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Evaluation of fungicide timing for management of Phoma black stem of sunflowers

Michelle Gilley: North Dakota State University

<div>Phoma black stem, caused by <em>Phoma macdonaldii</em>, was identified in 80% of North American sunflower fields surveyed in 2015. Preliminary data suggested that FRAC 11 fungicides may reduce disease severity, but the economic return and timing of fungicide application was unclear. The objective of this study was to evaluate the timing of pyraclostrobin application on two oil type sunflower hybrids under natural infection. In May 2017, two adjacent fungicide trials were established in North Dakota. Both trials were planted in four-row plots and arranged in a randomized complete block design with four replicates. Fungicide was applied to the middle two rows of plots singly and in combination at V8, R1 and R4. A disease severity index was calculated by multiplying incidence (number of stems infected) and severity (mean number of stem lesions) on ten arbitrarily selected plants in each plot. Infection developed in both trials with 100% incidence in the non-treated control (NTC) plots. All fungicide timing combinations that included an application at R1, as well as a double application at V8 and R4, significantly (P ≤ 0.05) reduced disease severity for both hybrids. Yield was significantly (P ≤ 0.05) higher than the NTC for all fungicide treatments except V8 and R4 alone in one of the trials; whereas, no significant yield differences were observed for the other trial. Under high disease pressure, fungicide applied prior to flowering may improve yield on some hybrids.</div>