Agriculture Reference
In-Depth Information
First application of fungicide: if the threshold is reached, spray timing depends on the
occurrence of rain events in the past 14 days. If a rain event is recorded, a fungicide
has to be applied within 10-17 days (dependent on cultivar). If no rain event occurs,
treatment should follow the next infection event or when GS 51 is reached.
Rain event:
- more than 10 mm within 2 or 3 consecutive days
- more than 5 mm in one day followed by 2 days leaf wetness.
Cultivar effects:
Susceptibility
max. interval
latest application trigger
high
10
GS 71
moderate
14
GS 65
low
17
GS 59
Second application: necessary, if 10-20 days (dependent on protectant fungicide
activity) after the first spray, there has been more that 10 mm rain within 2 or 3
consecutive days. If the first spray was at GS 47/51 or later, no second applications
required.
The advantage of this scheme is that it does take account of inoculum, although
in the UK inoculum is rarely limiting. Probably more importantly, the scheme
attempts to take account of cultivar resistance, even if it is on a rather crude basis.
9.7.4 Eyespot
Eyespot forecasting was based on an assessment of the presence of the disease in the
spring from the leaf sheath erect stage (GS 30) to first node (GS 31). Crops should be
treated if the crop has obvious symptoms affecting more than 20% tillers and lesions
penetrating at least two leaf sheaths (Anon., 1986). This could only be an insurance
measure, because if subsequent weather turned out to be dry, the affected leaf sheaths
would desiccate, become detached from the stem and remove the contact necessary to
enable the disease to progress to the stem where it could cause damage.
Early drilling increases the risk from eyespot and a system of accumulated day-
degrees was investigated by Croxall (1978). This was based on accumulation day-
degrees above 6°C with a starting point on the first day after crop emergence with
measurable rain and a mean temperature above 0°C. Croxall determined that visible
eyespot lesions should be detectable at 100-125 day-degrees above 6°C and the
optimum spray date between 200 and 250 days. However, he concluded, after four
years of research, that it was impossible to predict when spraying against eyespot
was certain to be beneficial. The most reliable advice, he thought, was a negative
prognosis by eliminating crops that were unlikely to benefit from eyespot control.
He listed a three-year break from cereals or following two years of grass, crops
drilled too late to accumulate 250 day degrees above 6°C. Even after all this
equivocation, he suggested taking into account cultivar susceptibility, crops with
