PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Diagnostics.
Symptoms on Impatiens are easily confused with
nutritional deficiencies or mite damage
Many hosts are not susceptible to downy mildews.
With a hand lens check for the characteristic downy
growth usually on underside of leaves, it may
escape notice until spores form. See page 345 for
comparison with powdery mildews.
If still unsure incubate suspect tissue in moist
chambers for about 48 hrs to encourage
development of downy growth on leaf undersides.
Microscopic examination by experts will identify
characteristic tree-like spore structures (which varies
for each species of downy mildew) and distinguish it
from powdery mildew and gray mould.
Systemic infection characterized by discolouration
and stunting of growing points.
DNA is not commonly used to identify downy
mildews.
Disease cycle
For downy mildew of grape (see Fig. 189). Shortlived
zoospores are produced at night and released the
following morning as air dries out. Spores germinate
within 4 hours in water and can produce more spores
in 3 days. Many downy mildews can only reproduce
on living plants.
‘Overwintering’
As systemic infections in some plants, eg roses.
Infections on perennial crops, eg grape, roses
infected buds and stems. Infected regrowth.
Fallen leaves, etc in soil, growing media, compost,
crop debris. Some spores remain viable for years
(oospores), others (zoospores) for only a few days.
Alternate or weed hosts, volunteer plants.
Old infected seedlings in nurseries, stock plants.
Contaminated seed, propagation material, cuttings.
Spread
‘Overwintering’ spores may wash from infected
plant debris into soil. Short-lived zoospores spread
by wind, water, sprinkler or rain splash.
By movement of infected plants, seeds, cuttings,
bulbs, etc. before symptoms are apparent.
Infected crop debris returned to soil and distributed
with irrigation or flood water.
There is some evidence that downy mildew
could be seedborne on some hosts.
Conditions favoring
Leaves wet for long periods. Spores require free
moisture on the leaf surface to establish infection.
Cool nights, wet warm days, extended periods of
cool, wet weather during spring and autumn.
Can be devastating on seedlings in crowded seedbeds
during cool, moist, dull weather in unheated,
poorly ventilated glasshouses. Only checked when
weather becomes hot and dry.
Temperature requirements vary with the species, eg
Brassicas 8-24 o C, pansies 13-18 o C.
Weather warning systems predict when outbreaks
may occur, eg downy mildew of grapevine.
Fig. 189. Disease cycle of downy mildew of grapes
(Plasmopara viticola) (adapted from Agrios, 1997).
Fungal diseases - Examples of fungal diseases 349