DIAGRAMMATIC SCALE FOR ASSESSMENT OF SOYBEAN DOWNY MILDEW SEVERITY

Abstract

The downy mildew, caused by Peronospora manshurica, is a secondary disease in soybean culture, however in humid cold regions has occurred with more frequency. In this study a diagrammatic scale was developed and valided a diagramatic scale forto quantify the severity of the disease Peronospora manshuricain soybean. Of the 200 collected leaves in Castro (PR), were selected 100 leaves dissected dried herbarium in better conservation’s state with different levels of severity and after were numbered to determinate the lesion and leaf area to calculate the severity percentage per each leaf. The scale considered the minimum and maximum limits of severity observed in the field and the intermediate values followed logarithmic increments, obtaining eight levels: 0,08; 0,30; 1,10; 3,39; 12,85; 34,92; 66,13 e 87,65%. In validation seven appraisersraters, with and without previous practice estimated the lesion area before and after the use of the scale. Accuracy and precision were determined by linear regression, the results of accuracy and precision were satisfactory. The coefficient of determination values after the use of the scale demonstrated increase getting levels of 0,97, in relation to the error dispersion graphic, improved 100% of the cases, decreasing the number of the notes more then 10% of error of 16 for 8,2. With the Test F (Assistat program) was observed the statistic difference in R² when it was used the scale, which provided adequate and it will can be utilized to validated the severity in others experiments.