|dc.description.abstract||Maize grey leaf spot (GLS), caused by Cercospora zeae-maydis, and potato late blight
(LB), caused by Phytophthora infestans, are foliar diseases of maize and potato, two
of the most widely grown crops in KwaZulu-Natal (KZN), after sugarcane and timber.
Commercial maize in KZN accounts for just on 4.3% of the national maize crop. This
is worth R563 million using an average of the yellow and white maize price for the
2001/02 season (at R1 332.87 ton(-1)).
In 2003 KZN produced about 5% of the national
potato crop (summer crop: 7531 300 10kg pockets from 2243 hectares). This equates
to a gross value of R89.4 million based on an average price of R1 188 ton(-1) in 2001.
Successful commercial production of maize and potatoes depends upon control of these
diseases by translaminar fungicides with highly specific modes of action.
This study extends an existing model available for timing of fungicide sprays for GLS
and tests and compares two LB models for two calendar-based spray programmes. The
study also evaluated the use of an early blight model which is caused by Alternaria
solani, and over the single season of evaluation showed potential for use in KZN. For
the GLS model it was found that a number of refinements are needed, e.g., the amount
of infected maize stubble at planting and not the total amount of maize residue at
Based on two years' data, it was found that for the LB models there are no significant
differences in levels of control between using a predicted fungicide programme and a
calendar-based programme. The importance of knowing initial infection sites, and
hence initial inoculum, was demonstrated. This led to the creation of a KZN LB
incidence map, now being used to more accurately time the start of a preventative spray
programme and to time the inclusion of systemic fungicides in the preventative spray
This study has contributed to the further development and expansion of the Automatic
Weather Station Network (AWSN) at Cedara, which now comprises 15 automatic
weather stations in KZN. The AWSN is currently used to aid farmers and advisers in
decision-making regarding fungicide spray timing for GLS and LB.||en