Some viral diseases attack our plants and there are sometimes no cure for them and we end up destroying the plants together with the virus at the same time. This is what you should know about viral diseases;
There are wide differences in length of time that different viruses retain their infectivity. In nature, viruses are perpetuated and transmitted principally within plant parts or in insects. The particles therefore are rarely exposed directly to the physical factors of the environment except in finely broken up plant parts, like tobacco dust.
In expressed plant juices some viruses retain their infectivity only a few minutes; others retain it several hours, and still other for several months. Their longevity in insect vectors also differs greatly. Most viruses can survive high temperatures than their host plants can tolerate. The viruses that causes curly top of sugar beets is inactivated by a temperature of 1670 to 1760 F., and tobacco mosaic viruses withstand temperatures up to 1900 F.
On the hand, peach yellows virus is inactivated when its insect vector is kept at 310 C. for 12 days. Resistance to drying differs also. Many viruses are inactivated when plants are dried, but the curly top virus and tobacco mosaic viruses are notable exceptions, as they have been recovered from dried plant parts after 8 and 50 years respectively.
Effects of temperature on viral diseases
The effect of temperature on viral diseases may be great enough to determine their seasonal and regional occurrence. It can also affect the abundance of infection, the length of the incubation period, the concentration of the virus in the host, the degree of expression or suppression of symptoms, the kinds of effects produced, the prevalence of different strains of the same virus, and the degree of injury to the host.
Peach yellows is restricted largely to the cooler regions of the United States because the virus is inactivated at temperatures of about 350 C or higher, which are likely to prevail for considerable periods in the South but not in the North. According to Hutchins, the virus of phony peach is restricted to the roots of peach trees in southern United States and Kunkel suggest that this may be due to the inactivation of the virus at the higher temperature in the above-ground parts of the peach tree.
There are many reports on the effects of temperature on the abundance of infection by certain viruses in the field. Sometimes, of the effect is directly on the number and activity of the vector. But the effect may be directly on the host also. Tobacco mosaic virus, as one example produces more lesions more quickly on Nicotiana glutinosa at 200 C. than at 150, according to best.
The effect of temperature on symptoms and on severity of infection varies with different virus. And sometimes with different strains of the same virus. In general, symptoms are likely to be most pronounced at low temperature and to decrease progressively with increasing temperature. But the reverse can also be true. The severity of tobacco mosaic virus is greatest at lower temperatures, provided they exceed a minimum of 10 0 C. The severity decreases as temperature increases, and symptoms may be masked at 350 C.
Long exposure of infected plants at this temperature may result in the elimination of virulent strains and. The persistent predominance of less virulent ones, according to Holmes. The kinds of symptoms produced by some plants may vary considerably with temperature.
For example, tobacco mosaic virus produces small, necrotic, discrete lesions on Nicotiana glutinosa at low temperature. The lesion increase in size with increasing temperature, and between 280 and 350 C. they tend to coalesce. At 350 and above there is no necrosis; instead, chlorotic blotches are formed, and the virus becomes systemic, according to Samuel.