As a prelude to remote sensing of rhizomania, hyper-spectral leaf reflectance and multi-spectral canopy reflectance were used to study the physiological differences between healthy sugar beets and beets infested with Beet necrotic yellow vein virus. This study was conducted over time in the presence of declining nitrogen levels. Total leaf nitrogen was significantly lower in symptomatic beets than in healthy beets. Chlorophyll and carotenoid levels were reduced in symptomatic beets. Vegetative indices calculated from leaf spectra showed reductions in chlorophyll and carotenoids in symptomatic beets. Betacyanin levels estimated from leaf spectra were decreased at the end of the 2000 season and not in 2001. The ratio of betacyanins to chlorophyll, estimated from canopy spectra, was increased in symptomatic beets at four of seven sampling dates. Differences in betacyanin and carotenoid levels appeared to be related to disease and not nitrogen content. Vegetative indices calculated from multi-spectra...

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PHYTO.2003.93.6.720

Remote detection of rhizomania in sugar beets.

The biotrophic fungus Puccinia striiformis f. sp. tritici is the causal agent of the yellow rust in wheat. Between the years 2010–2013 a new strain of this pathogen (Warrior/Ambition), against which the present cultivated wheat varieties have no resistance, appeared and spread rapidly. It threatens cereal production in most of Europe. The search for sources of resistance to this strain is proposed as the most efficient and safe solution to ensure high grain production. This will be helped by the development of high performance and low cost techniques for field phenotyping. In this study we analyzed vegetation indices in the Red, Green, Blue (RGB) images of crop canopies under field conditions. We evaluated their accuracy in predicting grain yield and assessing disease severity in comparison to other field measurements including the Normalized Difference Vegetation Index (NDVI), leaf chlorophyll content, stomatal conductance, and canopy temperature. We also discuss yield components and agronomic parameters in relation to grain yield and disease severity. RGB-based indices proved to be accurate predictors of grain yield and grain yield losses associated with yellow rust ( R 2  = 0.581 and R 2  = 0.536, respectively), far surpassing the predictive ability of NDVI ( R 2  = 0.118 and R 2  = 0.128, respectively). In comparison to potential yield, we found the presence of disease to be correlated with reductions in the number of grains per spike, grains per square meter, kernel weight and harvest index. Grain yield losses in the presence of yellow rust were also greater in later heading varieties. The combination of RGB-based indices and days to heading together explained 70.9% of the variability in grain yield and 62.7% of the yield losses.

Grain yield losses in yellow-rusted durum wheat estimated using digital and conventional parameters under field conditions

Blast, caused by Pyricularia oryzae, has become an economically important disease in wheat in Brazil, but little effort has been devoted to understanding the wheat–P. oryzae interaction. This study was intended to determine the effects of P. oryzae infection on the photosynthetic process in wheat plants using a susceptible (BR 18) and a partially resistant cultivar (BRS 229). It was found that the net carbon assimilation rate (A), stomatal conductance (gs), and transpiration rate were dramatically reduced in both cultivars due to P. oryzae infection but to a lesser degree in BRS 229. Photosynthesis was impaired in asymptomatic leaf tissues, indicating that blast severity is not an acceptable indicator for predicting P. oryzae-induced reductions in A. The proportionally larger decreases in A than in gs, in parallel with increases in internal CO2 concentration (Ci), suggest that the lower influx of CO2 into the diseased leaves caused by stomatal closure was not a prominent factor associated with th...

https://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-01-13-0024-R

Limitations to photosynthesis in leaves of wheat plants infected by Pyricularia oryzae.

Quantification of the damaging effects of pathogens on diseased plants and inclusion of these damaging functions in crop simulation models is of great importance for a more complete understanding of yield response to diseases. In this study, the effect of Septoria tritici blotch (STB) on net photosynthetic and dark respiration rates of wheat flag leaves was quantified. Bastiaans' model: Y=(1-x)beta was used to characterize the relationship between relative leaf photosynthesis (Y, considering Ynet and Ygross) and STB severity (with x the proportion of the diseased area). The value of beta indicates whether the effect of disease on photosynthesis is larger (beta >1), lower (beta 1 is due to a combination of a decrease in the gross photosynthesis (betagross still >1) and to an increase in the dark respiration rate (betagross<betanet). Finally, it is discussed how the results could improve the prediction of crop loss caused by an STB epidemic in wheat fields.

/pdf/quantification-of-the-effects-of-septoria-tritici-blotch-on-2uq046c4y7.pdf

Quantification of the effects of Septoria tritici blotch on wheat leaf gas exchange with respect to lesion age, leaf number, and leaf nitrogen status

Low-cost assessment of wheat resistance to yellow rust through conventional RGB images

Alteration of physiological processes in wheat flag leaves caused by stem rust and leaf rust.

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Alteration of physiological processes in wheat flag leaves caused by stem rust and leaf rust.