Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/15/2016
Publication Date: 1/1/2017
Citation: Twizeyimana, M., Hartman, G.L. 2017. Sensitivity of Phakopsora pachyrhizi (soybean rust) isolates to fungicides and the reduction of fungal sporulation based on fungicide and timing of application. Plant Disease. 101:121-128. doi.org/10.1094/PDIS-04-16-0552-RE.

Interpretive Summary: Soybean rust is a damaging foliar fungal disease of soybean in many soybean-growing areas throughout the world including the southern states in the United States. Strategies to manage soybean rust include the use of foliar fungicides. Fungicides types, the rate of product application, and the number and timing of applications are critical components for successful rust management. The objective of this study was to determine if the fungus causing soybean rust would be effectively controlled by fungicides that are classified as preventative, curative, or a mixture of both. On greenhouse grown plants, all three fungicide types reduced the amount of rust compared to plants without a fungicide treatment. When the fungicides were applied before or after inoculating plants with the fungus causing soybean rust, their effectiveness was reduced when the applications occurred 8 days before and 8 days after inoculating plants. This shows the importance of fungicide application timing in the management of soybean rust. This information is useful to scientists and producers interested in research on managing soybean rust through the use of fungicides.

Technical Abstract: Soybean rust is a damaging foliar fungal disease of soybean in many soybean-growing areas throughout the world. Strategies to manage soybean rust include the use of foliar fungicides. Fungicides types, the rate of product application, and the number and timing of applications are critical components for successful rust management. The objectives of this study were to determine (i) the sensitivity of P. pachyrhizi isolates collected in the U.S. to a range of fungicides and (ii) the reduction of fungal sporulation based on fungicide type and timing of applications on soybean. There were differences (P < 0.05) in effective concentration (EC50) values among the fungicides tested. Azoxystrobin had low EC50 values for both urediniospore germination and fungal sporulation on inoculated leaves. There were differences (P < 0.05) in fungal sporulation for application times, fungicide treatments, and their interaction when the fungus was inoculated on plants. All application times and nearly all fungicide treatments reduced (a = 0.05) fungal sporulation compared to the non-fungicide control. Information on fungicide sensitivity of P. pachyrhizi isolates and the preventive and curative effects of different fungicides are important in the management of soybean rust.