Diagrammatic scale for phyllachora in Australian red cedar

Abstract

The Australian red cedar (Toona ciliata), a forest species native to southwestern Asia, has aroused commercial interest for its high-quality timber. Phyllachora was recently reported in this species in Brazil, but there is a lack of knowledge about its epidemiology and control. Currently, no methodologies exist for quantifying the severity of this disease in cedar trees. In the present study, a diagrammatic scale comprising five levels of disease severity was developed. Validation of the proposed diagrammatic scale was performed by having 13 inexperienced evaluators conduct three assessments, separated by seven-day intervals, of samples with various levels of disease severity. In the first assessment, the evaluators did not use the diagrammatic scale and estimated severity as a percentage. In the second and third assessments, levels were assigned with the aid of the scale proposed in this study. The accuracy and precision of the severity estimates of each evaluator were analyzed by linear regression and Lin’s statistics. The reproducibility of the estimates was evaluated using the coefficient of determination of linear regressions of data from pairs of evaluators. Of 200 leaflets collected to develop the diagrammatic scale, 96% showed severity below 9%, and only 4% showed severity between 9 and 15%. The assessments performed with the scale yielded greater precision, accuracy, reproducibility and reproducibility of estimates than did those performed without the scale. Therefore, it is recommended that the proposed scale be used for the quantification of severity of phyllachora.