Agriculture Reference
In-Depth Information
satisfied. The high-risk period started with a high risk day and stopped when none,
or only one of the factors had been satisfied. This scheme is not in current use, as it
was recognised that the presence of mildew in the crop at the 3-5% level was itself
an indication that conditions were generally suitable for the start of the epidemic.
This advice then changed to when mildew was 'present' in the crop, as being able to
detect mildew on plants at the tillering stage, without having search for it, or for it to
be very obvious, equated to about 3-5%. This is a classic example of a complicated
system being refined to simple practical advice.
9.7.2 Barley brown rust
A simple forecasting scheme for barley brown rust (caused by
Puccinia hordei
) was
proposed by King and Polley (1976). Simkin and Wheeler (1974) reported that
urediospores did not germinate at a relative humidity below 100%, germination was
complete over a range of temperatures in 6 h and subsequent development was
solely dependent on temperature. King and Polley (1976), suggested that a dew
period of at least 5 h following a day when the maximum temperature was more
than 15°C would indicate that susceptible crops were at risk.
9.7.3 Septoria
Septoria diseases of wheat (causal agents
Phaeosphaeria nodorum
and
Mycosphaerella graminicola
) are wet-weather diseases and ADAS developed a
forecasting scheme based on a 'wet period' (Anon., 1984a) to assist in the timing of
fungicide applications. A wet period was defined simply as 1 mm or more of rain
occurring on any 4 days in the previous 14 days. However, under UK conditions, it
was rare that these criteria were not satisfied, and there were risks of spraying crops
prematurely or too frequently. Septoria spores are distributed largely in rain splash
(Shaw, 1987), and Thomas
et al
. (1989) revised the scheme to take account of
'splashy' rain events so that: 'a total of 10 mm or more in up to 3 consecutive rain
days, once the canopy has reach full height, although over 5 mm on any one day
may be sufficient in shorter crops where stem elongation is incomplete'. Provision
was made for a second spray if these conditions occurred after flag leaf emergence,
or the protectant activity of the first spray had been exceeded.
Schöfl
et al
. (1994) proposed a scheme not only based on rainfall but also
incorporating inoculum in the risk assessment:
Decision period: GS 33/37 to GS 55-71 (depending on cultivar)
Threshold value: 40-50% incidence per indication leaf layer infected (presence of
disease)
Growth stage
Indication leaf layer
GS 33/37-43
leaf four
leaf three
GS 45-65 (71)
