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Measurements of leaf area are important in yields damage studies, but the equipment needed may not be available for every one. Therefore, the use indirect comparisons with weight of fresh or dried leaves may help in estimating leaf area. This research studied the relationship between leaf weight and leaf area in plants sampled at the growth stage R7.1 from 64 field plots of Nidera 5909 RG soybean cultivar. The leaves were weighted soon after sampling for fresh weight and again after 48 hours of incubation at 65° C for dried weight. Leaf area was measured with a Licor digital leaf area meter. The model equations were significant (p < 0.0001, R2 0.74 to 0.97) for each plant part (lower, medium, or upper third) and for the whole plant. In average, leaf area (y) was estimated as y = 45.53x + 19.03 from fresh weight (x) and as y = 176.17x - 75.30 for dried weight (x). This tool was shown potentially viable to estimate plant leaf area. The method based on dried weight is more labor consuming but does not require leaves being weight soon after sampling. The use of such comparisons for other soybean cultivars may require additional validation studies. [ABSTRACT FROM AUTHOR]

Asian soybean rust (ASR) is the principal disease that affects the soybean crop. Fungicides are most effective in controlling ASR. However, the intrinsic factors related to the soybean cultivar, suitability of the application technology to the target (leaves) and environmental factors directly affect the application effectiveness control. This study investigated the effect of the combined use of spray volumes (70, 110 and 150 L ha−1) and droplet sizes (fine and medium) on the chemical control of ASR in two cultivars with different leaf area indices (LAI) [BMX Lança cv. (LAI<6.0) and BMX Garra cv. (LAI>6.0)]. For the BMX Lança, in 2017, when the maximum LAI achieved was 5.8, the use of increasing spray volumes, according to the crop growth, and fine droplets (in the applications 2 and 3) presented an area under the disease progress curve (AUDPC) and productivity results similar to the use of the 150 L ha−1 rate for all the applications, irrespective of the droplet size employed. For the BMX Garra, which reached a maximum LAI of 7.1 in the same year, the highest seed yield and lowest AUDPC were obtained for the spray volume of 150 L ha−1 in all the applications, using both droplet sizes. In 2018, when the LAI of the cultivars were reduced because of lack of rainfall, BMX Lança and BMX Garra showed maximum LAI of 4.9 and 5.5, respectively, and the same treatments that stood out in the 2017 season for both cultivars presented the lowest values regarding the AUDPC. Otherwise, none or minor statistical differences were detected between the combinations of spray volumes and droplet spectra for both cultivars in 2018. The spray volume and droplet size should be determined according to the leaf area of the crop at the time of the application and under conditions of disease pressure. • Application technology should consider the crop leaf area at the spraying time. • The larger the leaf area the greater the dependence on the use of fine droplets. • The larger the leaf area the greater the need to increase the spray volume. • These practices are necessary to overcome crop shielding and enhance soybean rust control. [ABSTRACT FROM AUTHOR]