PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
SPREAD
Air has many fungal
spores floating in it
CONDITIONS
FAVOURING
H 2 O
Weather
monitoring
WIND
Fungal spores are produced at or near the surface of the host ensuring prompt spread
by wind and air currents. Spores of wheat rust can be carried 500 km. Some even
further, poplar rust is thought to have spread by wind to NZ.
WATER
Rain and irrigation water splash spores from leaf to leaf and from plant to plant, eg
black spot of rose. Drainage water washes spores and other fungal bodies of soilborne
fungi downhill, eg Sclerotium stem rot, Phytophthora. Phytophthora zoospores have
flagella and can ‘swim’ a few mm or cm.
SOIL, POTTING MIXES, DUST
Soilborne fungi may be transported in dust, soil eroded by water, mud on implements,
vehicles, footwear, soil in deliveries and containers, eg Fusarium.
SEED
If a fungal disease is seed-borne, then any agency that spreads seeds of infected plants,
eg humans, wind, water, will also spread the fungal disease.
INFECTED PLANTS, NURSERY STOCK
Infected susceptible plants, plant parts, nursery stock, eg peach leaf curl, shothole of
stone fruits, Phytophthora root rot.
OTHER METHODS
Insects are not a common method of spread. Driedfruit beetles and caterpillars of
the oriental fruit moth spread brown rot in stone fruits. Fungus gnat larvae spread
Pythium. Overseas, Dutch elm disease is spread by the European elm bark beetle.
Birds and other animals are not an important method of spread.
Pruning wounds, eg Eutypa dieback of apricots and grapevines.
Hyphal growth, eg during postharvest storage of fruit.
Infected germplasm. In SE Asia, leaf blight and stem cankers (Kirramyces
spp.) of eucalypts may have spread around the region on infected germplasm.
EACH DISEASE IS DIFFERENT
Each fungal disease has its own optimum environmental needs for spore
formation and germination, host plant infection, disease development, eg
– Moisture. Downy mildew spores germinate in a thin film of rain or dew on the
plant surface, powdery mildew spores on a dry surface in humid conditions.
– Temperature. Most fungal spores germinate at 15-30 o C. Free mycelium survive
from -5 o C to 45 o C when in contact with moist surfaces, inside or outside the host.
Most spores can survive broader ranges of temperature.
– Weather monitoring. Knowledge of temperature, rain, humidity, etc necessary for
spore germination, host plant infection and disease development, means that
epidemics can be forecast with fewer but more effective pesticide applications.
– Others, eg poor light; deficiencies or toxicities can increase disease risk.
Stage of crop development affects seriousness of disease outbreaks, eg leaf
disease at the end of harvest of a tomato crop is not serious – unless the disease can
spread to new plantings.
Lack of crop rotation. A mature susceptible crop may withstand a disease but
a following planting of the same susceptible crop in the same ground will certainly
develop a damaging infection while still young.
Injury to produce during harvesting favours infection by disease organisms
causing postharvest rots.
Vegetatively propagated plants have greater uniformity. The severity of a
disease outbreak is greatest when the uniformity of the host is increased.
Lush new growth favours certain diseases, eg powdery mildews.
ENVIRONMENT
Does it favour the host or leaf spot fungus?
SUSCEPTIBLE
LEAF SPOT SPORES
HOST PLANT PRESENT
PRESENT
Fig. 180. Disease triangle.
326 Fungal diseases - Biology, identification and classification