Tan spot is one of the main diseases of wheat in Brazil reducing grain yield up to 80%. Although the lesion expansion is an important component of the epidemics, there are few studies on its control by curative sprays of fungicide. To know how different fungicides may control lesion expansion and to improve disease management, four assays were carried out at the Universidade de Passo Fundo (UPF) in 2011. All assays were organized as randomized blocks, with four replicates, inside a growth chamber with control of light and temperature. In two assays a triazole (propiconazole) and a strobylurin (azoxystrobin) fungicides, as well as their mixture, were sprayed from one to 12 day intervals after inoculation of the wheat cultivars 'Fundacep Horizonte' and 'Quartzo'. Based upon measurements of the lesion size, the strobylurin did not reduce the rate of lesion growth (0.176 to 0.180 mm².day-1) in comparison to the non-sprayed control (0.175 to 0.184 mm².day-1), whereas the triazole (0.080 mm².day-1) and the fungicide mix (0.060 to 0.080 mm².day-1) did it if sprayed within 10 or 12 days after inoculation. In two other assays, commercial fungicides (epoxiconazole, propiconazole, tebuconazole, azoxystrobin, pyraclostrobin, cyproconazole + azoxystrobin, epoxiconazole + pyraclostrobin, protioconazole + trifloxystrobin and tebuconazole + trifloxystrobin) were sprayed onto plants at three, five and eight days after inoculation. Spraying of the fungicides epoxiconazole, propiconazole, and protioconazole + trifloxystrobin at five days reduced the rate of lesion expansion (0.07 to 0.14 mm².day-1) over the control non-sprayed plants (0.29 mm².day-1). Fungicide applications eight days had no effect on the rate of progress of lesions. The final lesion size was smaller (3.76 to 5.89 mm²) on plants sprayed with tebuconazole + trifloxystrobin, protioconazole + trifloxystrobin, tebuconazol, epoxiconazole + pyraclostrobin, and propiconazole, but higher (8.46 to 11.70 mm²) for cyproconazole + azoxystrobin, azoxystrobin, epoxiconazole, and pyraclostrobin, which did not differ from the non-sprayed control. These results show how difficult is to limit tan spot increase after disease establishment and indicate that is management requires the adoption of preventive control strategies.
Pyrenophora tritici-repentis; triazoles; strobylurins
