There are several symptoms that show on plant leaves, stems and flowers
that are not
caused by pests or diseases. The main causes are: nutrient
deficiencies, excess fertilizer, frost, high temperature, lack of light,
overwatering and underwatering.
Each nutrient (the commonest being nitrogen, phosphorus, potassium,
calcium and magnesium) is required in the correct amounts to enable the
plant to carry out its chemical processes. When amounts present are too
low, deficiencies begin to show, usually by means of leaf symptoms (see Plant nutrition
Care should be taken to provide regular applications of a suitable
fertilizer, especially during the summer months and in situations where
the roots are restricted (as in pots).
|Figure 15.22 Blossom end rot in tomato.
The fruit at
the opposite end from the stalk
has a typical black sunken
Two common horticultural problems should be noted. In tomatoes and
peppers, blossom end rot
(see Figure 15.22) produces a symptom of a
black, concave lesion which looks at first sight like a fungal disease. It
is caused by an imbalance between potassium and calcium in the soil
or compost. It occurs most often when the soil or compost is allowed to
dry out while the fruits are swelling. It is seen more often in greenhouse
container-grown plants than with plants growing in the open garden or greenhouse borders. It is most common when plants
are raised in grow bags, where they have a small, shallow
root run that dries out easily. Although there is no cure for
blossom end rot once the symptoms begin to appear, the
obvious recommendation is that fruiting crops should never
be allowed to have dry roots.
A second problem is bitter pit
in apples. Here the fruit
develop many small, dark-brown, sunken pits. The tissues
below are stained to depth of about 2 mm. Cultivars such
as ‘Bramley’s Seedling’ and ‘Egremont Russet’ are most
susceptible. Young over-bearing trees show the worst
effects. The disorder is caused by low calcium levels in the
fruit, influenced by irregular water supply in the tree. Four
recommendations are given for this problem.
- Ensure a steady water supply to the tree during dry spells.
- Mulch around the tree to help moisture retention.
- Summer prune young, vigorous trees especially when they are holding
too many fruit.
- Occasionally use foliar sprays of calcium nitrate plus detergent in the
evening during summer to help prevent this problem.
When fertilizers are present at too high levels, roots are scorched and
are unable to provide nutrients for the other parts of the plant, often
resulting in the plant’s death. This condition is described in Plant nutrition
. Careful consideration of the appropriate frequency and
amounts of fertilizer will prevent this embarrassing situation.
Plants differ in their tolerance to low temperatures. Low temperatures
slow down the plant’s growth. Frost often causes the above-ground parts
of sensitive plants to collapse into a mess of green tissue after ice has
formed inside the plant and fractured all the cells.
Plants may become exposed to very high temperatures in greenhouses,
where growth may be weak and ‘leggy’. Their leaves also may become
dry and brittle, especially if they are touching the glass sides or roof
of the greenhouse. Regular attention to ventilators or the use of the
automatic ventilators available to amateur growers avoids this problem.
Lack of light
House plant species are sometimes placed in parts of the house
unsuitable for their ideal growth. For example, a poinsettia needs high
light levels. Plants outdoors may be subjected to the same oversight. Pelargoniums
used as bedding plants should be given full sunlight and will develop a pale foliage colour if placed in a shady place. Impatiens,
on the other hand, is able to withstand considerable shade and maintain
its rich dark-green foliage.
Overwatering replaces the air spaces in soil and growing composts with
water, thus preventing root respiration which is needed to supply energy
for root growth and nutrient uptake. Overwatering symptoms may
include the following.
- The whole plant may wilt, the lower leaves turn yellow and drop.
- New foliage may have brown spots.
- The whole plant may become stunted, and stems and roots become
brown and decayed.
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 in some
plant tissues. In some plant species, leaves change from shiny to dull
as a first signal of water stress and also may change from bright green to
a grey green. New leaves wilt, but in species such as holly and conifers
only the very youngest leaves wilt. Flowers may fade quickly and fall
prematurely. Older leaves often turn brown, dry and fall off. Digging
a few centimetres into the soil may indicate the need for watering with
shallow rooted perennials and annual border plants. Shrubs with deep
roots rarely need watering, although transplanted older shrubs may
show summer water-stress for a number of years (see also Transport in the plant
and Soil water
|Figure 15.23 Raised oedema
lower leaf surface of
superficially resembles rust
Oedema is seen as raised corky spots on the undersurface of leaves.
Species such as pelargonium (see Figure 15.23), rhododendrons,
begonias, pansies, violets and some fleshy-leaved plants such as Peperomia
are affected. Orchids can show oedema on their petals.
Oedema occurs when the roots’ ability to supply water exceeds the
leaves’ ability to release the water by transpiration. Conditions favouring
oedema occur most commonly in late winter and early spring especially
during extended periods of cool, cloudy weather. Warm, moist soil
occurring alongside cool, moist air brings on the condition most
severely. The symptoms are commonly seen in unheated greenhouses.
The problem can be greatly reduced by glasshouse heating and
Symptoms of disease and physiological disorders
Below in Table 15.2 is a summary of the most important symptoms to help
the reader ‘home-in’ on disease problems and physiological disorders.
|Table 15.2 Some symptoms of diseases and physiological disorders