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