STRI Tech Notes Issue 12 - Fundamentals of fungicide selection
Understanding how fungicides work is paramount to getting the
best value for your money from any application. The different modes
of action of fungicides, or the way that they tackle disease within
the plant, allow you to decide which product to use depending on
the disease activity at the time of application, reports Dr Ruth
Mann of STRI.
The action of the fungicide and the disease target is also
important in deciding the optimum application technique to deliver
the treatment where it is required.
Mode of action
The 'topical' mode of action concerns where on the grass plant
the fungicide will remain after application:
CONTACTS, such as chlorothalonil
(DACONIL WEATHER STIK) and fludioxonil (in INSTRATA), remain on the
outside of the grass plant at the point of application, forming a
shield to protect the leaf from pathogen attack. They will also
help prevent the spread of disease, by preventing the pathogen
moving to unaffected plants.
LOCALISED PENETRANTS, such as
iprodione can move slightly from the point of application. For
example, they may slightly penetrate into the grass leaf or move a
short distance around the leaf surface.
SYSTEMICS, such as azoxystrobin
(HERITAGE) and propiconazole (BANNER MAXX) can move from the point
of application in the xylem (the part of the grass plant that water
moves in). These are termed acropetal penetrants and, once absorbed
by the leaf, move up through the plant and are redistributed in the
leaf. Fungicides applied on the leaf continue to move over time,
protecting new leaf as it grows.
It is worth noting that the reason azoxystrobin is effective
against take-all is that it also absorbed by the roots and so can
prevent growth of the pathogen where it is found in the plant.
The topical mode of action is important for which product will
give you the best control depending on the immediate activity of
the pathogen. For example, in a high risk situation, where the
disease is present and possibly spreading, a fast acting contact
fungicide may be most appropriate to immediately protect
surrounding clean leaf.
For this reason an acropetal penetrant is likely to give its
best results when applied during high-risk situations, but before
the disease is observed and spreading. This gives time for the
active to be taken up and redistributed within the leaf for much
more effective control - preventing the pathogen from infecting the
plants.
Biochemical action
The 'biochemical' mode of action concerns how the fungicide
prevents or kills the pathogen:
MULTISITE fungicides, such as
chlorothalonil and fludioxynil, affect more than one biochemical
process within the pathogen.
SINGLE SITE fungicides, such as
propiconazole and azoxystrobin affect one biochemical process
within the pathogen.
This is important to understand in designing fungicide
programmes to prevent resistance developing - which could result in
a reduction in the optimal control provided by the available
fungicides.
It is very difficult for the pathogen to overcome two or more
biochemical processes at once, and so the risk of resistance
developing is low for multisite fungicides. Remember that using
products with multiple active ingredients from different chemical
families acts in a similar way to using a multisite active
ingredient, as the product supplies more than one biochemical mode
of action.
However, alternating two different products with the same type
of activity will not have the desired effect to fight fungicide
resistance. For example, azoxystrobin is a strobilurin from the QoI
class of fungicides, as are the actives pyraclostrobin and
trifloxystrobin. Since they all affect the same biochemical pathway
in fungi - fungal respiration at the Qo site - resistance to one of
these actives will confer resistance to them all. Therefore,
care must be taken to ensure these active ingredients are used
within a planned resistance management programme that alternates
products from different chemical groups, to prevent resistance
occurring.
Application timing
The third thing you need to consider is application timing.
PREVENTATIVE application is when
weather conditions conducive to disease development have occurred
and there is a high chance that spores will be active on the leaf,
but before the pathogen has actually infected the grass plant.
Preventative control stops the pathogen before there is any loss in
turf quality. It also minimises the risk of further disease
spread.
The challenge is to know when to apply preventatively, without
wastefully applying when there was little or no disease risk -
which is where the combination of good local knowledge and disease
forecasting can prove so effective.
CURATIVE application occurs when the
pathogen has germinated or just began to grow into the grass plant.
Curative activity ends at the very first sign of disease.
It should be remembered that in most cases this is the
final timing for optimal control with most available turf
fungicides.
Most curative fungicides also have preventative activity, so
uninfected leaves treated at the same will also be kept clean.
ERADICANT application occurs when the
disease is visible and probably spreading. In many instances this
is mistakenly referred to as 'curative', but in effect the damage
to turf quality has already been done. Most fungicides have only
limited effect in curing disease pathogen at this stage, although
they can stop its spread by protecting surrounding clean leaf from
new infection. Although the fungicide may adequately control the
pathogen, you need soil temperatures high enough to allow recovery
from the infection. This is why some Fusarium (microdochium patch)
scars remain all winter, even though the disease has been
controlled.
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