Plants can become sickened in several ways. They can become stressed from climate conditions, attacked by fungi or bacteria, or even catch a virus. Viruses can enter a plant through a grafting or pruning wound (or a wound made by a yard tool). They can arrive on the backs of insects or even on the spores of fungi. Viruses affect plants in different ways and with different levels of seriousness. While some viruses only weaken or disfigure a plant, others can quickly kill it.
Potato Yellow Dwarf Virus (PYDV)
Potato yellow dwarf virus (PYDV) affects tubers, most notably and seriously potatoes. The virus can affect the plant in any number of different ways, including stunting the growth of the potato and yellowing of both the foliage and root. Cracking of the tuber is also common. Insects are known to transport and transmit this virus; in particular, the common leaf-hopper. Like all viruses, once a plant is infected with this virus, it usually dies and the focus becomes keeping the virus from spreading to other plants.
Cherry Leaf Roll Virus
This virus not only attacks cherry trees, but also American elm and walnut trees, in which it is usually called walnut blackline disease. In cherry trees, the virus causes the leaves to roll up and prematurely drop from the tree. Some cherry trees, especially those weakened from other problems, may die.
In elm trees, the disease causes the leaves to become spotted with a ring-like pattern, after which the tree will begin to experience dieback. Dieback refers to when a tree dies from the outer parts inward, with the tips of branches beginning to turn brown, wither and die before the rest of the tree.
In walnut trees, the cherry leaf roll virus causes grafting tissue to die. Black lines appear on the wood where grafting has occurred. The dead tissue girdles the grafted branch, choking the life out of it, and causing dieback to occur.
Tomato-Tobacco Mosaic Virus Disease
The tomato-tobacco mosaic virus (often it is simply called the tomato, mosaic or tobacco virus) can be found all over the world, according to F. L. Pfleger and R. J. Zeyen, master gardeners with the University of Minnesota. The virus is known to affect over 150 types of plant categories, including weeds, ornamental and crop plants.
While the virus affects plants in different ways, one of the common symptoms includes a mosaic or mottled pattern of light-colored splotches that appear on the fruit or leaves of the plant. The virus enters plants through tiny wounds such as those caused by chewing insects or slight wounds caused by basic handling of the plants. Other symptoms include stunted growth or shriveling of the plant's flowers and leaves.