Evaluating the effects of fungicides for the control of Exserohilum turcicum on sorghum in South Africa

Abstract

Sorghum (Sorghum bicolor (L.) Moench) is one of the five most widely cultivated grasses in Sub- Saharan Africa. Exserohilum turcicum (Pass.) Leonard and Suggs is a fungal foliar pathogen that causes leaf blight (LB) on both maize and sorghum, with losses of up to 50% recorded on sorghum. Certain fungicides used to control the disease have been shown to have plant growthregulating properties leading to increased yield and delayed leaf senescence. The merit of these growth-regulating properties of fungicides when applied prophylactically has not been evaluated under South African climatic conditions. Accordingly, field trials were conducted in three major sorghum cultivation sites (Greytown, Potchefstroom and Standerton) during the 2013/14 and 2014/15 growing seasons. A glasshouse trial to evaluate certain biochemical and physiological parameter was also conducted during 2015. Two fungicides (azoxystrobin/difenoconazole and epoxiconazole/pyraclostrobin) were applied to four sorghum cultivars at 6, 8, 10, 6 & 8 and 8 & 10 weeks after planting to study agronomic, physiological and biochemical responses of the plants in response to such application. Significant differences in yield were only shown in response to one treatment at Standerton during the 2013/14 planting season. Clear delays in senescence were observed with fungicide application at Greytown for the entire duration of the 2013/14 season, as well as the first senescence observation (S1) during 2014/15, but this effect was not observed at any of the other sites. In the glasshouse, no response to fungicide application was obtained with photosynthetic capacity parameters measured. Enzyme responses were visible with anti-oxidant enzymes showing increased activity with fungicide application. Delays in senescence may be associated with delays in development, as black layer formation was delayed in some cases. These delays in senescence did however not result in increased sugar acculmilation in the seed. To conclude, both senescence observations and enzyme function resulted in supporting evidence that fungicides affect plant function, but with no clear indication that prophylactic application will provide financial return in the form of yield increases which will support increased production cost to the producer