Pest and
disease control
During the growing season the major fungus diseases are late blight and early
blight.
An adequate spraying with protective fungicides against late blight
also protects against early blight.
Late blight:
For chemical control quite a
number of effective fungicides is available. It concerns a.o. the groups of the
dithiocarbamates zineb, maneb and mancozeb and the copper compounds, which are
extensively used.
At present new types of fungicides are being produced
which are introduced as they are more friendly to the environment
None of these products is able to effectively control late blight for more than
six to eight weeks under severe disease pressure, which is considerably shorter
than the growing season.
Therefore it is essential to prevent early
infections by removing infection sources and by an early start of spraying.
Crops should be protected when conditions are favourable for Late blight
CHEMICALS LATE BLIGHT----------------EARLY BLIGHT
Late blight favourable conditions:
Late blight
favourable conditions are marked by one or more of the following:
- night temperatures do not drop below 7 °C.
- temperatures between 15 and 21 °C are the most favourable for
lesion development and sporulation, whereas at temperatures over 29 °C the
pathogen does not develop.
- leafwet periods (dew and rain) over 6 hours are favourable for new
infections.
- leafwet periods over 8 hours are very critical.
- in case of a severe late blight crop infection, seed tubers may
carry over a
large amount of inoculum to their following growing season. Massive late
blight
outbreaks will occur when conditions are favourable to disease
development, hence
start spraying earlier and more frequently. At times when conditions are
favourable for the development of late blight crops should be protected with
the fungicides.
Spraying schedules for late blight
control
Until recently, the most common rule for chemical late blight control was:
"Start spraying against late blight when the leaves of the plants within the row
touch and repeat spraying every 7 days when conditions for late blight are
favourable and every 10 days when conditions for late blight are not
favourable".
With the presently more aggressive populations of P. infestans
this rule of thumb has become obsolete.
In stead of 7 days the generation
time of the pathogen is now 4 days. Hence, spraying should be done when
conditions for late blight development are favourable and intervals may be as
short as 4 days.
The start of the sprayings against Late bight is
influenced by:
- infection pressure
- susceptibility of the cultivar grown
- growth of the plants
- the existing weather conditions
Infection pressure
If infection in the area is low
and when there is no risk of infections from oospores start spraying later.
Use of a, for the local conditions, adequate disease- forecasting-system is
very useful.
Susceptibility cultivar
Susceptible cultivars are
to be sprayed more frequently than cultivars with a higher level of resistance.
For a susceptible and a more resistant cultivar the interval may respectively
be 4 and 10 days.
Growth of the plants
Crops with fast growing haulm
need more frequent sprays than when growth is slow.
Leaves that developed after the last spray are unprotected. With fast growing
haulms, fungicides with a systemic component are recommended.
Weather conditions
In periods that conditions are
favourable for Late blight spraying is frequent. Efficiency of chemical control
interacts with rain. Some hours of dry weather are necessary to allow the
chemical to stick to the leaves.
INTERACTION WITH RAIN