DIAGRAMMATIC SCALE OF SEVERITY FOR WHITE THREAD BLIGHT DISEASE IN YERBA MATE (Ilex paraguariensis SAINT HILAIRE)

VERESCHUK L.M.; DOMINGUEZ F.G.; ALVARENGA A. E.; ZAPATA P.D.

Congreso; Reunion conjunta 2020 SAIB- SAMIGE; 2020

Yerba mate?s (Ilex paraguariensis St. Hil.) industry is one of the most important economic activities in the province of Misiones, Argentina. White thread blight disease (Ceratobasidium niltonzousanum) affects yerba mate crop, reducing its quality and productivity. The measurement of the intensity of a disease is an indispensable requirement in basic epidemiological studies. The use of diagrammatic scale is the most widely standardized method used to categorize different degrees of disease intensity. To date, there is no known scale to assess the damage caused by C. niltonsouzanum in yerba mate. Therefore, this study aimed to develop and validate a logarithmic diagrammatic scale to quantify the severity of white thread blight. Yerba mate branches with and without symptoms were collected from a field in the north of the province of Misiones. The leaves of each branch were digitilazed and the infected leaf area of each branch was determined using the ImageJ 1.47v program. The percentage of the real severity of each branch was obtained from the quotient between the total leaf area and the infected leaf area. A scale with six levels was developed using the DOSLOG program, based on the Weber-Fechner visual stimulus law. The validation was carried out by twenty evaluators who estimated the severity of 21 photos of branches with different intensity of symptoms. One evaluation without a diagrammatic scale and two evaluations with the scale were carried out at 14-day intervals. The accuracy, precision and reproducibility of the estimates were evaluated by linear regressions and correlations by pairs. Compared to the results obtained without the scale, using the diagrammatic scale developed in this work, the evaluators were able to improve the accuracy and precision of the estimates, and the reproducibility of the scale improved by 94.74%. The need for training prior to the use of the scale was also demonstrated to correctly recognize the signs and symptoms of the disease in the field and improve the estimation of severity. This scale is an important tool for assessing the damage caused by white thread blight in yerba mate in Misiones, Argentina.