, wheat breeders have options to use aerial technologies in their phenotyping programs with the availability of unmanned aerial vehicles (UAV) with the capacity to have NDVI, multispectral or thermal cameras. Ultimately, the development of a high throughput tool using UAV that accurately predicts the tolerance of a cultivar to P. thornei will enable more rapid development by researchers and plant breeders of germplasm and adapted cultivars with superior tolerance to P. thornei.