Diagrammatic Scale Severity for Tar Spot Complex in Maize

Abstract

The tar spot complex of maize, induced by the fungi Phyllachora maydis and Monographella maydis, is a disease of economic importance in Mexico and Central America because it causes severe yield losses. Given the growing importance, standardized methods are required to quantify disease that allow for reproducible epidemiological methods. For this purpose a logarithmic scale of severity was developed with intervals and midpoints of 0(0-0), 3(1-6), 12(7-22), 38(23-55), 72(56-84), 91(85-95) and 98(96-100)% of the affected leaf area, analyzing accuracy, precision and reproducibility. The evaluation considered ten evaluators and 50 plants with different levels of severity, determined by the Image Tool 3.0 Software. The precision and accuracy was determined by a simple linear regression analysis between actual and estimated severity. All evaluators tended to overestimate the disease, and recorded values of the b₁ coefficient statistically equal to 1 (P < 0.01), so that the disease evaluation with the proposed scale gives values close to real results. High levels of accuracy were observed; r² was comprised between the range of 0.71 and 0.91, indicating that the scale can be successfully implemented in the evaluation of the tar spot complex disease.