PLANT PROTECTION 4

PLANT PROTECTION 4 – How to Diagnose Plant Problems
WEEDS
Cardamine
Liverworts
Weeds can be
affected by pests and
diseases in the same
ways as crop plants
WEEDS
1. Weeds may be defined in many ways but
the most apt definition is ‘a plant growing
where it is not desirable or wanted’.
2. Most plants including those usually
considered beneficial, eg herbs, may be
weeds at times. Many plants previously
used as ornamentals or crops are today's
weeds.
3. Weeds generally reproduce, spread and
photosynthesize efficiently, surviving
under unfavourable conditions.
4. Weeds are a major economic and
environmental problem in Australia.
12 Background briefing – Causes of plant problems
Red-flowered mallow
Signs
The presence of the weed itself, weed seedlings, seeds, roots, bulbs, rhizomes and
stolons. Some weeds can be difficult to identify.
Prickly pear (perennial
environmental weed)
Winter grass
(annual weed)
Some weed seeds persist
in soil (seed banks)
Rhizomes, as well as roots, tubers and
corms persist in soil (bud banks)
Indirect effects of weeds
Some are toxic to stock, eg Paterson’s curse is especially toxic to horses and pigs.
As pest and disease sinks, ie alternate hosts which harbour pests and diseases that can
spread into a crop, eg cineraria leafminer (see below).
Weeds increase humidity in a crop, which encourages pests and diseases.
They compete with the crop for nutrients and water so that the crop appears unhealthy.
Weed seeds may contaminate crop seed and grain.
Interfere with machinery and cropping.
Devalue land due to the presence of difficult-to-control weeds.
Cause mechanical injury to animals and humans, eg thorny seed.
Affect appearance of landscapes and containers for sale.
Reduce biodiversity, taking up valuable cropping areas or bush land.
Harbour vermin and can be a fire hazard.
Indicate soil conditions which can be changed to discourage weed growth, eg yellow
nutsedge (Cyperus esculentus) indicates excessive water perhaps due to broken
irrigation pipes.
Cineraria leafminer damage –
sowthistle
Rust pustules on leaf
undersurface – mallow
Fasciation – Paterson’s curse;
arguably a genetic abnormality