Agriculture Reference
In-Depth Information
worthwhile because it will so rarely differ from routine spraying”. This is true in
part but there are still savings to be made in defining the start of the programme in
situations where inoculum is limiting. All blight forecasting schemes assume
presence of inoculum. This is quite clearly not the case when a range of forecasting
schemes trigger application based on weather weeks ahead of first occurrences.
9.7 CEREALS
The rapid and widespread introduction of fungicides to the cereal crop in the early
1970s has resulted in an increase in the number of schemes aimed at improving the
efficacy of fungicide application. Unlike disease control in potatoes, failure to apply
a fungicide, or to time an application with precision, will not result in total crop
failure. The cereal crop has a lower profit margin than potatoes and therefore
spraying at set and frequent intervals would not necessarily be economic,
particularly at full doses. There are also stages in the growth of the cereal plant
where the presence of disease is not damaging to yield and therefore control is not
worthwhile. For winter wheat, control of disease in the autumn and generally up to
the first node development stage (GS 31, Zadoks
et al
., 1974) is unlikely to be
necessary or economic.
A few schemes have been developed over the years in an attempt to define
conditions where fungicide application is likely to be economic. Again, as with
potato blight, it was the development of fungicides that were not phytotoxic to the
crop which triggered an interest in disease forecasting and control.
9.7.1 Powdery mildew
Mildew of spring barley was probably the first disease targeted for fungicide control
and the Polley Period was devised to indicate the optimum timing of sprays to control
the disease (Polley and King, 1973; Polley and Smith, 1973). The barley mildew high-
risk period was designed to detect days on which there was a high risk of a large
number of spores being released in barley crops. It was intended that the criteria be
used together with observing the presence of mildew in the crop. This was assessed as
between 3 and 5% on the lower leaves before a spray was recommended.
A high-risk period occurred when:
•
day maximum temperature >15.6°C;
•
at 09.00 h the dew point deficit >5 °C;
•
day rainfall <1 mm;
•
day run of wind >246 km.
A high-risk period occurred on a day when all four criteria were satisfied, or the
second day when three of the four factors occurred or the third day when at least two
of the factors occurred but with one or two out of the three days having three factors
