Diagrammatic scale for assessment of alternaria brown spot severity on tangerine leaves

Abstract

Alternaria brown spot (ABS) is one of the main diseases of tangerine worldwide. It is necessary to use a practical and reliable method to quantify the disease severity of ABS and thus suggest management strategies. Thus, a diagrammatic scale (DS) constructed with color photographs was developed and validated to quantify the disease severity. Leaves were collected from crops and a nursery with different levels of ABS symptoms to determine the real severity digitally. The scale levels were determined according to the minimum and maximum severity values found and based on the frequency distribution of severity values. The DS validation was carried out by eleven assessors who estimated the severity of 50 leaves with different symptom levels. One assessment was performed without the DS and two using the DS at seven day intervals. The precision, accuracy, repeatability and reproducibility of the estimates were measured. The developed DS has nine score levels: 0 (0%), 1 (0.1–0.49%), 2 (0.5–1.0%), 3 (1.1–2.0%), 4 (2.1–4.0%), 5 (4.1–8%), 6 (8.1–18%), 7 (18.1–30%) and 8 (30.1–50%). The linear regression and Lin’s concordance correlation coefficient (LCCC) demonstrated an improved accuracy, precision repeatability and reproducibility of the estimates using the DS. The scale is appropriate, useful and reliable to visually estimate with the ABS severity in tangerine leaves.