Bacteria, Fungi, and Insects

J. G. Leach.

Insects influence fungus and bacterial diseases of plants in several ways.

Without disseminating the microorganisms themselves, insects may make wounds on a plant through which fungi or bacteria may enter. An example is white grubs, which often feed on the roots of raspberry plants and make wounds through which the bacteria that cause crown gall gain entrance from the soil. We have no evidence, however, that the white grubs are involved in the spread of the bacteria. Curculios that feed on young peaches and plums likewise make wounds through which wind-borne spores of the brown rot fungus enter and cause infection.

But some insects spread pathogens from diseased plants to healthy plants without wounding the plants. When bees transmit the bacteria causing fire blight of apples and pears from diseased to healthy blossoms, they make no wounds on the blossoms but introduce the bacteria into the nectar, where they grow and later penetrate the tissues through the nectar glands. Another example is the relationship between flies and the ergot disease of rye. In early stages of infection, the fungus produces large quantities of spores in a sugary exudate, which has a foul and carrion-like odor and attracts flies. The flies feed on it and become contaminated with the fungus spores. They transport the spores to healthy flowers, where infection takes place without the need of any wounds. More efficient is the insect that transmits the pathogen from plant to plant and also makes the wound through which infection takes place. That kind of relationship exists between the elm bark beetles and the Dutch elm disease, the striped cucumber beetle and the bacterial wilt of cucurbits, and many other diseases transmitted by insects that feed upon the plants by chewing the tissues or sucking the sap.

In a few instances insects may influence the development of a disease although they neither disseminate the pathogen nor make wounds through which it enters the plant. So it is when beetles of Monochamus species bore into the heartwood of trees or logs already infected with wood-rotting fungi and hasten the growth of the fungi and consequently the rotting of the log.

Almost everywhere, except possibly the humid Tropics, some period of the year is unfavorable for the growth of fungi and bacteria. Then the pathogen has the problem of survival. In the North the critical period is winter, when temperatures are too low for growth of both pathogen and host. In the warmer, drier regions, heat and drought may be limiting factors. The successful pathogens are the ones that have some special adaptations that enable them to survive winter cold and summer heat so that they are ready to cause infection when conditions again are favorable. Some pathogens are adapted to survival in the soil. Others survive in roots or stems of perennial plants or in the seeds of annual plants. Some produce resistant spores that withstand unfavorable conditions.

Some fungus and bacterial pathogens that are transmitted primarily by insects can survive within the body of the insects that transmit them. The bacteria that cause wilt of sweet corn survive the winter in the bodies of the corn flea beetle. Those that cause the wilt of cucurbits survive in the bodies of the striped and spotted cucumber beetles. The fungus that causes the Dutch elm disease may overwinter in the body of the elm bark beetle. Fungi that cause the blue stain disease of pines survive in the bodies of the pine bark beetles. Pathogens like these, which survive within the bodies of their insect vectors, withstand the digestive fluids in the insects, while many other micro-organisms are killed and digested.

When fungi and bacteria survive in the bodies of their insect carriers, there is often a mutualistic symbiotic relationship a mutual aid arrangement. The fungi or bacteria may supply digestive enzymes or vitamins that the insect needs. They may condition trees so that insects may breed in them or they may provide a more concentrated source of nitrogen for the insects. In return, the insect protects the microorganism against unfavorable environments, transmits them to susceptible plants, and often makes the wounds through which they infect.

Such associations between insects and plant pathogens are not matters of chance. They are the result of a long evolutionary process. Sometimes insects have developed special organs in their bodies for the purpose of harboring the micro-organism in relatively pure culture. The females of some species that have become dependent upon micro-organisms have special organs and devices in their bodies designed to contaminate the eggs so that the new generation will have a supply of the necessary micro-organisms. A rot of apples associated with the apple maggot fly is caused by bacteria that are transmitted in this way. The female fly has special pouches in the walls of the oviduct that harbor the bacteria and are so arranged that they provide a mechanism that contaminates each egg with the bacteria as it is deposited into the tissues of the apple.

Wind, water, man, and animals also spread plant diseases. Only a few fungus and bacterial diseases depend entirely on insects for their spread and development. Wind scatters spores of pathogenic fungi, but only a small percentage of them fall upon the proper plant under conditions necessary for infection. Most of them are wasted. But spores that are adapted to dissemination by insects are taken usually directly to a plant that is susceptible to their attack and are often deposited in wounds where infection occurs immediately. Thus, wind dissemination is much more wasteful of inoculum and is more subject to the vagaries of the weather; insect dissemination leaves less to chance and is more economical of inoculum. The situation is like the pollination of flowering plants by wind and by insects. The highly developed adaptative mechanisms of insect pollination have evolved from the more primitive and less efficient mechanisms of wind pollination. A similar evolutionary trend appears to exist in the methods of dissemination of fungus and bacterial diseases.

A more striking parallel with insect pollination is found in the rust fungi, in which insects are instrumental in transporting the sexual elements of the pathogen in a manner quite comparable to insect pollination of flowering plants. Many of the rust fungi are heterothallic, which means that they form two kinds of mycelium that differ sexually. Before the rust fungus can complete its entire life cycle, pycniospores of one sex must be transported to receptive hyphae of the opposite sex so that fertilization can be accomplished.

The phenomenon has been studied in the black stem rust of cereals (Puccinia graminis). The rust is heteroecious, which means that it lives part of its life on cereals and grasses and part on the common barberry (Berberis vulgaris). The spores that infect the barberry leaf are of two sexes, designated as + and . They are in the haploid condition and neither one can complete the life cycle until there has been a sexual fusion of the two. The + or spore infects the barberry leaf and forms a mycelium of its own kind (+ or ). From them are formed (on the upper side of the barberry leaf) small fruiting bodies (pycnia), each of which produces myriads of pycniospores of the corresponding sex. Each pycnium produces also a number of short hyphae, which protrude from the pycnium and serve as receptive organs comparable to the stigma of flowering plants. They are known as receptive hyphae. Before aeciospores can be produced on the under side of the leaf, a pycniospore of one sex must reach a receptive hyphae of the opposite sex and fuse with it. The nucleus from the pynciospore passes into the receptive hypha and fuses with a nucleus of the opposite sex. In this process it fertilizes or "diploidizes" the mycelium within the leaf tissues so that aeciospores can be formed.

Since the pycnia producing these sexual organs of different sexes are often separated on a leaf or occur on different leaves, a way is needed to bring the pycniospores to the right kind of receptive hypha. Nature has provided a mechanism that tends to insure successful fertilization. The pycnia are produced on the upper leaf surface on a bright orange spot. The pycniospores are liberated in a drop of nectar that has a high sugar content and a fragrant odor. The bright spots, the fragrance, and the food attract insects of many kinds, especially flies. The insects in feeding move from one pycnium to another and by transporting the spores to receptive hyphae bring the opposite sexes together.

Since individual diseases may be transmitted in several ways, it is desirable to know the relative importance of the different methods. Such information often is necessary for working Out effective control measures. It has been proved that the bacteria causing wilt of cucurbits survive the winter only within the bodies of cucumber beetles and in nature are transmitted Only by the beetles. It is obvious therefore that effective control of the beetles Will control the disease. But when insect transmission is one of several methods, the relative importance of each must be established and control measures modified accordingly. An example: The seed-corn maggot is known to transmit blackleg of potatoes and other bacterial soft rots, but it is only one of several means of transmission and even though the insects were completely controlled the diseases would be spread to some extent by other means.

It is especially difficult to determine accurately the importance of an insect vector in relation to other means of spread if one cannot control the vector. We knew for a long time, for example, that the brown rot of peaches might have some connection with the feeding and egg-laying of the curculio, but since no effective means of controlling the insect was available we could get no accurate data as to its importance. When the newer organic insecticides like DDT became available, however, better control of the curculio was possible, and we learned that less brown rot occurred in orchards in which the curculio was checked.

We have no evidence that the curculio is of great importance in disseminating the spores of the brown rot fungus, which are readily wind-borne, but it is obvious that the curculio influences the development of brown rot by making wounds on immature fruits through which the wind-blown spores are able to infect. The brown rot fungus has difficulty in infecting immature fruits if the skin is uninjured, but it grows readily in punctures made by the curculio. Spores formed early on the injured green fruits provide an abundant source of inoculum for the ripening fruit later in the season.

Often more than one kind of insect may transmit the same disease. Since 1881, when M. B. Waite, a pioneer in the Department of Agriculture, first showed that the honey bee could transmit fire blight of orchard fruits, much has been written, pro and con, about the role of bees in the spread of the disease. The importance of the bee is often overemphasized by failure to recognize that other insects regularly transmit the disease and that wind, rain, and other agents also are involved. The bacteria commonly live over winter in cankers on the larger branches of the tree from which they are liberated in early spring in a sticky exudate.