Optimum timing of propiconazole to manage narrow brown leaf spot in the main and ratoon rice crops in Texas

Narrow brown leaf spot (NBLS) is a foliar disease of rice caused by Cercospora janseana. The present study was conducted to evaluate the effects of frequency and timing of propiconazole (PropiMax) applications on rice NBLS severity and grain yield in the main and ratoon (second) crops. Field experiments were established at Eagle Lake and Beaumont, Texas in the years 2010–2012 and 2014 to 2016 respectively. The Eagle Lake trials evaluated two rates of propiconazole (0.16 and 0.32 kg a.i. ha⁻¹) at two application timings (boot and heading growth stages) on the main crop, in combination with or without a single application at the boot stage on the ratoon crop. The Beaumont trials evaluated four main crop application timings (7 days after panicle differentiation, boot, heading, and 7 days after heading) at 0.32 kg a.i. ha⁻¹ in combination with a single application at boot stage on the ratoon crop. Fungicide application frequency of up to three main-crop applications with a single ratoon-crop application was also evaluated in the Beaumont trials. NBLS severity on the rice flag leaves was visually evaluated prior to harvest in the main and ratoon crops. At both locations, fungicide treatments significantly reduced NBLS severity in the main and ratoon crops compared to the non-sprayed control in each of year evaluated. Among the single fungicide application treatments, NBLS severity in the main crop was consistently lower when the fungicide was applied at the heading stage than at any other application stages. Fungicide application at any stage of the main crop significantly reduced NBLS severity in the ratoon crop. An additional fungicide application at the boot stage of the ratoon crop further reduced NBLS severity in four of the six seasons tested. In the Eagle Lake trials, both rates of the fungicide evaluated were comparable in their efficacy in reducing NBLS severity. In Beaumont trials, where multiple fungicide applications were evaluated, main-crop NBLS severity decreased as the number of applications increased. At these low to moderate NBLS severity levels, rice grain yields were not significantly affected, at either of the locations.