Agriculture Reference
In-Depth Information
Spray materials should be applied during environmental conditions that maximise
interception of spray droplets by the target plant. Whereas the potential for phyto-
toxicity by some chemicals is increased when there is slow drying of spray droplets,
it is not known whether these same conditions would enhance or reduce the efficacy
of compost tea. Indeed, environmental conditions during and immediately after ap-
plication of compost tea, including the temperature, relative humidity, wind speed
and UV index, could be investigated in relation to spatio-temporal changes in mi-
crobial populations on plant surfaces.
Once the first spray has been applied, the interval between spray applications
will depend on crop phenology and disease susceptibility, plus environmental con-
ditions affecting disease development. If the presence of live microorganisms in
compost tea contributes to its efficacy, then environmental conditions are also likely
to affect their survival on plant surfaces. In a field trial of ACT applied to grape-
vines, there was an increase in the number of bacteria on Riesling leaves between
5 and 13 days post application, which might have been promoted by rainfall 6 days
following ACT application (Evans et al.
2013
).
Plant growth also influences the spray interval. New leaf tissue emerging since
the previous spray application may be unprotected from infection by those patho-
gens that colonise green, juvenile tissue (e.g., mildews). If a compost tea inhibits
a plant pathogen only in the location in which it is deposited, like a 'contact' fun-
gicide, then the interval between spray applications needs to be relatively short
during periods of rapid shoot growth. Similarly, there may be key phenological
stages when a crop is highly susceptible to infection by a plant pathogen. Again, the
interval of any 'contact' spray material may need to be as short as possible during
these periods.
The integration of biological control with other disease management practices
is especially important when biocontrol is effective only when disease severity in
the absence of crop protection is low to moderate. Farmers who understand crop
cultural factors conducive to disease development can sometimes manipulate these
to reduce disease development; for example, orchardists and vineyard managers
can adopt measures to increase air circulation around fruit, which in turn reduces
the duration of surface wetness and lowers relative humidity (Thomas et al.
1988
).
These measures might include selection of appropriate sites and row orientation at
planting, and pruning, feeding and watering plants for a good balance between crop
vegetative growth and fruit production (crop load). The extent of shading within a
crop canopy can also be manipulated to increase exposure of foliage and fruit to UV
light, which is known to reduce the severity of powdery mildew by reducing the
colonisation of epiphytic fungal hyphae (Austin et al.
2011
; Willocquet et al.
1996
).
If there is a commitment to the adoption of organic practices, then farmers need
to plan their disease management strategy in growing seasons that are highly condu-
cive to disease development. Disease forecasting systems and long-range weather
forecasts can provide alerts to looming risks. If significant crop loss or failure can
be tolerated, then farming practices can proceed as before. Otherwise, every known
cultural and biological control practice needs to be implemented when disease risk
is very high, assuming multiple measures will be additive or synergistic in their
effects.
