The troubles which arise in the culture of crops in the greenhouse may be divided into several groups
a) failure to supply the essential factors for optimum growth such as light, moisture, carbon dioxide and heat in amounts necessary for each individual crop
b) fertilizer deficiencies
c) fertilizer excesses
d) toxic gases
e) attacks by insects, animals, and allied pests and
f) susceptibility to fungus, bacteria and virus troubles.
Symptoms of deficiencies of various fertilizers have been studied over a period of years with plants in greenhouses.
This is a term used to denote the loss of normal green colour from the foliage whether it is on the older, more mature leaves or the younger foliage. The entire leaf may be affected, or just areas between the veins, in which case the yellowing is most usually in irregular patches shading into the green colour. Sometimes only the margin of the leaf or leaflets may be yellow, while the centre of the foliage is almost a normal green.
This refers to the death of the area severely affected by chlorosis. Necrotic spots or areas can also be caused by spray or aerosol damage, sunscald and other such factors which may have no relation of fertilizer.
Generally the entire plant becomes lighter green, but the effect will be most noticeable on the older foliage. Gradually the oldest leaves loose their green colour, and most plants become yellow. The flowers are smaller and may lack well-developed colour.
A purplish coloration developing first on the underside of the petiole, or leaf stem, which spreads to the main veins of the leaf is characteristic of this deficiency.
The margins of the leaves of the older foliage become yellow, and the chlorosis progresses toward the mid-portion of the foliage as this deficiency increases in severity. The older leaves may drop in extreme cases of deficiency. Certain fumigants may cause marginal burning or chlorosis, and sometimes droplets of spray or fumigant may result in spots or blotches of chlorotic or necrotic nature.
In sand culture, a typical symptom is the development of short clubby roots followed in a matter of several weeks by their death. In many cases insufficient calcium is associated with a low pH of the soil.
This is a rather common trouble although an actual lack of iron may not be the primary cause. As iron deficiency becomes more intense, necrotic areas appear on scattered portions of the yellow coloured leaves and the affected foliage may drop. Iron can become deficient in soil, but often the symptoms of this deficiency are induced by other causes from injury to the roots by over-watering or over fertilization. Nematodes, or other soil pests interfering with root growth can also induce iron chlorosis symptoms.
The number of cases where this is a limiting factor are few, and most of them are with certain rose and carnation varieties. The new foliage is thick or leathery and quickly becomes chlorotic. The rose flowers are usually very malformed. The stem tip dies, giving rise to growth of shoots immediately below, which in turn die at the tip, and a ‘witches broom’ effect is observed. Because deficiency symptoms can sometimes be confused with the effects of some other environmental factor of cultural practice, a thorough review of fertilizer application, soil testing, soil type, watering practices, and other procedures is warranted before hasty conclusions are reached.
An unfortunate belief among many growers is that when a plant does not grow under apparently favorable conditions, the trouble can be overcome by applications of fertilizer. This practice has resulted in untold damage or loss of crops, as more often than not the original trouble could have been too much fertilizer in the soil. If additional fertilizer is applied when no more is needed, the results can be very injurious. Sometimes the difference between a high but safe nutrient level and an injurious nutrient level is not very great and the margin of safety may be extremely small. Therefore, it behaves the grower to test the soil in case of doubt to determine the advisability of fertilizer application.
The plants exhibit heavy, rank growth, with large, dark green leaves that are often crisp and break easily. Additional nitrogen may inhibit root action, causing typical symptoms of iron chlorosis. If the root system is killed, the plants wilt excessively and never recover. This yellowing of the top foliage is very common in chrysanthemums and snapdragons. Over 75 ppm of nitrates is not safe.
Over doses of phosphorus precipitate the iron from the soil solution and make it insoluble and unavailable, causing iron chlorosis to develop. Over 25 ppm may cause trouble.
Up to a certain point, excessive potash apparently is not injurious. Greater amounts inhibit root action and may cause chlorosis, wilting, or immediate death of the plant. Over 60ppm is dangerous.
Usually the pH of the soil will rise when there is excessive calcium. This causes iron chlorosis in many plants and has been called over liming injury. Over 300 ppm is high.
In the normal pH range of soil, there is little danger of excessive iron since phosphorus or calcium will precipitate it from the soil solution. At ph 5.0 or lower, iron becomes very soluble, and on hydrangeas brown dots appear on the leaves, indicating iron is being precipitated as water vapor is lost by transpiration.
A low pH may often be characteristic of soils high in sulfates. Sometimes high sulfates are encountered at pH 6.5. Over 600 ppm is toxic to most plants.
This trace elements is often found in soil to which unleached cinders have been added or where boric acid has been applied indiscriminately. Certain water supplies may have rather high amounts of boron. On roses, the serrations on the margin of the leaflets of the lower leaves turn black and the remaining leaves turn yellow and drop.
This is not troublesome except on hydrangeas that are being blued. Roots are burnt, and the plants wilt.
Soluble salts excess
Too much fertilizers in the soil injures or kills roots, and plant growth is severely reduced.
Remedial measures for excess fertilizer
When fertilizer levels rise to the point where they become toxic, immediate steps must be taken to remove the excessive materials. Excessive nitrogen leaching with heavy waterings and application of of straw mulch Excessive phosphorus can not be leached. Only by addition of lime or iron sulfate Excessive potassium leaching may wash some quantity, but in clay soils removal may be almost impossible. Excessive calcium Acidifying the soil excessive iron Raising the pH or by addition of phosphorus excessive sulfates Leaching and avoiding the use of sulfate forms of fertilizers excessive boron Water glass, or sodium silicate can be dissolved in water at the rate of 100 cubic centimeters per gallon and applied to the soil. Excessive aluminum raising the pH or by adding phosphorus excessive soluble salts leaching.
Injury by toxic gases
This gas usually contains 95 per cent methane and 4 to 5 per cent ethane and frequently causes injury to greenhouse plants. The common source of injury is from corroded or leaky gas pipes inside or outside the greenhouse. This injuries usually occur during winter when the ventilators kept closed. Very small concentrations of gas are sufficient to cause damage to plants, its detection is difficult by sense of smell. One part to 350 will cause a headache after 2 hours, whereas 1 part of natural gas in 10,000 to 100,000 of air will injure many plants. The best method of detection is through the use of tomato plants. In the presence of gas the leaves of tomato turn downward, because of epinastic response, which causes petioles of leaves to grow more rapidly on the upper side. Carnations exposed to low concentrations will develop long stigmas, but this may also occur in bright weather in unshaded greenhouses. In case of prolonged exposure of young buds even 1 part to 100,000 may kill the buds and flowers fail to develop. Rose foliage on the upper shoots exhibits epinasty, or a bending downward of the petiole. Severe leaf drop may follow and the flower colour often fades. Bulbous plants usually develop twisted foliage and the flowers do not open properly.
This gas apparently is a by-product of metabolic processes and is given off in very small quantities by plants or their parts. Dropping of flowers after pollination is thought to be associated with ethylene vapors. The carnation flowers close or may appear “sleepy”, that is, the ends of the petals curl inward due to ethylene gas.
In very low concentrations this gas is toxic to plants. Sulphur dioxide enters the leaf through open stomata and kills the cells nearby, thus showing patches of dead tissues scattered over the leaf and frequently affecting the margins. Middle-aged leaves are more susceptible than young leaves.
Damage from sulphur dioxide is commonly found in localities where coal is burnt in large quantities. Foggy days are particularly dangerous. The common practice of using sulphur on heating pipes in rose houses to control mildew is responsible for leaf drop on some varieties.
Many plants are quickly damaged by vapors from metallic mercury. This damage is manifested in roses by peduncles of young buds turning yellow and later black. The color of flowers turns dark and leaves are scorched. Breaking of mercury thermometers or the use of bi chloride of mercury on beds as a disinfectant is the usual way in which mercury may be released. Control measures consist of removing all possible traces of mercury and covering the areas where mercury was spilled or applied with a 2-inch thickness of iron filings. Paint containing mercury as a fungicide should not be applied to rose houses.
The fumes of 2,4- dichlorophenoxy acetic acid and related compounds used as weed killers cause bending, curling, and other malformations of leaves, stems, flowers or bracts. Fumes or ‘drift’ from a spray applied along the side of a greenhouse may enter through the doors or side vents cause trouble. Therefore, it is well to prevent trouble by keeping such sources of potential damage out of the greenhouse, boiler room, potting shed, or any place where the fumes could conceivably enter a greenhouse.
Many materials containing phenol or its derivatives are toxic to plants. Tar, carbolic acid, pentachlorophenol and many others of similar nature should never be used under glass. Treatment of wooden bench members with wood preservatives containing phenol compounds results in severe damage to the plants.