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 Chapter 21, p369).
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).
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
Figure 15.21 Reversion disease of blackcurrant. Note that the infected leaf (bottom) has fewer main veins and leaf lobes than the healthy leaf (top)
A physiological disorder is a condition in the plant resulting from a non-living (abiotic) factor such as nutrient or water being present at the incorrect level.
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.
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