How insect spread diseases of plants and development
Insect cause in the spread and development of plant disease in the following 6 ways
1) Direct production of disease without the help of pathogen.
2) Dissemination of the pathogen.
3) Inoculation of the suscept with the pathogen.
4) Ingression of the pathogen into the suscept.
5) Invasion of the suscept by the pathogen.
6) Preservation of the pathogen.
1) Direct production of disease without the help of pathogen: Toxicogenic insect and toxicosis. For example, Potato leaf hopper (Empoasca fabae) causing “Hopper burn” of potato. After inject phytotoxic substances, potato leaves become burn. Squash bug causing “Anasa wilt” of cucurbits.
2) Dissemination of pathogen: Transfer of pathogen from disease suscept to another suscept/place is called dissemination. We know that pathogen can not cause disease in all stages of it’s life cycle. For example; A fungus can cause disease only in its spore forming stage which is called inoculating stage. If that inoculum is not sprayed widely, diseased production will be less or totally not. That inoculum has not any organ by which they can disseminate. For their dissemination, they depends on the some agents such as wind, water, insect, man and other animals. Though wind is the most important and more agency for spore dissemination but there are some pathogen which can’t be disseminated by wind rather they are entirely depend on insect for their dissemination. For example, ‘Gummosis’ of sugarcane (bacterial disease) caused by flies. The disease first occur in leaves and sticky substances are produced. Their which contains bacterial cells, this sticky substances is attractive to the flies. They eat when sticky substances secret from the leaf but they can’t digest it. As a result, they exerts it to another healthy leaf. Another example of such dissemination is ‘Tungro disease’ of rice and green leaf hopper (Nephotettix virescens) is the only insect which can disseminate tungro virus. Therefore, we can say that insects can help in the spread of plant diseases which can’t be transferred by other agent.
3) Inoculation of the suscept with pathogen: Inoculation means the transportation of pathogen to a particular part of the plant where infection may result. For disease development, Inoculation must be occurred in a particular part. For example, ‘Powdery mildew’ is occurred only in the leaves not other parts of the plant.
Diseased cucurbit → Spore of the pathogen → Leaf of other cucurbit → Disease occur.
Diseased cucurbit → Spore of the pathogen → Soil → Disease not occur.
Diseased cucurbit → Spore of the pathogen → Root → Disease not occur.
Diseased cucurbit → Spore of the pathogen → Fruit/ Flower → Disease not occur.
However, insects play significant role in inoculation. For example, ‘Blossom blight’ of fruits and honey bees. It is a bacterial disease which effects floral parts. Honey bee during collection of nectars from infected flower also collected inoculum and then transfer to other healthy flower. Thus it help in inoculation of “blossom blight”. Another example of insect inoculation is, ‘Sigmatomycosis’ of cotton and Red cotton bug. This disease is also known as “internal boll rot”.
4) Ingression of the pathogen into the suscept: Ingression is the process of growing entrance of the pathogen into the suscept. Most of the pathogen fail to entrance into the suscept through plant cuticle as it is composed of cutin, lignin, Cellulose, hemicellulose, waxy materials etc. That is why pathogen try to enter into the host through wounds or other natural openings. Insects favour this entrance by causing wounds. For example, Green Leaf Hopper and Fungal and bacterial infection occur. In this case, feeding and ovipositional mark predispose in rice plants and fungal and bacterial infection may occur. Other example, White grub and crown gall disease. White grub are soil inhibiting insect, they feed on root by cutting it. When root are cut and open, they are exposed to crown gall pathogen.
5) Invasion of the suscept by the pathogen: Invasion means spreading of the pathogen into the inner tissues of the suscept. Insects help in invasion of the suscept. For example, Brinjal Fruit and shoot borer and sooty mould fungus. These insect makes tunnel into the fruits. Another example, Wood boring beetle and wood rotting fungus. Two species of wood boring beetle (Monochamus scutellatus, Monochamus notatus) eat up the pith portion of the log of the trees. The wood boring fungus invade there by the way made by the beetle and damage the wood.
6) Preservation of the pathogen: All living organism want to survive. Pathogen is not an exception. During adverse weather, the winter insect survive by diapause and also help all the pathogen to survive or preserve. For example, cucumber beetle and bacterial wilt of Cucurbits. Cucumber beetle is the pest of cucurbits and winter is its adverse weather. Winter is also adverse to wilt bacteria when beetles feed on cucurbits. The bacteria into the digestive system of beetle and remain their inactive in winter. Next when summer come, the bacteria become active with the beetle and come through out anus of the beetle and infect cucurbits again. e.g. Corn flea beetles and bacterial wilts of corn.