PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
CONTROL
METHODS
(contd)
WEED-TESTED PLANTING MATERIAL.
Do not plant crop seed, use hay contaminated with broomrape, dodder or
witchweed seed, or strip seed or cut hay from contaminated crops.
Certified seed. For some crops certified seed is available and crops grown for
seed or hay, eg lucerne, must be inspected for signs of infestation before harvest.
Dodder (Cuscuta). Some states and the federal government have Seeds Acts and
regulations against the importation and/or sale of infested seed and weed seed limits
(max. no. seeds per kg) are in place for castor oil plant seed (nil dodder seeds).
Seed treatments with herbicides of differing toxicity are being researched.
PHYSICAL AND MECHANICAL METHODS.
Pre-plant solarization controls broomrape, also nematodes, weeds and soil
fungi, but its high cost precludes its use in most situations.
Burning (fire) is used to destroy small isolated patches of dodder in lucerne crops
to reduce the amount of seed which is set and seed already shed. Mistletoe could be
scalded rather than burnt with flame throwers.
Rifle shooting branches with mistletoe that are beyond the reach of other
methods may be useful in certain circumstances.
Heat treatments have been used to devitalize niger seed (Guizotia abyssinica)
contaminated with dodder imported into the USA for feeding of wild birds.
HERBICIDES.
Obtain recommendations for registered herbicides for specific parasitic
weeds from your local Department of Primary Industries. Table 69 below indicates a
few of the problems associated with the use of herbicides to manage parasitic plants.
Broomrape has developed resistance to some herbicides.
Germination stimulants promote suicidal germination of seeds, ie in the
absence of hosts the germinating seeds die, reducing the seed bank. Overseas a
synthetic germinating agent for broomrapes is being researched.
Pre-emergence herbicides (for preventing attachment) can be used to
control dodder seedlings. Fumigants may also be used to kill seed in soil.
Post-emergence herbicides (treatment after attachment).
– May be applied selectively, non-selectively and as a directed spray.
– Anti-transpirants. Most parasitic flowering plants have high transpiration rates
associated with the stomates that remain open under most, if not all conditions.
This cools the plant under hot conditions. Anti-transpirants which mechanically
impede water loss cause leaf temperatures to rise and rapidly kill emerged
witchweed plants during hot dry conditions.
Southern New England Landcare (2008) has published comparisons of the
use of various herbicides and other treatment for mistletoe control (avail. online).
Table 69. Parasitic flowering plants Permits are often required
What to use?
SOIL
Pre-emergent herbicides (seeds)
Group D, eg Dacthal (chlorthal-dimethyl)
Fumigation
POST-EMERGENT HERBICIDES
Non-selective directed
Selective
Group B, eg metsulfuron-methyl is registered for control of
brush and broad leaved weeds including
golden dodder (Cuscuta australis) as a spot
spray in native pasture, rights of way,
commercial, industrial areas.
Group I, eg various 2,4-D sprays in experimental work
in spring or summer have killed more than 50%
of the mistletoes with little injury to the hosts.
Tree injection, frill and daub, painting
When and how to apply?
Dacthal is registered for dodder control in various crops.
Not really an option
Non-selective sprays kill the parasite and the crop.
Glyphosate has been used as a directed spray to control
broomrape in some crops.
No truly selective herbicides are available to control
parasitic plants in broadleaved crops. Some are available
for certain types of pasture and as spot spray.
2,4-D herbicides are not registered for mistletoe control
and broad scale spraying of these herbicides is not
permitted today.
Tree injection for mistletoe control has had varied
success. Mistletoe has been painted with glyphosate when
it emerges from ‘roots’ within stems but this is generally
impractical.
Parasitic flowering plants 385