PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
1
Table 76. Near desired plants. (contd)
What to use?
When and how to apply?
RESIDUAL WEED CONTROL---------- 3-6 MONTHS
Shrub beds, around trees, roses, along domestic fencelines. Care should be taken when spraying around
newly planted trees and shrubs.
Annual beds, containers (pages 463, 464).
PRE-EMERGENT RESIDUAL HERBICIDES
Existing emerged weeds should be controlled either
by hand weeding, spot spraying or combining a preemergent
with a post-emergent knockdown herbicide,
eg Glypho , Roundup (glyphosate) (pages 453, 457).
Residual activity is the length of time a herbicide
provides control. Most pre-emergent herbicides are
effective for 3-4 months (some up to 6 months),
depending on the uniformity of coverage, weed
pressure, reintroduction of seed to the area and amount
of hand weeding carried out.
Pre-emergent herbicides kill germinating seeds
of both target weeds and non-target plants (herbicide
must contact germinating seeds; seedlings may emerge
before they die).
Registered uses. The pre-emergent must be
registered for the particular weed in the crop to which
it is to be applied otherwise apply for a permit.
Weed type. Most pre-emergents are more effective
against the germinating seeds of some weed species
than others, eg broadleaved weeds or grass weeds.
Spectrum of activity. Formulations combining
herbicides, eg Rout (oryzalin + oxyfluorfen), are
useful as they can increase the weed control spectrum
and the residual activity of a herbicide application,
thereby reducing the number of applications required.
Timing of application. Apply pre-emergents before
time of maximum weed seed germination or very soon
after, eg Surflan (oryzalin), simazine (low rates).
Apply at correct rate and at the correct frequency.
Even application, incorporation and activation
are essential:
– Evenness of application. Uniform coverage of soil
surfaces outdoors and in containers, ensures that the
effectiveness of pre-emergent herbicides is maximized
and cost savings achieved.
– Incorporation into the soil may be required to prevent
loss of activity through volatility or prevent later crop
damage. Many pre-emergents need rainfall or irrigation
for incorporation but too much causes them to
decompose too rapidly, with too little rain the herbicide
stays on the surface and volatilizes, degraded by
sunlight. Poor soil moisture conditions probably
cause more failures with soil-applied herbicides than
anything else. A few pre-emergents are incorporated by
mechanical means (page 447). Later cultivation may
enhance or decrease herbicide activity.
– Activation. Pre-emergents, need to be activated by
rain or irrigation. Pre-emergent herbicides act when
weed seeds germinate as a result of moisture.
– Soil disturbance can dramatically reduce the
effectiveness of pre-emergents.
COMBINING PRE-EMERGENTS AND POST-EMERGENTS
Always check mixtures are compatible
Post-emergent (knockdown) eg
Group M, eg Roundup (glyphosate)
Pre-emergent (kills germinating weed seeds), eg
Group D, eg Surflan (oryzalin)
OR
Group C, eg Gesatop (simazine) (low rate only)
Post-emergent (knockdown), eg
Group L, eg Spray.Seed , Tryquat (diquat + paraquat)
DANGEROUS POISON
Pre-emergent (kills germinating weed seeds), eg
Group G, eg Goal (oxyfluorfen)
For effective application of pre-emergents:
Weather. Check weather, rainfall, temperature, wind,
volatilization, photo-degradation.
Seedbeds should be clump-free, ie free of weeds,
trash and clods at time of application.
Formulations. Pre-emergent herbicides may be
applied as a directed spray or as granules and either
before or after planting the crop.
Plant injury (phytotoxicity). All pre-emergents may
damage some species of plants. Some pre-emergent
herbicides at higher concentrations damage established
plants, eg simazine at low concentrations is a preemergent
while at high concentrations it can kill
established plants. Others, eg Surflan (oryzalin) have a
much wider margin of plant safety. Pre-emergents may
be sprayed over the top, or as a directed spray before or
after planting, so there is scope for injury.
The crop or the emerging crop must be tolerant.
Know which species may be injured, check label.
– Injury can occur with woody plants but is more likely
with soft-foliaged annuals and herbaceous perennials.
– The use of pre-emergent herbicides on annual and
herbaceous perennial flower crops has been
slower to develop and are frequently not quite as
effective as those used around woody trees and shrubs,
fruit trees, vines and in turf areas. This is mainly
because annual and herbaceous perennial crops are
more susceptible to injury due to:
Roots being more superficial in the soil.
– Foliage is softer and closer to the ground and more
easily damaged by herbicides.
– Application in polytunnels where the
atmosphere is enclosed and temperature is higher,
vapour may injure non-tolerant species.
– Being grown in closed recycling systems.
More applications than label recommendations.
Season when applied affects effectiveness.
Many pre-emergents are more effective against the
same weed species during summer than during winter.
Cost of herbicide. Some pre-emergent herbicides, eg
Ronstar (oxadiazon) are much more expensive than
others, eg simazine.
Labels supply compatibility information.
Systemic, foliage absorbed, non-selective herbicide
translocated down into roots, kills emerged weeds.
Soil residuals
Some broadleaved and grass weeds.
Some broadleaved and grass weeds.
Non-systemic, contact, non-selective foliage
herbicide, kills foliage on contact, does not kill roots.
Soil residual
Grasses and some broadleaved weeds
458 Weeds - Examples of weed situations