Agriculture Reference
In-Depth Information
Spore germinating
Spore landing on
the leaf surface
The infected tuber (1) containing
mycelium and sexual spores which
survive in the store or field
A - infection by the spore
through the leaf epidermis
(magnified 500)
(3)
SPORES being
produced for
wind and water
dispersal
The infected shoot is the source of
disease for the subsequent crop
(2)
(4)
(1)
Disease reaches the leaf (2), moves
from leaf to leaf (3), and eventually
infects the tubers (4)
HYPHAE
B - Hyphae obtaining food from the
leaf for spore production
(magnified 500)
Figure 19.5 Potato blight life cycle
quickly. Later in the crop, badly infected plants may
have tuber infection as rainfall washes down spores
into the soil.
Control . The gardener should use clean seed bought
from a reputable source, choose resistant varieties and
apply a protective fungicide such as copper sulphate
(Bordeaux mixture) before damp conditions appear.
It is strongly recommended that a full cover of the
upper and lower surfaces of the leaves be achieved to
prevent even the smallest of initial infections, because
this fungus has such a potential for rapid increase.
There is, currently, no systemic fungicide (see p. 204)
available to gardeners for the control of potato blight.
Early potato cultivars usually complete tuber
production before serious blight attacks. Resistant
potato cultivars such as 'Sarpo Axona' prevent rapid
build-up of disease, although resistance may be
overcome by newly occurring fungal strains.
(Defra) scientists give a forecast to growers
when they have measured a 'Smith period'
(conditions most favourable for spore infection).
This period requires two consecutive days
where the minimum temperature is 10°C or
above and when each day has at least 11 hours
with a relative humidity greater than 90%. In
this way, protectant sprays of chemicals can be
applied before infection takes place.
19
Damping off ( Pythium and
Phytophthora species )
These two 'fungi' belong to the Zygomycota.
Damage . These two similar genera of 'fungi' cause
considerable losses to the delicate seedling stage. The
infection may occur below the soil surface, but most
commonly the emerging seedling plumule is infected
at the soil surface, causing it to topple (Figure 19.6).
Occasionally the roots of mature plants such as
cucumbers are infected, turn brown and soggy, and
the plants die.
Rose plants often have high levels of Pythium (along
with parasitic nematodes) around their roots as they
Predicting blight outbreaks
Nationally broadcast warnings are given on
the radio, based on temperature and humidity
measurements. UK government department
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