Mosaic Viruses
Simple Mosaic
Symptoms -- Plants mottle in shades of light and dark green which is more evident in cool and cloudy weather as in the spring (Figure). There is no leaf wrinkling, no vine stunting, and no tuber defects.
Pathogens are Potato Virus M (PVX), Potato Virus M (PVM) and Potato Virus S (PVS) and in combinations. Spread can be mechanically through brushing and rubbing against by equipment and people. Also spread by plant to plant contact by leaves, roots and seed pieces. Cutting knives can also pick up the viruses and introduce them to uninfected tubers pieces.
Control -- Planting certified seed is a must, and field roguing is helpful. Control of Green Peach Aphid will help reduce spread of PVM.
Mild Mosaic
 |
| Mild mosaic mottling |
Symptoms -- Plants are mottled as with Simple Mosaic but more pronounced and obvious. Leaves have light green to yellowish patchiness and there is slight leaf wrinkling. Leaves tend to be smaller. Plants may be slightly dwarfed. There are no tuber defects. Pathogens are a combination of Potato Virus A (PVA) with PVX. Spread can be mechanical but PVA is also spread by aphids.
Control -- Some varieties are resistant to PVA and PVX. Certification and roguing are key. Aphid control may be needed against mid and late season spread.
Rogose and Common Mosaic
Symptoms -- Black streaks appear in leaf veins and on stems. Early-season infection shows shriveled leaves that hang from the stem by a thread of dead tissue. Later in season, the plants become bare with a few leaves on top. Late-season infection does not show any symptoms. Plants from infected seed tubers have mottled and wrinkled leaves that are distorted ("rough") and reduced. Stems are brittle and dwarfing is common. Harvested tuber size is greatly reduced. The primary pathogen is Potato Virus Y (PVY) which may act alone or in conjunction with PVX. There are many strains of PVY with differing characteristics and behavior. PVY is spread by both seed and aphids.
Control requires certification, roguing and aphid control. Control of nightshades in and around potato field is necessary as these host PVY. Other crops that are PVY strain hosts are tobacco, tomato and pepper.
Viruses