Agriculture Reference
In-Depth Information
of the only fungicide available for control at the time - copper. Copper tended to be
phytotoxic to young developing foliage and in dry years where blight was not a
problem, spraying could reduce yield by 3% (Large, 1959a). Even spraying crops
with the later-developed fungicides can also have a deleterious affect on yield in the
absence of late blight (Taylor
et al
., 2000). Delaying the first application until
growth of the potato plant was nearly mature resulted in less damage from scorch.
The development of a forecasting scheme was therefore important in delaying
spraying until absolutely necessary to avoid too early an application and consequent
damage to the potato foliage in drier and lower risk years.
The prime aim in devising a forecasting scheme was to define a set of
conditions that, when satisfied, would lead to an outbreak of blight within the next
14 days (Large, 1959b). Under normal circumstances, this would allow for the
timely application of a protectant fungicide. It is perhaps surprising to read in
Large's (1959a) account of the major blight epidemic of 1958 that “Good
commercial spraying, in most blight years, can be expected to give on average
about two week's prolongation of useful growth of the haulm”. The increase in
yield from an attack that did not defoliate the crop until the end of August was
estimated at 15%. Today this would seem marginal and the expectation is for a
longer delay in defoliation.
It is important when considering a forecasting scheme to have an understanding
of the aetiology of the pathogen. Most of the blight forecasting schemes have been
based on the work of Crosier (1934). This showed that the fungus grows most
rapidly at 18-21°C and slowly at below 9°C and above 24°C. Also, and importantly,
in the initial phase of the disease, the growth of the fungus in an infected potato
plant, either from a planted seed tuber, cull pile or from volunteers, is only governed
by temperature. It is only when sporulation occurs that humidity, leaf wetness and
their duration affects disease progress.
There is a major history to blight forecasting and this can be traced by looking at
the increasing sophistication of systems devised over the years as technology has
improved.
9.6.2 Dutch rules (van Everdingen, 1926)
The first national blight forecasting scheme was probably that devised by van
Everdingen (1926) who analysed a number of weather parameters on the
development of blight.
Four criteria (known as the Dutch rules), when satisfied, indicated the time to
apply control measures:
•
the occurrence of dew for at least four hours at night;
•
a minimum temperature of 10°C;
•
a mean cloudiness on the next day of 0.8 or more;
•
at least 0.1 mm of rain in the following 24 h.
