Precise assessment of angular leaf spot severity using the Leaf Doctor app for common beans

Abstract

Angular leaf spot (ALS) is one of the most severe diseases of common bean. However, there is as yet no standardization of the phenological stage, inoculum concentration, diagrammatic scale (DS), and day of assessment for screening ALS resistance in bean lines. The present study aimed to evaluate the potential for assessment of ALS severity in common beans using the Leaf Doctor app and inoculation at the V2 phenological stage under two inoculation concentrations. The experiment was carried out in a completely randomized design in a 3 × 2 × 2 factorial arrangement with three cultivars (AND 277, IAC-Milênio, and IAC-Carioca), two monosporic isolates (31-31 and 10-59), and two inoculum concentrations (2 × 10 4 and 4 × 10 4 conidia/mL). Disease severity was evaluated from 11 to 15 days after inoculation (d.a.i) using a diagrammatic scale and the Leaf Doctor. The results showed that 15 d.a.i., the concentration of the inoculum was no longer significant and that the isolates showed no significant difference in terms of pathogenicity. Moreover, in 15 d.a.i. it was possible to separate the cultivars into three phenotypic classes: resistant (AND-277), moderately susceptible (IAC-Milênio) and susceptible (IAC-Carioca). Considering the diagrammatic scale of notes, the cultivar AND 277 had a score of 2.3, while for the image evaluation, it was observed that only 0.65% of the total leaf area was affected by the disease. For this reason, 15 d.a.i. was recommended as the better day for evaluation, and the concentration of 2 × 10 4 conidia/mL may be adopted in order to reduce the amount of inoculum needed for the experiment. In addition, the evaluation with the Leaf Doctor app increases accuracy and improves discrimination between resistant and susceptible cultivars.