Viral Plant Diseases

diseases of plants caused by viruses. Various families of flowering, coniferous, fern, water, and fungus plants are subject to infection. Often the harvest of agricultural crops and its quality is sharply lowered.

Viral plant diseases are divided into mosaic and jaundice diseases. The basic symptom of mosaic disease is mosaic-like (uneven) coloring of the leaves, caused by the destruction of the assimilative parenchyma in the plastoid apparatus of the cells of the leaves. The most harmful diseases of this group include tobacco mosaic, tomato mosaic, tomato streak, freckle and stripe potato mosaic, and beet mosaic.

A general chlorosis of the leaves is characteristic in jaundice diseases. Also characteristic are growth derangement (frequently dwarfism), twisting and rolling of the leaves, and excessive accumulation of carbohydrates in the leaves, causing them to be hard and brittle. Beet jaundice, pupation on grains, rolling up of potato leaves, and so forth are caused by the jaundices. Aster jaundice and tomato bigbud are two conditions that used to be in the jaundice group and are now excluded from viral plant diseases, since they are caused by microplasm-like pathogenes.

Mosaic is easily transmitted in the sap of diseased plants during the thinning of seedlings, during later thinning, and when sick and healthy plants touch and there is slight mutual traumatization (for example, caused by wind); it can be transmitted occasionally through seeds, as well as by sucking insects (mostly aphids). Transmission of the virus in these instances takes place purely mechanically. Jaundices are spread mostly by insect-carriers, primarily cicadas. Here, transmission of viruses takes place biologically after a preliminary multiplication of the virus in the body of the insect (the incubation period). Herbivorous ticks, nematodes, and lower fungi can also be carriers of viral plant diseases. It is possible for dodder to carry viral diseases. Almost all viral plant diseases are easily passed to descendants in vegetative multiplication and grafts.

Viruses winter in plants, in their frozen remains, in the carriers, and in sowing and planting material. The age of the plant (most susceptible are young plants), conditions of feeding, and other factors of the external environment have a large influence on the speed of viral multiplication and the appearance of the symptoms of the disease.

Two methods of fighting these diseases are the use of immune varieties and regulation of the period of sowing and harvesting—for instance, in southern regions early varieties of potatoes, when planted and picked early (in July) are less susceptible to disease. Other methods include cleaning the seed remnants of sick plants, fighting carriers and weeds, warming inoculated materials, and other special measures.