Absorbed radiation and radiation use efficiency as affected by foliar diseases in relation to their vertical position into the canopy in wheat

The impact of foliar diseases on crop yield losses is better understood if considering ecophysiological variables together with pathological variables. Although wheat crop losses due to foliar diseases have already been studied with an ecophysiological approach, none of these studies analyzed the vertical diseases distribution into the canopy leaf layers in relation to the canopy size (leaf area index—LAI) and its architecture (light extinction coefficient-k value). Thus, the objectives of the present study were: (i) to understand the importance of the vertical distribution of the foliar diseases in relation to radiation absorption efficiency (RAE) for different LAI levels and canopy architectures and; (ii) to analyze and compare two ways of radiation capture estimations, considering LAI as a total or LAI of each leaf layer separately. The wheat cultivar Klein Pegaso was grown in plots under field conditions during three growing seasons using different nitrogen supply levels and a wide range of severity diseases by using protected and unprotected crops (applying or not fungicides, respectively). Unprotected plots were inoculated with biotrophic and/or necrotrophic pathogens. Biotrophic (Puccinia triticina-leaf rust) and necrotrophic pathogens (Drechslera tritici-repentis-tan spot; Alternaria triticina-leaf blight) constituted the “diseases complex” with different levels and proportions depending on the year and N treatment. Results showed that foliar diseases reduced LAI and GLAI (green leaf area index). GLAI was not only diminished by LAI reductions, but also by increases in NGLAI (non-green leaf area index) due to lesion coverage which reduced light absorption. In spite of the differences observed in LAI between protected and unprotected crops, radiation interception was not affected until LAI dropped down its critical value (i.e. when crop intercept 95% of the maximum radiation possible to be intercepted). The results demonstrated that assuming a uniform distribution of the diseases, lead to underestimations of accumulated absorbed radiation up to 21%, and as a consequence to overestimations of radiation use efficiency (RUE) up to 29% when diseases were concentrated in the lower leaf layers into the canopy. Together with the severity of the pathogen, at the time to decide controlling diseases, farmers should take into account: (i) LAI level, mainly in those crop situations where, during the critical period for yield determination, the LAI is close to or below the critical value; (ii) canopy architecture (k) associated with light distribution into the canopy and (iii) vertical diseases distribution into the crop.