PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
HARMFUL
EFFECTS
OF WEEDS
(weed impacts)
Lantana alone threatens
1246 plant species and
41 animal species
,
is toxic to stock,
especially
toxic to horses
Seeds are
often the
most toxic part
Control methods, eg
cultivation, burning,
herbicides may have
adverse effects on
soil, crops, and the
environment.
DIRECT EFFECTS.
Weeds cost Australia around $4 billion per year (2008) in cost of control, lost
production and contamination and rank with salinity as one of Australia’s most serious
problems environmentally. In 2006-2007 farmers spent more than $1.6 billion on weed
control alone. Weeds degrade our environment and ecosystems, threaten native flora and
fauna and reduce amenity for humans.
Weeds compete strongly with crop plants for moisture, nutrients and light, reducing
yields and/or quality to the extent that an operation may no longer be profitable.
Weeds occupy potentially useful space.
Presence of weeds can devalue land in rural areas. A history of cape
tulip or Paterson’s curse may result in additional management costs.
Appearance. Customer tolerance of weeds in containers in nurseries is low. Weeds
are offensive to look at, interrupt views and crowd out desirable species.
Biodiversity. Introduced weeds (and animals) are second only to habitat clearing as the
greatest threat to biodiversity in bush land. Weeds displace plants found naturally in a
particular area and cause habitat loss.
Waterways. Riparian weeds, eg willows, impede water flows and reduce access by
stock and humans. Aquatic weeds, eg, salvinia blocks waterways and impede recreation
activities. Herbicides in drainage water from treated areas may contaminate water ways.
Contaminate produce, eg weed seeds lower the value of cereal grain for sowing in
clean areas. It is illegal to sell contaminated grain or fodder. Weed seeds are often found
in coarse grains used for feeding pigs, some are harmful to pigs, eg potato weed
(Heliotropium europaeum), Mexican poppy (Argemore ochroleuca and A. Mexicana).
Some weeds have an offensive odour, eg some thornapples (Datura spp.). Milk and meat
of animals grazing on certain plants may be tainted. Wild garlic will flavour milk within
4-5 minutes of feeding.
Interfere with agricultural operations, eg burry seeds are problems for
shearers and pickers. Skeleton weed and wild melons become tangled in machinery.
Weeds interfere with transport and recreation. Weeds under power lines, on railways and
obscuring road signs must be suppressed. Blackberries are impenetrable to live-stock,
vehicles and bush walkers. Boneseed and bridal creeper impede beach users.
Domestic animal losses are not uncommon. Cape tulip can cause losses in
stock newly introduced to it. Annual rye grass toxicity (ARGT) affects cattle grazing on
Wimmera rye grass infected by nematode-carrying bacteria which produce a toxin.
Animals with light colored skins feeding on St John's wort or lantana become more
sensitive to sunlight which may lead to skin diseases and eventual death.
.SOME OTHER EFFECTS. Almost infinite
Human fatalities are rare. Few plants have been known to cause human death, eg
angel’s trumpet, Datura (Brugmansia spp.), arum lily (Zantedeschia aethiooica), lantana
(Lantana camara), oleander (Nerium oleander), poison hemlock (Conium maculatum),
white cedar (Melia azerdarach), yellow oleander (Thevetia peruviana).
Hay fever and dermatitis. Pollen of many grasses and weeds cause hay fever in
susceptible people, eg annual ryegrass, plantain, privet, capeweed. The majority of plants
producing pollen which trigger hay fever were introduced from the northern hemisphere.
Rashes, swellings, dermatitis, pain, localized burning or infections may occur in
susceptible people when some weeds are handled or brushed against, eg St John’s wort,
Bathurst Burr, scarlet rhus, some Grevillea spp., poison ivy.
Mechanical injury. Spiny leaves, stems and seed heads of thistles, galvanized burr,
etc, may injure feet, legs, mouthparts, ears and eyes and other parts of animals. Burry
or corksrew seeds may adhere to the wool, hair and feathers of animals and trouser
legs/socks of humans. Nettles sting animals and humans.
Harbour diseases, pests and vermin. Prickly lettuce is a host of powdery
mildew of cucurbits, brassica weeds for cabbage aphids, common sowthistle for
cineraria leafminer, white clover for western flower thrips (WFT) and tomato spotted
wilt virus (TSWV). Thickets of blackberry harbour rabbits.
A fire hazard when bulky perennial grass weeds dry off in spring/summer, eg
mission grass (Pennistemon polystachion).
Genetic pollution. Pollen disperses more widely than seed. Garden and crop
plants can be improved by genetic engineering for drought hardiness, however, this may
also increase their chances of becoming weeds.
Some weeds release chemicals into the soil that retards crop growth
(allelopathy), eg aqueous extracts of the pasture weed, lippia (Phyla canescens), can
inhibit seed germination of certain pasture and crop species.
BENEFICIAL
EFFECTS
OF WEEDS
Leguminous weeds add nitrogen to the soil, eg white clover.
Provide fodder for animals, eg weed grasses, Salvation Jane, during hard times.
Provide pollen and nectar for bees, eg Salvation Jane.
Source of food for beneficial insects, encourage a diversity of beneficial insects.
Prevent or reduce soil erosion and rain compaction where there is no other vegetation,
eg bitou bush.
May be a source of food for humans, eg chicory.
Some weeds are reputed to produce beneficial exudates.
May improve drainage, soil structures, add organic matter. Deep rooted species may
retrieve scarce nutrients from the subsoil.
Certain weeds act as indicators of nutrient imbalances or soil problems, eg sorrel
indicates acidity.
Weeds - Biology, classification and identification 411