By Emma Leonard
In New Zealand the dense crops and moist growing conditions are an ideal
inducement of fungal disease, especially stripe rust in wheat and barley
scald. Although rotation and variety choice are control options, fungicides
are seen as the backbone for disease control.
Nick Pyke, of the Foundation of Arable Research (FAR), believes New Zealand
farmers have swung from using too little, to (in many cases) too much
fungicide, but better information is helping growers to make improved
assessments of fungicide use.
How can Australian growers gain from this knowledge?
In Australia, only the systemic fungicides are currently registered for
use in grain crops. Systemic fungicides are also referred to as eradicant
or curative fungicides. For example, triazoles are able to control disease
up to a certain number of days after infection. This is referred to as
“kickback” since the fungicide is protecting foliage that
was infected before the application was made.
Mr Pyke explains: “The advantage of the systemic fungicides is
that they are taken into the plant and translocated to varying degrees.
“This means that most of these products have an ability to eradicate
or cure infection that is developing within the leaf, but which may not
be visible on the surface of the leaf.”
The period of disease development before symptoms are seen is called
the latent or hidden period. Diseases vary in their length of latent period,
all of which are temperature dependent to some extent.
For example, under optimum conditions, Septoria tritici has a latent
period of 400 degree days. Degree days (°Cd) is the sum of ‘mean’
daily temperature in degrees centigrade above a base temperature of 10°C.
Septoria nodorum has a latent period of only 250°Cd.
After a known infection period with Septoria tritici, it will be about
21 days before the characteristic symptoms of the disease become visible.
Different triazoles have different periods of kickback: 10 to 14 days
is the maximum for most triazoles. Mr Pyke says that the warmer the weather,
the shorter the life cycle and smaller the kickback.
Most systemic compounds move in the xylem, the tissue through which most
of the water and minerals are transported from the roots. Thus, if fungicides
are applied to only part of the leaf, most fungicidal protection will
be between that point and the tip of the leaf.
Former chair of FAR, Stuart Wright, suggests that based on New Zealand
costs and returns, crop yields need to be about eight tonnes/ha to justify
the use of a full fungicide program.