PVY (important virus in France)

Potato virus Y (PVY) has long been a common virus of tobacco in France. The use of resistant varieties has reduced its incidence in this country. It is known that this virus can cause a great variety of lesions, such as :
 

• abnormal colours

They can be mottling, veinal or interveinal yellowing of leaves (figures1-4), and darker green areas of the lamina along the veins ("vein banding",figure 3). Sometimes plants show numerous ringspots and /or bright yellow spots in the middle leaves (figure 4). These symptoms, already reported by several authors, could be attributed to a particular PVY strain.; 
 

• necrotic lesions

 - either white spots, but mostly beige to brown ones of different appearance and size, sometimes growing near the veins (figure5a) (see Different viruses in the section of Leaf spots). Virginia tobacco varieties, with a resistance gene to Meloidogyneincognita, show rather specific symptoms when infected with PVY. This is often verified by the appearance of numerous small ecrotic lesions reminding of "etch" (figure 6b);
- or browning of the midrib and /or secondary veins of the lamina (figure 5b and 6 to 8). The origin of the European name of the virus, "brown rib disease" is based on this syndrome. The described syndrome is expressed rather weakly on Burley tobacco, but much more strongly on dark and Virginia tobacco types. When the infections are very severe, the vessels, the stem and the pith show brown to black necrosis (figure 6).
 
On certain genotypes, leaf necroses are more superficial and diffuse, giving to the lamina a tanned appearance. This symptom is due to the death of mesophyll cells of tobacco leaves infected with the virus that are exposed to sun. It may  also be associated with a particular strain of PVY present on mature leaves. Infected leaves are sometimes small, crinkled and curled. If the infection occurs at  early growth stage, it can be greatly reduced (figure 13). The nature and intensity of the symptoms can vary depending on the tobacco type and the variety grown, but mainly on the virus strain involved.
 
Similar to CMV, strains of PVY are classified in multiple  groups, including strains of ordinary Y and necrotic YN, according to their symptoms on tobacco. Among the necrotic strains, some are able to infect  PVY resistant varieties. Those strains have been detected in France particularly in Alsace; they exist also in other European countries (several Eastern European countries, such as Italy,and Switzerland, figures 9 and 10). The occurrence of these strains in the field is not yet very important, but it may explain in some incidences failure of resistance. Comparable isolates were detected  occasionally in, Tennessee, USA causing significant damage on the resistant varieties TN86, TN90 and Polalta. They have been found in other countries of the American continent, including Chile and Argentina.
 
All types of tobacco grown are not equally susceptibleto CMV and PVY; bright tobaccos are by far the most sensitive, especially Burley tobacco and Virginia tobacco to a lesser extent. Dark tobaccos, because of their hardiness and resistance to PVY (including all Paraguay varieties, PBD6, and ITB1000 grown in France), are less susceptible. Those types of tobacco occasionally express quite specific symptoms in the presence of CMV.
 
Simultaneous infections of PVY-CMV are quite common, especially during the summer time. They are responsible for very dramatic symptoms, sometimes resulting in lethal necrosis.
 
Finally, potato virus Y, like CMV is transmitted by aphid vectors in a non-persistent manner. Outbreaks of isolated virus-infected plants can be observed in tobacco fields, before the virus symptoms are widely observed in the field. For further information see the fact sheet of potato virus Y (PVY).