Alternative products to control late season diseases in soybeans

ABSTRACT: In the last crop seasons, the complex of late season diseases (CLSD) of soybean (Glycine max L. (Merrill)), has been causing considerable reductions in the crop yield. Currently, there are no cultivars resistant to all pathogens that causes CLSD. The present study evaluated the effect of applying the acibenzolar-S-methyl resistance inducer, alternative products and fungicide on the severity of CLSD in the soybean cultivar BMX Potência RR during the 2013/2014 and 2014/2015 crops, in the field. The treatments for the experiments were: 1 - control (water); 2 - acibenzolar-S-methyl; 3 - calcium; 4 - micronutrients: copper, manganese and zinc; 5 - micronutrients: manganese, zinc and molybdenum; 6 - nitrogen-potassium fertilizer; 7 - Ascophyllum nodosum and 8 - azoxystrobin + cyproconazole with the addition of the adjuvant. Four applications of alternative products and two of fungicide were carried out in both harvests. A diagrammatic scale assessed the severity of CLSD at the phenological stage R7.1. The acibenzolar-S-methyl resistance inducer, alternative products (macro and micronutrients) and A. nodosum had no effect on the severity of CLSD in the two harvests. The fungicide (azoxystrobin + cyproconazole) reduced the severity of CLSD and prevented damage to productivity in both experiments.

Diagrammatic scale for assessment the severity of Cercospora leaf blight on soybean (Glycine max) leaflets

Introduction. Cercospora leaf blight (CLB) caused by Cercospora kikuchii is an endemic soybean crop disease in Argentina. Objective. To develop and validate a scale to quantify the severity of CLB in leaflets of soybean (Glycine max) for the evaluation of foliar fungicides and cultivar trials. Materials and methods. A diagrammatic scale to assess the severity of CLB on soybean leaflets (Glycine max) was developed. The scale was based on the evaluation of 50 leaflets. The different severity levels of the leaflets were determined according to Weber-Fechner’s stimulus-response law. The proposed scale included severity levels of 1, 4.5, 17.5, 50, 82.2, 95, and 99 %. The validation of the scale was performed by 23 raters (3 experienced and 20 inexperienced) who assessed the severity of CLB on 50 leaflets with and without the use of the scale. Results. The results from the three experienced raters were more precise than those obtained by the inexperienced raters. The scale improved the visual interpretation to assess the severity of CLB for the inexperienced raters, who obtained estimated severity values close to the actual severity values. When the inexperienced raters used the proposed scale to estimate the severity of CLB on leaflets, they were accurate having determination coefficients (R2) between 0.74 and 0.91. According to the absolute errors, the greatest deviation was observed between -2.49 and 2.46. However, these deviations were not significantly different from the population average. To speed the process of cultivar evaluations we also propose the use of scales based on grades. Conclusions. The proposed scale to assess the severity of CLB on leaflets can be used for the assessment of foliar fungicide trials and soybean cultivars. The scale showed specificity for assessing the severity of Cercospora leaf blight (CFB) in soybean leaflets.

Development and validation of diagrammatic scales to evaluate damage by the two-spotted spider mite (Tetranychus urticae) in peanut

Argentina is the second largest peanut (Arachis hypogaea L.) exporter in the world. The main peanut pest in our country is the two-spotted spider mite (Tetranychus urticae Koch 1836). To date, there is no validated method to quantify this pest. The aim of this work was to develop and validate a logarithmic diagrammatic scale to assess damage by the two-spotted spider mite in peanut. In 2015-16, 200 leaflets were collected from a plot infested with the pest. Damaged leaf area (DLA) was calculated using SisCob software. Six-, seven-, eight- and nine-class scales were proposed. Mean values for each class and scale were obtained with 2-LOG. Leaflets were analyzed by 13 raters who used the four proposed scales. Precision and accuracy were determined by simple linear regression between the DLA and estimated damaged leaf area. Reproducibility was determined by linear regression between estimates of raters combined in pairs. The seven-class scale was the best validated one for all the parameters. Most raters showed constant deviations and overestimated DLA, whereas only one rater presented systematic deviations. This seven-class scale is the first developed and validated one to evaluate two-spotted spider mite damage to peanut in Argentina. Highlights According to the validation results, the seven-class diagrammatic scale was the best one in terms of accuracy, precision and reproducibility with values above 0.80, 0.75 and 0.75 respectively. The scales are useful tools for pest assessment, fundamental parameters for integrated pest management. The seven-class scale is the first one developed and validated for evaluating damage by the two-spotted spider mite in peanut crop in Argentina. With this scale a loss function will be calculate which will allow the estimation of the EIL of this pest.

Diagrammatic scale for assessment of anthracnose severity in feijoa fruit

ABSTRACT: A diagrammatic scale of anthracnose in feijoa fruit was elaborated and validated in order to standardize disease severity assessments. The proposed scale showed six disease severity levels: 2, 10, 20, 40, 70 and 100% of the injured fruit surface. The scale took into account the minimum and maximum limits of disease severity observed in the field and the intermediate values followed logarithmic increments according to the Weber-Fechner stimulus-response law. Eight inexperienced raters validated the scale by quantifying the disease severity (using/not using the scale) of 50 feijoas with anthracnose symptoms. In conclusion, the scale improved the assessment of anthracnose in feijoa. Eight genotypes from different crosses were tolerant to anthracnose.

Development of a Protocol and a Diagrammatic Scale for Quantification of Bacterial Leaf Streak Disease on Young Plants of Maize

Bacterial leaf streak (BLS), caused Xanthomonas vasicola pv. vasculorum (Xvv), has become a major concern for maize production, mainly in the United States and South America. Therefore, this study aimed to establish a protocol for Xvv inoculation in young maize plants under controlled conditions and to develop and validate a diagrammatic scale for evaluation of maize hybrids in regard to BLS resistance. The study was carried out in three steps: the establishment of a protocol for inoculation of Xvv in young maize plants under controlled conditions; the development and validation of a diagrammatic scale for BLS severity evaluation; and the screening for BLS resistance of 45 hybrids using the proposed protocol for bacterial inoculation and the diagrammatic scale developed in this study. Besides reproducing a more natural Xvv infection, the bacterial suspension spraying without injury inoculation method induced higher disease incidence and severity, as well as reproducibility of results under the experimental conditions established in this study. The proposed diagrammatic scale allowed evaluating BLS severity with up to 97.49% of the leaf area affected by the disease. Further, the use of the diagrammatic scale resulted in an increase of accuracy from 0.909 up to 0.992. The reaction of 45 maize hybrids to BLS allowed establishing six major groups of susceptibility to the disease. The most resistant maize hybrids to BLS formed a group of 13 hybrids, with disease severity below 5%.

Validation of a diagrammatic scale for quantification of bacterial leaf blight (causal agent Pseudomonas avenae) in maize

Bacterial leaf blight occurs in almost the entire Brazilian territory and can cause significant economic damage. However, its effects can be curtailed with the following tools that aid in quantification for carrying out the best severity assessment that can be applied to the studies used in its control. The objective of this study was to develop and validate a diagrammatic scale to quantify the severity of bacterial leaf blight in maize caused by Pseudomonas avenae. Corn leaves were collected with different levels of disease severity, and the total leaf area as well as the injured area of the middle third of each leaf was measured. From this, the minimum and maximum limits of the disease and, subsequently, four intermediate levels were determined whilst taking into account the law of visual stimulus of Weber-Fechner. Levels 1.5%, 2.5%, 9%, 18%, 46%, and 70% were selected with validation performed by eight evaluators, four inexperienced and four experienced. These evaluators estimated the severity values, initially without the use of the proposed diagrammatic scale and in a second attempt with the use of the scale. We concluded that it was possible to improve the accuracy and precision of the evaluators when they used the scale as a tool to assess the severity of bacterial leaf blight on corn leaves.

Validation of a diagrammatic scale for quantification of bacterial leaf blight (causal agent Pseudomonas avenae) in maize

Bacterial leaf blight occurs in almost the entire Brazilian territory and can cause significant economic damage. However, its effects can be curtailed with the following tools that aid in quantification for carrying out the best severity assessment that can be applied to the studies used in its control. The objective of this study was to develop and validate a diagrammatic scale to quantify the severity of bacterial leaf blight in maize caused by Pseudomonas avenae. Corn leaves were collected with different levels of disease severity, and the total leaf area as well as the injured area of the middle third of each leaf was measured. From this, the minimum and maximum limits of the disease and, subsequently, four intermediate levels were determined whilst taking into account the law of visual stimulus of Weber-Fechner. Levels 1.5%, 2.5%, 9%, 18%, 46%, and 70% were selected with validation performed by eight evaluators, four inexperienced and four experienced. These evaluators estimated the severity values, initially without the use of the proposed diagrammatic scale and in a second attempt with the use of the scale. We concluded that it was possible to improve the accuracy and precision of the evaluators when they used the scale as a tool to assess the severity of bacterial leaf blight on corn leaves.