Downy Mildew (Plasmopara viticola) is known as one of the most important vineyard
diseases in the Western Cape, because it has the capability to develop and spread very
fast, and so cause large crop losses in certain years. In 1992 an Austrian researcher
developed the Metos automatic weather station and associated software, to predict the
occurrence of primary and secondary infection of downy mildew. This Metos weather
stationâs software was adapted for South African climatic conditions during 1995 and is
known as the Metos-2 model. The Metos-2 model however had some shortcomings that
needed to be improved. The most important of this was that the model was not sensitive
enough to accurately calculate infections, and furthermore it gives only a âYes/Noâ
warning of possible primary and/or secondary infections. The Metos-2 model makes use
of measured leaf wetness values from a leaf wetness sensor that is probably considered as
one of the most inaccurate meteorological sensors. During 1995 - 2005 the Metos-2
model has been thoroughly tested and used by the disease management division of ARC
Infruitec-Nietvoorbij, to warn the industry of possible downy mildew outbreaks. Results
over these years have shown that more sprays were needed within the preventative
spraying programs, as opposed to recommendations of the Metos-2 model, for the same
or even improved control of downy mildew. On the other hand the results of the Metos-2
model compared to the Metos model, gave similar warnings for both primary and
secondary infections. It is however very difficult to get clear similarities/differences
between what the Metos-2 model has calculated and what had really occurred in the
vineyards. This can be attributed mainly to the accumulation effect of downy mildew
infections. With the development of the Downy Mildew Early Warning Model (DSVWmodel),
two important changes were made, namely the leaf wetness was replaced with a
mathematical, non-linear regression and the Metos-2 modelâs âYes/Noâ warnings for
downy mildew infections were replaced with four classes of possible risks. The
calculated leaf wetness of the DSVW-model, that uses measured relative humidity and air
temperature as input values, had a significant coefficient of determination of 0.70,
compared with measured leaf wetness. The DSVW-modelâs four risk classes of possible
infections (primary and secondary) are as follows: zero infection (0 %), low infection (1 -
34 %), medium (35 - 74 %) and a high risk class (75 - 100 %). To test the DSVWmodelâs
accuracy and reliability, historical weather data (1998 - 2003) and measured
disease outbreak data in the Stellenbosch, Robertson and Paarl areas were used to run
both the Metos-2 and the DSVW-models. Primary as well as secondary infections were
predicted by the models. When the DSVW-model and the Metos-2 modelâs infection
warnings were correlated with disease outbreaks, of the two, the DSVW-model showed
consistently similar or better correlations with the measured disease outbreak data. The
DSVW-model also calculated on a regular basis more primary and secondary infections,
than the Metos-2 model, which at times did not warn of any downy mildew infections,
although outbreaks of downy mildew did occur soon after. Producers can use the new
DSVW-model with confidence, together with one or other prevention spray program, for
the control of downy mildew.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-06132007-151522 |
| Date | 13 June 2007 |
| Creators | Haasbroek, Pieter Daniel |
| Contributors | Prof S Walker |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-06132007-151522/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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