PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Disease cycle
Powdery mildew of rose is one example (Fig. 186).
Powdery mildews are obligate parasites and can
only multiply on living plants. The fungus grows
almost entirely externally on the surface and tiny
suckers called haustoria penetrate the outer cells of
the leaf to obtain food. Small black spots
(cleistothecia) form with cool weather in autumn.
‘Overwintering’
As mycelium and spores on buds, twigs, canes,
fruit and other plant parts, especially on perennials
such as roses and apples.
As active infections on host plants (in warm
climates and glasshouses). Infection of annuals
probably originates from out-of-season plants, held
over stock, etc. Infected volunteer plants.
Spores in fruiting bodies on infected crop debris.
Seedborne on some hosts, eg pea.
Spread
Spores spread by wind, air currents water splash.
By movement of infected plant material, seed.
‘Conditions favoring’
Night temperatures of about 15 o C and relative
humidity of 90-95% and day temperatures of
above 26 o C and relative humidity of 40-70%.
Epidemics are prompted by high humidity.
Most severe in spring and autumn during hot humid
weather. Many powdery mildews flourish in hot
dry conditions, dews at night provide sufficient
moisture for spore germination. Unlike downy
mildews, powdery mildews can flourish in fairly
dry weather.
Spores germinate and infect hosts without free
water and will not germinate in rainy weather.
3
Late crops may be severely affected, eg pea,
gerbera, days are warm and dry and nights cool
enough for dew to form and spores to germinate.
Shade, high plant densities and luxurious plant
growth due to high nitrogen levels provide plenty
green young tissue for powdery establishment.
Management (IDM)
Are you a commercial grower or home gardener?
1. Access/prepare a plan in advance.
2. Crop, region. Know if your crop is susceptible to
powdery mildew. Management programs are
available, eg AUSVEG, Ausvit, Cropwatch (Vic),
Rose Growers Assoc.
3. Identification. List the diseases the crop is
susceptible to. On some crops you may need expert
advice to confirm that it is powdery mildew and not
downy mildew as the fungicides used to control them
are often quite different. Consult a diagnostic service
if necessary (page xiv).
4. Monitor & detect disease especially in low light
sites, record results (page 327). You may need a x10
magnifying lens. Remember you need to know
when, where, what and how to monitor.
Monitor apple trees during winter and growing
seasons for small areas of white powdery growth.
Weekly inspections of glasshouses (1 plant in 30)
where mildew has not been found. If detected, scout
1 plant in 10 every week until plants are disease-free
for at least 3 weeks then go back to 1 plant in 30.
Disease warning services may be available.
Roses planted along the edge of vineyards in France
served as an early warning system for powdery
mildew of grapevines.
5. Threshold. How much damage is acceptable? Have
any thresholds been established? If so, what are they,
eg economic, aesthetic, environmental?
6. Action/Control. Take appropriate action when any
threshold is reached, eg cultural improvement, sprays.
7. Evaluation. Review program. Within a few days of
spraying check whether powdery mildew colonies are
active. Recommend improvements if required.
Fig. 186. Disease cycle of powdery mildew of rose
(Podosphaera pannosa) (adapted from Agrios, 1997).
346 Fungal diseases - Examples of fungal diseases