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on susceptible versus resistant cultivars. Field experiments were
conducted over four seasons at Roodebloem and Malmesbury
in the Western Cape. Two wheat cultivars namely SST 88 and
SST 056, which differ in their resistance to wheat rusts, were used in
all tests. SST 056 is moderately resistant to leaf rust and stripe rust
and moderately susceptible to stem rust. SST 88 is susceptible to
stem and leaf rust and moderately resistant to stripe rust. Different
fungicide treatments were applied at seven leaf and flag leaf growth
stages and finally, grain yields from fungicide treated and untreated
plots were compared.
Averaged over seasons and localities, fungicides resulted in varying
levels of yield increments compared to the untreated control on
the two cultivars. On both SST 88 and SST 056, Epoxiconazole +
Pyraclostrobin, Pyraclostrobin and Pyraclostrobin/Tebuconazole
provided better yields than the remaining treatments (
Table 1
).
About 0,571 t/ha and 0,285 t/ha yield increments over the control
were obtained by application of Epoxiconazole + Pyraclostrobin to
SST 88 and SST 056, respectively. Most of the remaining fungicide
treatments on the resistant cultivar SST 056 provided lower yield
improvements of less than 0,200 t/ha. Most fungicides which
increased yield, also reduced the severity of leaf and stem rust.
This suggests that the increased yield from fungicide treatments
was largely due to the effect of fungicides on foliar diseases. This
conclusion can be further supported by the higher yield response
recorded from the susceptible cultivar SST 88 than from the resistant
SST 056.
Economic analysis was performed to determine the benefits of the
different fungicide treatments on the two wheat cultivars. For this
purpose, average yields of each treatment over four seasons and
two localities were used. The profitability of each treatment was
calculated by subtracting costs of double fungicide application
from the gross benefit obtained by application of each fungicide
treatment. The gross benefits were calculated based on average
wheat price of R3 705/ton during March 2018.
Fungicide treatments resulted in different levels of profitability
(Table 1). On the susceptible cultivar SST 88, most of the fungicide
treatments resulted in positive net profits ranging from R121/ha to
R1 266/ha. However, a few fungicides gave negative net benefits
of R17/ha to R379/ha. Averaged over all fungicides and seasons,
application of fungicides to the susceptible cultivar gave an average
profit of R362/ha. On the resistant cultivar, only three fungicide
treatments out of eleven tested, gave positive net benefits of R26/ha
to R207/ha whereas the majority of the fungicides showed varying
levels of negative net benefits (losses). Averaged over four seasons,
application of fungicides to the resistant cultivar resulted in a loss of
approximately R250/ha.
These results showed that application of fungicide was profitable on
the susceptible cultivar while there was little or no benefit in applying
fungicides to the resistant cultivar. This indicates that when resistant
cultivars are available, they can be used for the control of foliar
diseases and protection of wheat yields and such resistant cultivars
may not require fungicide application. The use of fungicides should
be considered on susceptible, high yielding cultivars
which could suffer serious yield losses when disease
pressures are high.