Environmental, Nonparasitic Injuries

J. E. McMurtrey, Jr.

Bad weather, air pollution, growth regulators, and the deficiencies or excesses of minerals in the soil can cause a group of diseases of plants that we classify as environmental and non-parasitic.

They are related closely to diseases brought about by parasitic organisms. Often we can regulate plant growth so as to control them and thereby control somewhat the diseases attributed to parasites. Symptoms associated with nonparasitic diseases frequently are confused with those caused by fungi, bacteria, and viruses. Often the injury from nonparasitic disturbances permits fungi, bacteria, or viruses to enter and damage the plant.

The severity and type of injury vary with the plant, its stage of maturity when the disturbance occurs, and the part of the plant involved.

LIGHTNING, hail, wind-blown rain, drowning, frost, and drought are among the elements of weather that may harm plants.

Lightning may tear large trees apart or it may only injure a few limbs. It may kill the stem tissue of annual plants, such as tobacco, so that the leaves get a shrunken, dark midrib. Damage to plants by lightning is local and usually not extensive.

Hail may cause only small holes in a few leaves or complete defoliation and destruction of plants. A striking instance of hail damage is the destruction of an entire field of shade-grown tobacco and the shade cloth over it. Usually hail does damage in limited areas only.

Heavy rains may break young, tender leaves or puncture holes in them. Wind-blown rain also causes water soaking of the intercellular spaces of the leaves. Sometimes, if micro-organisms are present, the damage may be severe. Plants blow over; leaves and grain in contact with the soil may rot; it might be impossible to use machinery to harvest the crop.

Most crop plants grow well on relatively well-drained soil that may be subject to leaching or temporary flooding, but most plants will not survive persistent flooding, which drowns and destroys the root system. If a part of the root system is damaged, growth is reduced and micro-organisms may invade the tissues. Temporary wilting is often evident. In sandy soils the rapid loss of water by percolation-leaching causes the loss of soluble plant foods, particularly nitrogen and possibly magnesium.

Not uncommonly are plants injured on days of high temperature and bright sunshine. Sunscald is the permanent wilting of young leaves. Another type of injury results in a drying of the lower or older leaves. Such conditions are more common with temporary shortages of water. Corn, for example, first shows rolling of leaves; if the drought continues it may suffer to the degree that the upper part of the plant, including the staminate inflorescence, dries up and fertilization cannot take place. In extreme droughts trees and other plants may die.

In cold weather growth may be delayed so that parasites develop. Losses from late spring and early fall freezes are an ever-present threat in most of the Temperate Zone. Small grains, corn, and other crops often fail to reach proper maturity before being killed by freezes in some seasons in northern latitudes. Following frost and freezing injury, plants may suffer death of twigs and branches, splitting of trunks, and the loss of fruit crops when flowers are killed.

FACTORIES may release concentrations of gases that are toxic to plant growth. Sulfur dioxide is an example. In many places the vegetation around industrial establishments; such as factories that make sulfuric acid or smelters of sulfide ores has been almost entirely wiped out. Smoke from coal sometimes contains sulfur dioxide in amounts that may injure plants if it is not dissipated by wind.

Fluorine, as hydrofluoric acid gas, is injurious to plants near chemical works that release it into the air. The injury often appears only as marginal lesions or necrosis, but sometimes the entire leaf dies prematurely. Low concentrations of fluorine often cause leaves to turn yellow.

Smog is a still, heavy mixture of fog and various contaminants, such as sulfur oxides, ammonia, fluorides, filterable oils, gaseous hydrocarbons, oxides of nitrogen, and hydrogen sulfide. It is not known for certain which one of those gases is the culprit or whether two or more act simultaneously to injure plants. In the south coastal area of California, for example, Romaine lettuce, endive, and spinach suffer extreme injury from smog; beet, celery, oats, Swiss chard, and alfalfa suffer moderate injury. Barley, onion, parsley, radish, tomato, turnip, and rhubarb suffer slight injury. Cabbage, cantaloup, carrot, cauliflower, cucumber, pumpkin, squash, and broccoli suffer no injury. Bleaching and scorching sometimes are evident.

INSECTICIDES may injure plants. Arsenicals used improperly may cause shedding of leaves. The effect of arsenicals, particularly lead arsenate, may be cumulative and in time may kill fruit trees. Calcium arsenate, as Used on cotton, may temporarily sterilize the soil. Oil sprays may damage fruit trees. Parathion and some of the other newer synthetic insecticides may cause the russeting of some varieties of apples. Benzene hexachloride may cause the formation of strap-leaf and off-flavors, particularly in potatoes.

All parts of plants, notably orchard trees, might be injured by bordeaux sprays and dusts. The leaves may show burning, shot holing, spotting, discoloration, and defoliation. The blossoms may be injured so that no fruit is set. The fruit may show spotting, russeting, malformation, cracking, and shedding. The twigs may have injuries of various kinds, or the entire tree may die if damage is unusually severe. Lime-sulfur also might cause lesions on foliage or fruit and premature fruit drop. The most common injury is a dull-brown spotting of the leaves or burning of margins and tips.

Many injuries have followed the extensive use of the growth-regulator herbicides, especially 2,4 D. Minute amounts of them are enough to produce ill effects on plants even the tiny residue in a sprayer that has not been cleaned thoroughly with plenty of hot water and ammonia. Injury also may occur from the drift of herbicide spray when the wind is blowing. Leaf malformations occur in sensitive species around factories that prepare 2,4 D. The injury may be only a slight rat-tail type of growth of the leaves or the death of trees.

A DEFICIENCY of any one of the chemical elements necessary for plant growth may reduce total growth of plants. To distinguish the effect of one element from that of another, one must examine closely the affected plant. For example, it is not enough to say that a leaf is chlorotic; a detailed description of the chlorotic pattern is necessary and the age of the leaf must be known. A shortage of any one of the elements boron, calcium, copper, iron, magnesium, manganese, molybdenum, nitrogen, phosphorus, potassium, sulfur, and zinc may produce malnutritional diseases of plants.

A shortage of boron in the soil results in poor growth of tops and roots. Top sickness in tobacco, heart and dry rot of sugar beets, internal cork of apples, internal browning of cauliflower (which first occurs as small, concentric, water-soaked areas in the stem and central branches of the curd), and cracked stem of celery are boron-deficiency diseases. Poor growth, yellowing of the terminal growth, and death of the terminal buds are typical symptoms in most plants. The affected terminal growth becomes brittle, breaks easily, and shows discoloration of vascular tissues.

A deficiency of calcium shows up first near the growing point on the young leaves. The growing point dies and the young leaves often are severely distorted and show a hooked tip. When later growth takes place, the margins are irregular because of the failure in early development. The leaf petioles of many plants collapse when the growing points die. The floral parts, including corolla and calyx, may show abnormalities. Shedding and little or no seed set may follow. Tobacco, tomato, and potato plants show distinctive deficiency effects. Tomatoes show pronounced dieback of stems, leaves, and fruiting branches and blossom-end rot of fruits. Potatoes form few tubers, show bushy vines, and develop leaflets near the shoot tips that are small, chlorotic, and roll inward toward the midveins. Beans, peas, clovers, and other legumes have pale-green leaves with necrotic margins. The stems may collapse near growing points, petioles, and pedicels. Pods and seeds are few and poorly developed. The growing points of sugar beet, carrot, parsnip, and other root crops may die. The tree fruits undergo death of the terminal shoots. The tip leaves have a scorched and ragged appearance, and the margins roll inward. Calcium deficiency in most plants results in breakdown of the meristematic tissues in stem, root, or any part of the plant where the deficiency occurs. Extreme shortages of calcium commonly mean the early death of the plant.

Years ago growers learned that the dieback, or exanthema, of citrus trees in Florida could be corrected by the use of copper compounds, although the ailment was not recognized at first as due to a copper deficiency.

Apple, pear, and plum trees show much the same symptoms when they lack copper. Tobacco plants deficient in copper suffer a breakdown of older leaves and wilting of younger leaves. When the shortage of copper operates after flowering, the seed head cannot stand erect and the seed stalk bends to one side. The cereals show much the same symptoms withering of tips. of the younger leaves, wilting of the foliage, dwarfing, distorting of seed heads, and less formation of grain. The lower leaves and tillers on such plants tend to remain green. Copper is essential for normal color and growth of lettuce and onions, particularly when they are grown on peat soils.

The first deficiency disease of plants to be recognized was the one caused by too little iron. It was first reported in France, and the remedy was iron salts. Yellowing of the young growth is the first sign of the disease. Some necrosis may occur. In extreme cases the young leaves may become almost white. In milder cases there is a mottled pattern; the primary and secondary veins tend to retain the green: color. Sometimes there is drying or scorching of leaf tips and margins. In extreme cases dieback of twigs may extend to large branches of trees. Fruit and shade trees are often more commonly affected than field or vegetable crops. The typical chlorosis due to iron deficiency often occurs on soils of high lime content and has been termed "lime-induced." The typical chlorosis of pineapples in Hawaii occurs on soils high in manganese and has been corrected by the use of sprays that contain iron.