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  Section: Principles of Horticulture » Horticultural diseases and disorders
 
 
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Physiological disorders

 
     
 
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Horticultural diseases and disorders
  Fungi
  Leaf and flower diseases
  Stem diseases
  Root diseases
  Bacteria
  Viruses
  Physiological disorders

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.


Nutrient deficiencies

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 appearance
Figure 15.22 Blossom end rot in tomato.
The fruit at the opposite end from the stalk
has a typical black sunken appearance
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.

Excess fertilizer

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.


Low temperatures
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.


High temperatures
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
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.

Underwatering

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 spots on lower leaf surface of pelargonium. This symptom superficially resembles rust pustules
Figure 15.23 Raised oedema
spots on lower leaf surface of
pelargonium. This symptom
superficially resembles rust
pustules


Oedema

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 automatic venting.


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




 
     
 
 
     



     
 
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