Agriculture Reference
In-Depth Information
is spread by several species of aphids. The speed of
spread is quite slow because the virus is not able to
live and multiply in the aphid. Preventive control is the
only option. Certified planting stock should be used.
Aphid-controlling insecticides should be applied in late
spring, summer and autumn. Suspected infected trees
should be reported.
Arabis mosaic
This virus infects a wide range of horticultural
crops. On strawberries, yellow spots or mottling are
produced on the leaves, and certain cultivars become
severely stunted. On ornamental plants (e.g.
Daphne
odorata
), yellow rings and lines are seen on infected
leaves, and the plants may slowly die back, particularly
when this virus is associated with cucumber mosaic
inside the plant. Several weeds (e.g. chickweed and
grass species) may harbour this disease. The virus
is spread by a common soil-inhabiting nematode,
Xiphinema diversicaudatum
, which may retain the
virus in its body for several months. Certified virus-free
soft fruit planting material can be bought.
Figure 19.22
Frost-damaged plant, showing browning
of the petals
Low temperature
This usually cause slow plant growth, resulting from a
reduced rate of photosynthesis. Plant species vary in
their temperature preferences.
Cyclamen coum
grows
well under cool conditions. Snapdragons (
Antirrhinum
spp.)
grow better when night temperatures are no
higher than 12°C, while most bedding plants prefer a
higher night temperature.
High temperature
Plant disorders
The effects of high temperature may differ with
each species. As the species approaches its
upper level of tolerance, photosynthesis slows
down due to enzymes becoming less efficient.
Plants such as sweetcorn originating from the
tropics have different photosynthesis systems to
help cope with this problem (see p. 112). The direct
effects of high temperature are usually to cause leaf
margins to dry off before the whole leaf dies (see
Figure 19.23).
There are several symptoms that show on plant
leaves, stems and flowers that are not caused by
pests or diseases. The main causes are: frost, low
temperature, high temperature, shade, drought,
water-logging (see p. 151), high humidity (such as
'rose balling' and oedema), incorrect soil pH, nutrient
deficiencies and excess fertilizer (see p. 170),
fasciation (see also p. 271) and reversion
.
A plant disorder is a condition in the plant resulting
from a non-living (abiotic) factor such as an
environmental factor, a nutrient or water being present
at the incorrect level, or a genetic disorder.
Shade
House plant species are sometimes placed in parts
of the house unsuitable for their ideal growth. For
example, a poinsettia, needing high light, will grow
poorly in the back corner of a room. Plants outdoors
may suffer from the same oversight.
Pelargonium
used as bedding plants should be given full sunlight
and will develop a pale foliage colour if placed in a
shady spot.
Impatiens
, however, is able to withstand
considerable shade and maintain its rich, dark green
foliage.
Frost
Plants differ in their tolerance (hardiness) to low
temperatures. Low temperatures slow down the
plant's growth. Frost often causes the above-ground
parts of sensitive plants to blacken (see Figure 19.22)
and then collapse into a mess of dead tissue (after ice
has formed inside the plant and fractured all the cells).
Pelargonium
species left to grow outside in autumn will
die when the first frost occurs. At the beginning of the
growing season, potatoes planted too early in spring
may be killed by late frosts in June (see also p. 21).
A list of the hardiness of garden plants is summarized
on the
companion
website: www.routledge.com/cw/
adams.
Drought
The plant needs sufficient water to carry nutrients
around, to be present as an ingredient for making
sugar, to transpire from the leaf in order to keep a
desirable leaf temperature, and to maintain turgidity
