PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
‘Overwintering’
The disease cycle varies with the fungus (see
page for the diseases cycle of Phytophthora.
These pathogens are common soil and potting
mix inhabitants. They grow on undecomposed
organic matter and survive in soil for years.
In plant debris or soil, sometimes as resistant
spores or as sclerotia (black resistant fungal
bodies), depending on the fungus. Mycelium can
grow on plant debris in the soil as a saprophyte.
Seeds of some plants with Rhizoctonia.
Spread
Spores of some damping of fungi, eg Pc and
Pythium, are spread by water, eg rain, irrigation,
drainage and recycled untreated drainage water.
Movement of infested soil on machinery,
containers and tools; plant debris.
Movement of infected plants, cuttings.
Spores of some damping-off fungi, eg Botrytis
cinerea are spread by wind and air currents.
Botrytis spores are airborne, also spread by dust.
Seedborne on some hosts, eg Rhizoctonia.
Staff may carry spores on shoes, clothes, hands.
Pythium spores can be spread throughout the
greenhouse by fungus gnats and shore flies.
Pythium spores can be present in the growing
medium of plugs or prefinished plants arriving
from another greenhouse, or in soil clinging to
benches and used containers.
Conditions favoring
Damping off may be endemic in a nursery
without causing damage until conditions favour
it, eg high soil moisture, dense seedlings, etc.
Each species of fungus is favoured certain
temperatures, moisture, light, etc.
Seedlings and cuttings are most susceptible
during establishment.
Conditions unfavourable for growth of the seeds
or seedlings, and root development, eg
temperatures which are too low or too high.
Wet soils with poor drainage favour Pythium
and Phytophthora while dry soils favour
Rhizoctonia and Fusarium. Drainage water
running beneath pots. Under extremely moist
conditions Rhizoctonia can grow on above
ground parts webbing the seedlings together.
Overcrowding, seedbeds sown too thickly:
Acid soils with a pH of 5.2 or below.
Soils low in organic matter (such soils have low
populations of micro-organisms which might be
antagonistic to damping-off organisms).
Excessive amounts of nitrate fertilizers during
establishment favours Pythium.
Lack of crop rotation which can result in a
build-up in damping-off organisms in soil.
By undecomposed organic matter.
Pythium prefers young newly established plants.
Older established plants may become susceptible
when incorrect fertilization causes excessive salt
buildup in the root zone.
Management (IDM)
The Nursery Industry Accreditation Scheme Australia
(NIASA) is a national scheme for production nurseries
and grower media supplier businesses. NIASA Best
Management Practice Guidelines can be
purchased. The guidelines are regularly reviewed,
ensuring they cover relevant and current production
and environmental issues. Other publications on
managing with water, pesticide applications, the
environment and biosecurity (quarantine), are also
available from:
NGIA www.ngia.com.au/
1. Access/prepare a plan that fits your situation
including the management history of plants purchased.
Plan to implement preventative cultural and sanitation
measures to minimize the possibility of disease.
2. Crop, region. Recognize variations. Know which
damping-off diseases your crop is susceptible to.
3. Identification of the precise cause of damping-off is
difficult and it may be necessary to consult a diagnostic
service (page xiv). Identification of the fungus must be
carried out in a diagnostic lab by a pathologist.
4. Monitor. Remember know when, where, what
and how to monitor. If damping-off is a major
problem look for symptoms in seedlings. Test water
and media as well, record findings. A general
monitoring survey should be carried out on a regular
basis in small nurseries. In a large nursery about 10%
of the newly sewn nursery beds in the nursery could
be surveyed about 1 week after sewing just as
seedlings are emerging, using a visual assessment, eg
Scores
Nil
Low - up to 25% seedlings affected
Medium - 25-50% affected
Severe - More than 50% of seedlings affected
5. Threshold. How much damage can you accept? Do
you need to calculate your own threshold?
6. Action/control. Preventative measures should be in
place in all nurseries. If seedlings have not been
treated within the last month consider treating them or
transplanting them into larger containers, treat
afterwards. Take appropriate action when any
threshold is reached. Manage fungus gnats and shore
flies (page 75).
7. Evaluation. Review your program to see how well
it worked. Recommend improvements if required. If
necessary seek further advice.
Control methods
Pythium occurs in virtually all cultivated soils, so
eradicate is not really possible. Control fungus
gnats and shore flies in greenhouses.
Cultural methods.
Do not sow seedbeds or plant cutting beds too
thickly as this can encourage spread of disease.
Maintain optimum conditions for plant
growth, eg do not plant seeds or seedlings when
temperatures are too low for optimum growth.
Avoid overwatering. Water in the morning but
not late afternoon. Creating a humid atmosphere is a
fundamental technique of propagation. Provide
adequate moisture in the media (but not too much) to
prevent tops from drying out.
Provide good drainage and good air
circulation. Improve irrigation management and
surface drainage to reduce excess water lying in
bays. Use free draining mixes. Subirrigation may
result in spread of motile spores from infected to
healthy plants.
Avoid overfertilizing, especially with
nitrogenous fertilizers to avoid lush growth.
372 Fungal diseases - Examples of fungal diseases