PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
CONDITIONS
FAVOURING
Global proliferation of
environmental weeds
has coincided with the
huge population
explosion and led to an
increasing similarity of
plant species in regions
with similar climates.
Evolutionscientists
warn that the spread of
environment, where less
specialized animals and
plants such as
cockroaches, rats,
nettles & thistles will
flourish at the expense
of more specialized wild
organisms.
Weed Wizard warns of
potential weed problems
while they are still
avoidable. Wizard
simulates the interaction
between weather,
paddock management
and seed biology and so
tracks and predicts the
number, ages and soil
depths, dormancy levels,
viability and germination
of seeds in soil.
CONDITIONS FAVOURING WEED INFESTATIONS
The three most important factors influencing weediness of a plant are
invasiveness, impacts and potential distribution.
Poor crop planning, limited IWM (Integrated Weed Management).
Management practices, eg
– Weed floras change with changes in agricultural and horticultural practices.
Recent emphasis on crop diversification and reduced tillage has created new weed
problems, weed species which were previously controlled by tillage, including
perennial weeds and some annual grasses, are controlled by changes in chemical use
and possibly by periodic cultivation every few years.
– Reduced crop diversity. Many growers have moved away from crop rotations to
growing a single crop continuously.
– Grazing intensity and timing is often a major contributor to pasture decline and
weed invasion, which may result in more acid soil, water leakage, reduced organic
matter, reduced biodiversity above and below the surface, increased dryland salinity
and lower water quality all causing reduced productivity and profitability, droughts
exacerbate the weed problem.
– Disturbance is a precursor to invasion by some weeds, eg thin turf, over-grazing,
agriculture, fertilizers, erosion, trampling, clearing, landscaping, road making.
– Properties with uncropped areas always have greater weed problems than where
weeds are well controlled throughout. Weeds may first grow in wasteland around
buildings, paddocks and on fallow land, then produce seed or rhizomes which
spread to adjacent crops.
– Incorrect timing of weed control, eg cultivation, mowing, herbicide applications.
Temperature and moisture are critical for weed seed germination; weather
forecasting systems can help pin down when weeds will emerge.
Climate change. Simulation models predict future distribution of weeds. Weeds
may be favoured over native flora. Tropical weeds may extend their range, fewer
frosts mean some weeds will spread to new areas, alpine plants will decline, and
unpalatable grasses will grow more densely, creating a greater fire fuel load.
Bumble bees are efficient pollinators and if exotic species are introduced they may
better pollinate existing weed species.
Lack of knowledge of weed problems, eg large seed banks, which exist and
may increase, in present and future crops.
Herbicide resistance. Repeated use of many herbicides has lead to resistance
problems. so that many weeds are hard to control, eg annual ryegrass.
Empty niches, eg many grass weeds establish in sunny pockets in the bush. Sites
such as shady areas under trees, burnt out areas after fires.
Continuous cropping, resulting in volunteer crop plants regarded as weeds.
Fertilizer. Exotic weeds thrive in soils enriched by run-off from lawns, gardens,
crops, nurseries and where fertilizers are used in excess. High phosphorus content of
many fertilizers may kill or weaken native vegetation which competes with weeds.
In their natural habitat plants are often controlled by climate, predators, etc. In
regions with very cold winters many plants are killed each winter and do not have
time to become pests. When introduced to warmer climates such plants can grow
throughout the year or have only very short periods of dormancy with no natural
means of control, eg no imported pests and diseases.
Many garden plants still being purchased plants from hardware and garden
suppliers and nurseries, have the potential to become weeds, eg ornamental grasses.
ENVIRONMENT
Does it favour the crop or the weed?
CROP or SITE
Crop bed preparation right?
Crop seeding just right?
Crop vigor just right?
WEED
What is the weed in your crop?
Fig. 247. Weed triangle.
428 Weeds - Biology, classification and identification