PLANT PROTECTION 1 – Pests, Diseases and Weeds

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PLANT PROTECTION 1 – Pests, Diseases and Weeds
Conditions favoring
Continuous cropping of susceptible crops.
Fire. Jarrah dieback (Pc) in WA appears to be related
to a change from hot uncontrolled bush fires to less
hot controlled burning programs. This has led to a
change from an Acacia understory to one of Banksias
which is very susceptible to Pc relative to Acacia,
providing much inoculum that can infect jarrah.
Each soilborne disease is favoured by different
conditions.
Stress. Rate of disease development increases as
stress increases, eg avocado plants stressed by root
pruning develop cankers more readily than nonstressed
plants.
Water management.
– Prolonged periods of rain or excessive irrigation
over a long period of time.
– Poorly drained, waterlogged soils and drought will
stress plants. Tensiometers measure soil moisture and
improve irrigation management by accurately
determining when water should be applied to a crop to
maintain optimum growth and how much water should
be applied to avoid over-irrigating.
– Warm, wet winters followed by dry summers stress
plants. In WA areas with rainfall above 400mm are
most affected. Symptoms appear more rapidly when
plants are stressed by periodic drought, fluctuating
water tables and higher temperatures associated with
wet conditions.
– Warm moist aerated soils at >12 o C (optimum 25-
27 o C) with temporary flooding.
– Wet soil conditions and slow infiltration favour
many root pathogens, ie temporary flooding and
prolonged period of saturation that can occur following
heavy rainfall or overhead irrigation in soils with
structural decline.
– Planting azaleas with ‘balled’ roots in soil
different to the one into which it has been growing
favours Phytophthora root rot. Where 2 soil types
meet there is a natural water course, new roots
growing into it are readily infected with Pc.
Soils low in organic matter and micro-organisms.
Poor soil structure, chemical and physical
properties.
Nutrients deficiencies & toxicities. Highly
soluble salts can kill rootlets providing sites for
infection with Pc. The level of calcium carbonate
in soil can increase the level of Rhizoctonia disease.
Phosphorous and zinc deficiencies can be an issue in
some soils. Salinity will exacerbate Phytophthora.
Other infections, eg root-knot nematode damage
to roots allows the entry of Phytophthora spp., and
other fungal diseases of rice flower. These diseases,
along with other common problems such as stem or
root damage due to wind, root congestion and
longicorn borer damage, contribute to the early decline
and death of rice flower plantings.
Herbicides may have some affect but the situation is
unclear. Glyphosate is a broad spectrum inhibitor and
potent inhibitor of EPSPS, a key enzyme in the
synthesis of amino acids present in plants, fungi and
bacteria. So fungi and bacteria with glyphosatesensitive
EPSPS may be susceptible to the action of
glyphosate. Laboratory and field reports indicate that
glyphosate can cause temporary increases in Pythium
and other damping-off fungi in the soil but decreases
in Sclerotium and Fusarium.
Fig. 206. Disease cycle of Phytophthora root rot (Phytophthora cinnamomi).
Fungal diseases - Examples of fungal diseases 367