The exact nature of the causes of these diseases is not known. However, evidence indicates that they are caused by the so-called filterable viruses. A filterable virus capable of producing a plant diseaseis a microscopically invisible agent in the juices of the plantwhich has the ability to pass through filters that remove all ordinary bacteria and fungi. In addition to their peculiar filterable properties, these viruses are highly infectious, can propagate themselves rapidly and abundantly within the cells of the living plant,but not in any other type of medium. Recent information indicates that a certain few viruses can be separated out of the plant juices in a crystalline form and still retain their infectious naturefor an indefinite period.
The following virus diseases of potatoes are recognized: 1) mild mosaic, 2) crinkle mosaic, 3) rugose mosaic, 4) leaf roll, 5) witch's broom, 6) spindle tuber, 7) calico, and 8) giant hill.
Mild Mosaic. Mild mosaic is characterized by a mottling in the leaves of affected plants. Lighter green areas of irregular size are scattered throughout the normal green of the leaves, these areas being irregularly located in the leaf blades and not limited by the veins. A slight crinkling accompanies the mottling. This crinkling, if near the edge of the blades, causes the edges to be wary. The plants are slightly dwarfed. Tests have shown that the yield of infected plants is reduced by about one third in comparison with that of healthy plants.
Crinkle Mosaic. Even for the initiated it is sometimes difficult to distinguish between crinkle and mild mosaic. The symptoms of both troubles blend into each other to such an extent that differentiation is sometimes impossible. It could be said, therefore, that crinkle mosaic is an exaggerated case of mild mosaic — not that they are the same or that they are produced by the same virus. The mottling is more distinct and the lighter coloured areas are more numerous.