PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Conditions favoring
Bacterial leaf spots are more common in warm,
wet or humid climates, rather than dry, hot or
cold climates. However, some bacterial diseases
are common in wet cool winter, eg bacterial leaf
spot of lettuce.
Bacteria require free water for spread and
infection.
Plants can be infected through wounds.
Generally lush growth favours leaf spots but on
some hosts high fertilizer rates may reduce them.
Management (IDM)
Are you a commercial grower or home gardener?
1.Obtain/prepare plan that fits your situation.
2.Crop, region. Recognize variations in management
plans depending on the region.
3.Identification. Accurate and early detection is
essential for control and prevention of spread. Check
leaves and suspect spots carefully for bacterial ooze to
avoid confusion with other causes (page 310). If
unsure consult a diagnostic service (page xiv).
4.Monitor. Find out if a monitoring system is
available for your particular disease and crop.
Remember know when, where, what and
how to monitor.
5. Threshold. How much damage can you accept? Do
you need to calculate your own threshold, eg damage
threshold for your particular crop and region or are
there prescribed threshold?
6.Action. Take appropriate action when required,
including reduced irrigation, sanitation, etc.
7.Evaluation. Review IDM program to see how well
it worked. Recommend improvements if required.
Control methods
Disease outbreaks can be extremely destructive
and difficult to control.
Cultural methods.
Avoid wounding.
Avoid excessive nitrogenous fertilizer.
Practice crop rotation as bacteria can survive on
plant debris in the soil.
Provide adequate ventilation between plants,
avoid overcrowding nursery stock. Avoid
overwatering. Increased spacings between plants
may lower humidity.
Keep foliage dry. Control the growing
environment to reduce leaf wetness.
Avoid walking through, or working in,
susceptible crops while foliage is wet due to
irrigation or wet weather.
Sanitation.
Remove and destroy infected plants or leaves as
soon as they are observed.
Prune off affected branches at least 30-40 mm
below the damaged area and destroy.
If base of main stem is rotted remove and
destroy the whole plant as soon as detected.
Destroy systemically-infected plants.
Destroy self-sown plants.
Implement strict hygiene when growing highly
susceptible crops.
Staff must be trained in hygienic practices such
as washing hands after handling diseased plants
or soil, sterilization of tools, and wearing clean
uniforms.
Disinfect tools frequently when pruning.
Sterilize benches used for preparing cuttings
between batches to avoid chance contamination.
All debris must be removed between crops to
eliminate the risk of contamination.
Resistant varieties.
Grow if available and practical, varieties with
some resistance to bacterial leaf spots.
Grow highly susceptible crops in greenhouses
which keep out rain.
Plant quarantine.
If new planting material is suspected of being
infected grow separately from disease-free areas.
Keep stock plants separate.
Introduction of even one diseased cutting can
result in rapid spread if conditions are favorable.
Disease-tested planting material.
Only plant disease-tested seed, cuttings,
nursery stock.
Obtain planting material from a reputable source.
Do not take cuttings or save seeds from infected
plants.
Bactericides.
Copper-based fungicides are moderately effective
in reducing some bacterial diseases. However they
may be phytotoxic to some plants.
Table 57. Bacterial leaf spots – Some bactericides.
What to use?
NON-SYSTEMIC PROTECTANTS
Group M1, eg copper oxychloride; cupric hydroxide; cuprous
oxide; Liquicop (copper ammonium acetate); Tribase
Blue (tribasic copper sulphate) (page 341) -
copper fungicides are non-systemic
Group M1/M3, eg Mankocide (cupric hydroxide/mancozeb)
- both are non-systemic
When and how to use?
Copper-based and mancozeb fungicide sprays can
suppress bacteria on surfaces but will not eradicate
established infections or prevent re-infection if
conditions are favorable.
Bacterial diseases 311