The Relation of Insects to the Spread and Developmant of Plant Diseases

7/29/2019 The Relation of Insects to the Spread and Developmant of Plant Diseases

1/7

THE RELATION OF INSECTS TO THE SPREAD AND

DEVELOPMANT OF PLANT DISEASES

HISTORICAL

The parasitic nature of the disease was not known until the middle of nineteenthcentury. Before that, autogenic theory of plant diseases was prevailed. Fungi used to

be found associated with the diseased trees but they were thought to be the result of

disease rather than the cause. So, the association of the of the association of insects

and plant pathogens could not be understand at that time.

In 1853, De Bary publication conclusively proved that the fungi are theresponsible for diseases in plants.

During the same time Pasteur and Koch presented the germ theory of animaldiseases.

In 1881, Burrill showed that bacteria could cause animal diseases.After these discoveries the researchers started thinking that what are the factors

which are the factors which are responsible in the spread and development of

diseases.

In 1891, Waite proved experimentally insect transmission of plant diseases. 7 years earlier Forbes in 1884, observed tarnished plant bug feeding on

blighted pear twigs transferring disease.

In 1901, Takami showed dwarf disease of rice caused by feeding of leafhoppers.

In 1936, Katsura said Takami did not know the exact reason. He thought that itis due to the feeding of leaf hoppers rather than virus.

In 1912, Allard showed viral disease of tobacco is associated with Aphids.On the other hand research in the field of role of insects in the transmission of

animal diseases was also going on.

In 1878, Manson showed mosquitoes are the vectors of nematode wormcausing elephantiasis in man.

In 1893, Smith and Kilbourne published their classical work on transmissionof Texas fever of cattles by cattle ticks.

In 1895, Bruce proved association of tsetse fly and the nigna disease of cattle.


2/7

In 1898, Ross presented its classical theory of mosquitoes transferring malariain man.

In 1911, Smith reported transmission of bacterial wilt of cucurbits by thestriped cucumber beetle.

In 1920, Rand and Pierce published and article A coordination of ourknowledge of insect transmission of plant and animal diseases.

Appearance of Dutch elm disease alarmed plant pathologists andentomologists on the importance of joint work.

In 1935, Leach reviewed general subject insect in relation to plant diseases.


3/7

HOW INSECTS AIDS IN THE SPREAD AND DEVELOPMENT

OF PLANT DISEASES

Direct disease production Dissemination of the pathogen Inoculation Ingression Invasion Preservation of the pathogen


4/7

THE POSSIBLE ROLE OF INSECTS IN THE ORIGIN OF NEW

DISEASE AND THE EXTENSION OF THE OLD ONE

One another factor of the association of the insects and the plant pathogens we

usually ignore is the role of insects in the origin of new disease. We usually think

disease is due to an organism but it is a biological process which results due to the

interaction of two or more organisms under certain environmental factors. Theorganisms involved in the production of disease may have evolved slowly, but the

disease may have come into existence in different manner. For the purpose of

illustration consider an example of Dutch elm disease. This disease was appeared in

the 1919 in Holland. It was due to the association of a fungus Ceratostomella ulmi

and bark beetles. Ceratostomella ulmi is a fungus which can grow saprophytically on

dead organic matter and the bark beetles feed on the elm trees. Bark beetles during the

coarse of time found a symbiotic association with the fungus. Bark beetles can only

breed on weekened and drying elm trees. So, these bark beetles take the fungus into

the trees. Due to the action of this fungus the trees started to weekened and drying as a

result the bark beetles could breed easily. This example now shoes that how two

organisms found the association and using it in a symbiotic manner and cause a new

disease. With the increase in travel and trade organisms transfer from one area to

another, So, they could find another association which could be very lethal to us. So

we should also neglect that side of the mirror.

Another disease in Argentina is spread by another leaf hopper. The insect said

to be thrive in regions of relatively high humidity. If it should be introduced in USA

and prove to be a vector of curly top, there is a strong probability that the range of the

disease would be greatly extended.


5/7

THE CLASSIFICATION OF THE PHENOMENON OF INSECT

TRANSMISSION OF DISEASES

At various times authors have drawn up systems of classification of different types

of insect classification of diseases. Some of them have been more complete than

other; yet none of them is entirely satisfactory.

Rand and Pierce in 1920: External transmission of parasitic micro organism

External dissemination and direct inoculation by the insectvector

External dissemination and accidental infection without directinoculation

Infection through insect wounds without dissemination by thewounding agent

Internal transmission of the parasitic microorganism Karl Boning in 1929 rejected the first classification on the basis that the

internal and external basis of classification is an artificial one and involved too

much overlapping:

Dissemination without wounding Dissemination with wounding

Insect obligatory Insect not obligatory

A classification on the roles played by insects in the spread and development of

plant diseases was devised by the author and has been used for several years in

teaching a class in Insect in relation to plant diseases. The classification is as

follows:

Wounding of the plant by an insect without dissemination of a pathogen Insect toxicogenic (Psyllid yellows, mealy bug wilt of pine apples

etc.).

Insect not toxicogenic (White grubs and crown galls). Dissemination of a pathogen by an insect without wounding the plant

Mechanical dissemination only (sphinx moth and anther smut, flies andergot).

Biological dissemination Insect not obligatory (Flies and fire blight). Insect obligatory (Theoretically possible but no example

known).

Dissemination of a pathogen by an insect with wounding of the plants Mechanical dissemination only (Bark beetles and blue strain, elm bark

beetles and Dutch elm disease).

Biological dissemination Insect not obligatory (Diptrous insects and bacterial soft rot)


6/7

Insect obligatory (Cucumber beetles and bacterial wilt ofcucurbits).

THE BIOLOGIC AND EVOLUTIONARY SIGNIFICANCE OF

THE ASSOCIATION OF INSECTS AND PLANT PATHOGENS

Research work in phytopathology have, as a rule, considered insects merely as

one of several agents that disseminate inoculum, to be disposed of in the ordinary

routine study of a disease. It is the opinion of the author that the relation of insects to

plant diseases is too important and too complex to be handled in such a summery

manner. The association of insects with plant pathogens is, for the most part, is not a

mere matter of chance but rather a highly organized relationship which, in many

cases, may have arisen out of a close association of insects and microorganisms over a

period of 50 million years or more.

As a biological phenomenon the association of insects and plant pathogens is

in many respects similar to that of entomophilly i.e. insect pollination. All students of

botany and entomology are familiar with the remarkable evolutionary adaptations ofinsects and followers. There has been built up of a constant and regular association of

insects and plants that is mutually beneficial to both parties of the association.

Students of plants that our most primitive flowering land plants were wind pollinated.

Pollination was a matter of chance, and only a few pollen grains reach to the proper

stigmatic surface this necessitated the production of enormous quantities of pollen and

obviously resulted in a tremendous waste of materials. But in one fortunate period

long ago, insects discovered the food value of pollen and perhaps the associated

secretions of nectar and began to make regular visits to those plants producing it. In

this way, the pollen was transported with very little loss to the stigmatic surface of the

proper plant the association had a survival value for both insects and flowering plants

and consequently has persisted throughout the ages, developing to a state of very high

efficiency principle underlying this association is essentially the same as thatunderlying insect dissemination of plant pathogens. Some pathogens are disseminated

only by wind in which case large quantities of inoculums are produced but a relatively

small part of its reaches an infection court and survives. This type of association, like

insect pollination has a survival value and numerous cases have persisted with

remarkably complex adaptations on the part of both insect and pathogen. Survival

value does not predicate benefits by both insect and pathogen, but there are numerous

cases of striking mutual benefits and interdependence. As example may be mentioned

the association between the blue strain of fungi and bark beetles or that between the

bacteria causing soft rot and the seed corn maggot.

With the development of insect pollination the nature of the process lead to an

increase in cross pollination in comparison with self pollination. Cross pollinationincrease the frequency of hybridization, and this in tern stimulated variation a primary

factor in evolution. Thus insect pollination by stimulating hybridization and variation

has speeded up the development of entomophily itself. With the discovery of

heterothallism in fungi, the signification by hybridization in the evolution of fungi is

more clearly discernible.

Brodie (1931) has shown that flies are agents of diploidization in the fungus

Coprinus lagopus a process fundamentally analogus to insect pollination of higher


7/7

plants. Coprinus lagopus a mushroom commonly found on horse dung is a

heterothallic fungus. Each basidiospore formed on the Inky cap mushroom upon

germination forms a unisexual mycelium that is unable to produce the mushroom

stage until it has been fertilized by fusion with a mycelium of the opposite

complimentary sex. Each of these unisexual mycelium produced conidia in drops of

sticky solution that prevents them from being blown around by air currents but that is

attractive to flies, Flies by feeding on these droplets, transport the conidia from onemycelium to another. The conidia promptly germinate and fuse with mycelium of

opposite sex; dipoidization results, followed by the formation of the mushroom stage

the spores may pas uninjured through the digestive systems of the flies.

The extent to which insects are involved in the mechanics of diploidization in

other fungi is a problem for future study, because it has not received much attention

yet.

The Relation of Insects to the Spread and Developmant of Plant Diseases

Documents

Transcript of The Relation of Insects to the Spread and Developmant of Plant Diseases