Part 2

Insects, Bacteria, and Fungi

G. Leach.

Some insects do great damage by aiding in the spread and development of plant diseases.

The insect first proved by experiments to be a vector of a plant disease was the honey bee, which everyone considered completely beneficial; no one had thought of suspecting one of man's best friends.

The experiments that indicted the honey bee were a landmark in agricultural science. For many years the damage done to plants by insects had been measured only in terms of direct injury from their feeding and breeding.

M. B. Waite, an employee of the Department of Agriculture, discovered in 1891 and proved experimentally that the honey bee, while visiting apple and pear blossoms in search of nectar, became contaminated with the bacteria causing fire blight and transmitted the disease from blossom to blossom and from tree to tree.

That was a new idea, one that plant pathologists and entomologists were Slow to accept. The recognized importance of the honey bee in pollinating flowering plants and producing honey made many reluctant to believe that it could be guilty of transmitting a disease.

All this was discouraging to Waite, but his work was confirmed by J. C. Arthur, working at the New York Agricultural Experiment Station at Geneva. Soon plant pathologists and entomologists began to suspect other insects of transmitting plant diseases.

A few years later Erwin S. Smith, another pioneer worker of the Department of Agriculture, and his associates reported that a destructive bacterial wilt of cucumber and muskmelons was transmitted by two species of cucumber beetles. Further work has demonstrated that the bacteria causing the disease survive the winter within the bodies of the insects and that, in nature, the disease depends completely on the insects both for survival over winter and for spread from plant to plant in summer. A similar relationship exists between the bacterial wilt of sweet corn and two species of flea beetles.

Ergot of rye and related cereals and grasses was perhaps the earliest fungus disease to be recognized as transmitted by insects. The fungus affects the young flowers and replaces the normal seed with a hard, black mass called a sclerotium. In early stages of blossom infection, the fungus secretes a sugary fluid in which masses of spores are produced. The fluid has a foul odor that attracts flies. When the flies feed on the sugary solution, they become contaminated internally and externally with the ergot spores. Some of the flies also feed on the pollen grains of the healthy flowers. On them they deposit the ergot spores and thus spread the disease from plant to plant. In this instance, a mutually beneficial relationship exists between the fungus and its insect vector. The flies derive nourishment from the sugary fluid. In return for the food, the flies transmit the spores of the fungus from flower to flower and enable the fungus to survive. An association of this type is called mutualistic symbiosis.

Similar mutualistic symbiosis occurs in other instances of insect transmission. The seed-corn maggot and other dipterous insects carry the soft rot bacteria which affect many vegetable crops. The flies lay their eggs in the soil near vegetable tissue or directly on it. When the eggs hatch, the young maggots bore into the plant tissues, taking the soft rot bacteria with them. The maggots will not grow and develop normally in sterile plant tissue but grow rapidly when the tissues are decayed by the bacteria. Thus the bacteria are essential for the normal development of the insect. The bacteria may also provide essential vitamins for the insect and aid in the digestion of plant tissues. The soft rot bacteria are wound parasites and cannot penetrate uninjured plant tissues. The insects make the necessary wounds. The bacteria in return provide the necessary vitamins and aid the insects in deriving nourishment from the plant. Both the insect and the bacteria thus benefit from the association.

Because the young maggot would be helpless without the bacteria, which it may or may not obtain from the soil, the insect insures their presence when needed by harboring the bacteria within its body. The bacteria survive within the intestinal tract of the insect in all stages of metamorphosis. Freshly deposited eggs are usually contaminated. The insect carries with it at all times a culture of the bacteria that are essential for the nourishment of the young maggots. It is evident that the transmission of plant diseases by insects often is not a simple matter of chance but is a complicated association that has evolved over a long period.

Fire blight is a bacterial disease of orchard fruits, principally pears and apples. It chiefly affects blossoms and young tender shoots. It may also form destructive cankers on the trunk and larger branches. It is caused by bacteria that overwinter in the bark surrounding the cankers. Sap oozes in spring from the edges of infected cankers. A microscopic examination of the sap shows it to be teeming with the fire blight bacteria. Insects, principally ants and flies, feed on the ooze and then visit blossoms in search of nectar. Thus the bacteria are introduced into the nectar, from which they spread into the blossoms, causing the blossom-blight stage of the disease. Bees, wasps, and other insects that visit the flowers in search of nectar or pollen spread the bacteria from blossom to blossom and from one tree to another.

Shortly after the blossoms have been blighted, the young and tender shoots become infected, turn black or brown, and wither. Heavily infected trees look as if they had been scorched by fire, hence the name fire blight. The young shoots are inoculated with the bacteria by sucking insects, including several species of aphids and leafhoppers. These insects become contaminated by feeding upon or crawling over infected tissue. Later, when the contaminated insects pierce healthy twigs with their needle-like mouth parts, the bacteria are carried deep into the tissues and the twig is inoculated.

The bacteria of fire blight may be disseminated also by wind-blown rain and by pruning tools. Whatever the relative importance of the various methods of spread, it is agreed that if all dissemination by insects could be eliminated the disease would be much less serious.

Bacterial wilt of cucurbits damages cucumbers, muskmelons, and squashes in the North and East. The bacteria causing the disease are found in the water-conducting vessels of the plants, in which they grow in white, sticky masses and interfere with normal movement of water from root to leaves. Affected plants wilt as if suffering from drought and usually die before any fruits mature.

The bacteria gain entrance into the plant only through the feeding wounds made by two species of cucumber beetles, the striped cucumber beetle and the spotted cucumber beetle. The bacteria survive the winter within the bodies of the beetles and are introduced into the wounds from their mouth parts. Not all beetles are contaminated with the bacteria, but any beetle that feeds upon a diseased plant is likely to become contaminated. The beetles hibernate in the adult stage and in some years a relatively high percentage of overwintering beetles harbor the bacteria. Such beetles may transmit the disease to any susceptible plant on which they feed in the spring.

Striped cucumber beetle.

No other method of infection or survival over winter is known to occur in nature. The only way to control the disease is to prevent the beetles from feeding on the plants. The only satisfactory way to protect the plants used to be to grow them under insect-proof cages. Some organic insecticides, such as methoxychlor, have given promise against the beetles and may be a more practical means of controlling bacterial wilt.

Bacterial soft rot of vegetables is caused by several related strains of bacteria. It affects a variety of plants, including most plants with succulent tissue that is not too acid in reaction. The bacteria are strictly wound parasites and generally do not penetrate uninjured tissues. A wounded plant normally attempts to heal the wound by laying down a layer of cork cells, which will prevent infection. If conditions do not favor cork formation, the bacteria may infect and cause a rot before the healing action is completed. The soft rot bacteria can be found in Most agricultural soils. Any wound in susceptible tissue thus is a potential Point of infection for soft rot. If the wound heals quickly enough, infection may not take place, but if something interferes with wound-cork formation, the disease is likely to occur. It is common practice in some potato-growing regions therefore to store cut seed pieces under conditions that permit rapid healing or suberization of the cut surfaces.

Wounds made by insects on the roots of plants or on stems or leaves near the ground are common points of infection. Among the most effective insects in making the wounds are the dipterous insects, such as the seed-corn maggot, the cabbage maggot, and the onion maggot, which live in mutualistic symbiosis with bacteria. They harbor the bacteria within their bodies. When the maggots burrow into the plant tissues, they usually introduce the bacteria into the plant. Moreover, the maggots, by continually burrowing into the tissues, prevent the wound from healing or puncture each new layer of cork as it is formed.

Because the insects live in decaying plant tissue, they used to be considered harmless scavengers, coming in only after the plant tissues had already decayed. Actually, however, the insects, by transmitting the soft rot bacteria and making the necessary wound, inoculate the plant, and thereby produce their own rotted tissue.