PLANT PROTECTION 1 â Pests, Diseases and Weeds

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PLANT PROTECTION 1 – Pests, Diseases and Weeds HOW VIRUSES INFECT HOST PLANTS DISTRIBUTION WITHIN A PLANT It may be assumed that for all practical purposes, even though symptoms only appear on parts of the plant, all living cells within the plant are infected THROUGH WOUNDS MADE By vectors which are the most common and economically serious method of spread. The most important vectors are insects. Mechanically by plants rubbing against each other during handling or pruning. This is not so common or important (exceptions). DEPOSITION IN HOST PLANT MATERIAL Viruses may be deposited in plants via pollen. Scions may become infected when grafted onto infected rootstocks. MOVEMENT THROUGH PLANTS Viruses may move through plants in many ways including: Direct cell-to-cell invasion, eg in leaves viruses may move through 8-10 cells (about 1 mm) per day. Transportation through the phloem may occur rapidly, eg 15 cm in 6 minutes. Most viruses take 2-5 days to move from inoculated leaves. DISTRIBUTION WITHIN A PLANT Some viruses are fully systemic while others leave some tissues virus-free, eg the growing points. Infected plants usually remain infected for a lifetime (page 283). Plants propagated from such material are usually infected. Direction and rate of translocation of a virus in a plant (adapted from Agrios 1997). DETECTION AND IDENTIFICATION Symptoms of rose mosaic Virus particles Diseased bud to healthy plant New growth develops symptoms Virus protein injected into rabbit ELISA Testing DNA tests SYMPTOMS EXHIBITED BY THE HOST PLANT Some viruses cause distinctive symptoms in their hosts and so the disease and the virus can be identified from symptoms. However, frequently this is not possible. Some virus symptoms can be confused with nutrient deficiencies or excesses, herbicide or insect injury (page 275, Table 54). DETECTION AND IDENTIFICATION BY EXPERTS Experts test for the presence of virus in plants, parent stock and certification schemes, eg strawberry, cut flowers, potato, pome and stone fruit, grape. Testing is difficult, slow and expensive. Electron microscopy identifies the shape of viruses particles (rods, bullets or spheres), in plant sap or ultra-thin plant segments. For some viruses, though, the shape of particles is not a reliable means of identification. Indicator plants. Some herbaceous plants, eg tobacco, petunia, readily show symptoms when infected with many different plant viruses. A virus can be transferred by budding, grafting, mechanically rubbing the plant with sap, or by a vector, from a diseased host plant which does not show obvious symptoms, to a healthy indicator species, which does show characteristic symptoms. Serology. Virus protein is injected into a mammal, eg a rabbit, resulting in antibodies in the blood system which react specifically with the virus antigen injected. – ELISA (enzyme-linked immunosorbent assay) is a serological test in which one antibody carries with it an enzyme that releases a colored compound if virus is present. Kits identify some viruses in some hosts, and are a quick, sensitive and specific method of testing large numbers of plant samples. – More sensitive tests are being developed for viruses that accumulate in low amounts in their natural hosts and escape detection, eg carnation. DNA technology. – DNA can be used to detect unknown viruses for which there is no antiserum or information available. Can also be used for detecting woody plant viruses. – PCR (polymerase chain reaction) multiplies over a million times, a short segment of DNA, so that can be seen as a gel. A quick, simple, inexpensive generic test is being developed for the whole nepovirus (nematode-transmitted) group of viruses. They are a group of about 46 viruses that infect many plant families that cause probably the most serious viral diseases of horticultural crops, particularly perennial woody and bulb crops. Many have not been recorded in Australia so quarantine tests are important. 276 Virus and virus-like diseases
PLANT PROTECTION 1 – Pests, Diseases and Weeds VIRUS NAMES AND CLASSIFICATION Plant viruses were originally named after the first host on which they were studied followed by the most obvious symptom caused by the virus on that particular host, eg apple mosaic virus. The disease was called apple mosaic. Because many viruses were first studied on fruit, vegetable and field crops, many viruses affecting ornamental plants have names with fruit, vegetable and field crop connotations, eg turnip mosaic virus may infect stock. To further complicate matters the same virus may cause different symptoms: On the same host under different environmental conditions. On the same host if a different strain of the same virus is present. On a different host. Also on an individual plant, some ‘mosaics’ and other ‘viral symptoms’ may be caused by more than one virus. Symptoms of apple mosaic virus. PhotoCIT, Canberra (P.W.Unger). A virus causing apple mosaic symptoms on apple is called apple mosaic virus and the disease is called apple mosaic Some virus genera Genus names end in virus, eg Potexviruses, eg Cymbidium mosaic potexvirus Potyviruses, eg Potato potyvirus Y Sugancane mosaic potyvirus Turnip mosaic potyvirus Tulip breaking potyvirus Tospiviruses, eg Tomato spotted wilt tospivirus Ilarviruses, eg Apple mosic ilarvirus Rose mosaic ilarvirus Prunus necrotic ringspot ilarvirus Name of virus Name of disease Symptoms Tomato big bud phytoplasma Tomato big bud Tomato – vertical shoots, hard green fruit, split stems Petunia – greening of flower parts Apple mosaic virus Apple mosaic Mosaics are more pronounced in spring, new growth in summer on infected trees may only show mild symptoms or be symptom-free. First host on which the virus was studied Most obvious symptom caused by the virus on a particular host The International Committee on Taxonomy of Viruses (ICTV) oversees the naming and classification of viruses which now shares many features with the classification system used for other biological organisms, eg kingdoms, orders, families, genera and species. Note, however, that some aspects of the naming and classification of viruses differ from that of other biological organisms, eg the genus (ending in virus) comes after the species. All viruses belong to the Kingdom Viruses (Agrios 2005) within which they are divided: Into RNA viruses and DNA viruses then further divided on whether they contain 1 or 2 strands of DNA or RNA and the type of DNA and RNA. Other characteristics used in virus classification include type of protein units, size, and many physical, chemical or biological properties, eg host range, method of transmission, etc. Virus genera (like plant and animal genera) share some significant properties, eg structure, and composition, symptoms, and method of spread. Virus & virus-like organisms Common name of virus Turnip mosaic virus Tomato spotted wilt virus .SCIENTIFIC NAME. Genus & species Turnip mosaic potyvirus Tomato spotted wilt tospivirus Larger organisms belonging to the plant or animal kingdom Common name Humans Twospotted mite SCIENTIFIC NAME. Genus & species Homo sapiens Tetranychus urtica species genus genus species Animals and plants are generally classified according to their DNA, structural characteristics etc into classes, orders, families, genera and species Scientific names indicate the relationship of one plant or animal to other plants and animals Symptoms of tomato spotted wilt virus. PhotoCIT, Canberra (P.W.Unger). Virus and virus-like diseases 277
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Insect secretions PLANT PROTECTION
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Control methods Cultural methods.
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Root structure of dock (Rumex spp.)
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PLANT PROTECTION 1 â Pests, Diseases and Weeds

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PLANT PROTECTION 1 â Pests, Diseases and Weeds