Main Symptoms

Unlike what the American name of the disease, "vein banding", would suggest, the symptoms caused by potato virus Y (PVY) on different types of tobacco are numerous and variable. One can observe various patterns of discoloration (figures 1 to 5) resulting in more or less pronounced mottling, vein yellowing ("vein clearing") or greener lamina areas along the veins ("vein banding"). One may notice the presence of numerous rings and / or bright yellow spots on the middle leaves of some plants. These symptoms, previously reported by several authors, can be attributed to a particular PVY strain. Necrotic lesions also occur in several forms (figures 6 to 12):

* browning of the midrib and / or secondary veins of the lamina. This symptom is the origin of the European name of this problem "brown vein disease". When the infections are very severe, vessels, stem and pith show brown to black necrosis;

* mostly beige to brown, with some occasional white spots, of variable size, may appear close to the veins.

On certain tobacco genotypes, leaf necroses is more superficial and diffuse, giving the lamina a tanned appearance. This symptom is due to mesophyll cell death in the virus infected tobacco leaves that are exposed to sun. It may also be associated with a particular strain of PVY present in mature leaves.

Infected leaves are sometimes small, crinkled and curled. If the infection occurs at early stages, growth can be greatly reduced (figure 13). The nature and intensity of the symptoms depends primarily on the strain of PVY involved, but may also vary depending on the tobacco type and variety grown.

As noted earlier, simultaneous infections of PVY and CMV are quite common, especially during summer. They are responsible for far more dramatic symptoms, sometimes resulting in lethal necrosis.

Some of the information on the worldwide distribution of the disease is obtained from surveys conducted by CORESTA (Cooperation Centre for Scientific Research Relative to Tobacco). The reliability of the information is not confirmed in all countries.