Effect of pyraclostrobin on disease control, leaf physiology, seed yield and quality of sunflower

A two-year (2012-2013) field experiment was carried out to investigate the fungicidal activity of pyraclostrobin and its putative physiological effects on sunflower (Helianthus annuus L.) grown under rainfed, Mediterranean conditions. Five pyraclostrobin treatments were applied [single applications at rates of 100, 150 and 200 g active ingredient (ai) ha−1 at V6 growth stage (TV6/100, TV6/150, and TV6/200, respectively) and 200 g ai ha−1 at R1 growth stage (TR1/200) and a double application at V6 and R1 at a rate of 100 g ai ha−1 (TV6-R1/100)] and accompanied by untreated (TUC) and chemical control [iprodione at a rate of 750 g ai ha−1 at V6 (TCC)]. Diseases [charcoal rot (CR) caused by Macrophomina phaseolina, Phomopsis stem canker (SC) caused by Phomopsis helianthi, black stem (BS) caused by Phoma macdonaldii, and powdery mildew (PM) caused by Golovinomyces cichoracearum] were assessed twice. Leaf physiological traits [leaf chlorophyll content (SPAD), specific leaf area (SLA), carbon isotope discrimination (Δ), nitrogen (N) concentration, 15N natural abundance (δ15N), and carbon to nitrogen ratio (C/N)] were determined at R5 stage. Growth season had a significant effect on seed oil concentration (Oil) and oil yield (OY) and was higher in 2013; this season had low CR and SC incidence but high incidence of BS and severity of PM. Split application of pyraclostrobin (TV6-R1/100) or a high rate at the beginning of the reproductive phase (TR1/200) suppressed the diseases, mainly SC and BS. The former treatment (TV6-R1/100) showed the highest yields [seed yield (SY), OY, and protein yield (PY)]. All the physiological traits but Δ were not affected by fungicide treatments. The high-yielding TV6-R1/100 treatment had the highest Δ values over the growth seasons indicating a “waste” of water in turn to sustain open stomata and thus higher CO2 exchange rates. This swap leads to lower water use efficiency in the long-term but it indicates healthier and more active functioning sunflower plants that had greater yields. Phomopsis stem canker (SC) showed to be the most detrimental disease since its incidence was negatively correlated to yields (SY, OY) over the growth seasons. On the other hand, CR seemed to affect N uptake and translocation within sunflower plants. In conclusion, the physiological response (highest Δ) recorded for split application of pyraclostrobin (at V6 and R1 stages) was indirect; lower disease incidence secured water supply to sunflower plants resulting in higher yields.