Wednesday, 10 July 2019

The insect-pathogen connection

The insect-pathogen connection

insect-pathogen

Insects and similar organisms, such as mites and nematodes, are involved intimately and commonly in the facilitation, initiation, and development of many biotic and abiotic plant diseases. Some insects, e.g., gall-forming aphids and some mites, cause disease-like conditions in plants on which they feed. The importance of insect involvement in the development of pathogen-induced plant disease is so great that it can hardly be exaggerated. Insects become involved in disease development in plants primarily through the following four types of action. 
(1) Insects visit infected plant organs oozing bacteria or fungal spores or plants covered with fungal spores, become smeared with bacteria or spores, and, quite passively, transfer them to other plants where they might cause disease. 
(2) They cause wounds on plant organs (leaves, fruit, shoots, branches, stems, roots) on which they feed or deposit their eggs and these allow pathogens, primarily fungi and bacteria, to enter the plant. (3) By feeding on plants, especially perennial ones, insects weaken them and make them more vulnerable to attack by some pathogenic fungi. 
(4) Insects act as vectors of certain pathogens, including a few fungi and bacteria, many viruses, and all phytoplasmas and protozoa. Insects carry these pathogens from diseased to healthy plants where they initiate new disease. These pathogens depend totally on insects for transmission, i.e., in the absence of the insect vectors there is no spread of the pathogen and no new diseased plants.

The first type of incidental transfer of bacteria or fungal spores to other plants or organs where they might cause disease probably involves many types of crawling, walking, or flying insects. Some insects walk through or feed on flower nectar, as, for example, do bees in pear blossoms infected with the fireblight bacterium, or on sugars released in infected areas, such as cankers, on stems, or spots or powdery and downy mildews on leaves, or on spots on fruit still on the tree or after harvest. Such insects may include different types of fruit flies, aphids, leafhoppers, beetles, ants, and many others.

Numerous insects feed and cause feeding wounds on various plant organs, e.g., fruits and roots, and several insects cause wounds when they deposit their eggs into such organs. Fungal and, sometimes, bacterial pathogens, such as the soft rot bacterium of potatoes and many other fleshy organs, are facilitated greatly in entering these organs through the wounds made by the insects. For example, the plum curculio beetle creates wounds on fruit during ovipositing. The increased number of entry points for the fungus made on the fruit by insects makes it possible for fungi such as those causing brown rot of pome and stone fruits to be much more damaging in orchards where insect control is poor.

When insects feed on roots, leaves, or shoots of plants, especially perennial ones, the plants not only are wounded in numerous places and allow plant pathogenic fungi and bacteria to enter through the wounds and cause disease, they are also weakened greatly, especially in their ability to mobilize their defenses against pathogens and to protect themselves from becoming diseased. This situation is commonly observed on trees whose roots have been damaged by insects or have been defoliated by insects. In such trees, cankers or root rots, caused by fungi that are normally weak pathogens, develop much more rapidly and cause severe damage or may even kill the entire tree, something that would not have happened in the absence of the damage.

insect-pathogen

The fourth way in which insects influence the development of disease in plants is by forming close associations with certain pathogens. In such specific insect/pathogen associations, transmission and spread of certain pathogens from diseased to healthy plants depend almost entirely on the availability and involvement of one or a few specific insect vectors. For example, the corn flea beetle is the main vector of the bacteria causing bacterial wilt of corn, whereas the striped and spotted cucumber beetles are the main vectors of the cucurbit wilt bacteria. Similarly, without the vectoring ability of two species of elm bark beetles, Dutch elm disease, which is caused by a fungus, would not possibly occur. Certain insects have also formed symbiotic associations with phloem-inhabiting bacteria such as the citrus greening disease bacteria; with specific xylem-inhabiting bacteria, e.g., the planthoppers that transmit the bacterium that causes Pierce’s disease of grapevines; with the xylem-inhabiting nematode causing pine wilt; and with phloem-inhabiting plant pathogenic protozoa causing wilt diseases in coffee and palm trees.

insect-pathogen

The association of certain insects with specific pathogens, however, has reached its greatest frequency with the plant pathogenic phloem-inhabiting phytoplasmas that cause the yellows, proliferation, and decline diseases of numerous plants (e.g., aster yellows, apple proliferation, coconut palm lethal yellowing), and also with many of the phloeminhabiting plant viruses. Phytoplasmas are transmitted by the closely related leafhoppers, plant hoppers, and psyllid insects.

insect-pathogen

Plant viruses, however, are transmitted by one or a few species belonging to the following groups of insects: aphids transmit a large number of viruses, such as potato virus Y; leafhoppers and planthoppers vector numerous viruses, such as the rice grassy stunt virus (as well as phytoplasmas, spiroplasmas, and xylem and phloeminhabiting bacteria); and whiteflies vector geminiviruses, such as tomato yellow leaf curl virus. Other specific virus vectors include certain thrips, beetles, and mealybugs. The mechanisms of transmission of viruses by their insect vectors vary considerably. Although all phytoplasmas and most viruses transmitted by leafhoppers are taken up by the insect vector, circulated internally in its body, and multiply in some of its organs before they are injected into the phloem of new hosts, in many of the viruses, especially those transmitted by aphids, the virus is carried on or in the stylet of the vector and through it is deposited in phloem or parenchyma cells of the new host plant.

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