Yam mosaic virus

look at the young leaves. A healthy yam leaf is one even green. An infected leaf shows a mosaic — patches of light green and dark green mixed together like a shadow map, or a cloudy mottling you notice most when you hold the leaf up to the light.

the sick leaves become twisted and puckered (crinkled, not flat), sometimes narrower or crumpled at the edges. The veins darken into bands (dark lines against paler leaf), and the whole plant looks stunted — short vines that never climb the stake properly.

an infected plant makes fewer and smaller tubers, even though nothing looks rotten. The plant is simply weak from the start of the season to the end.

yellowing from hunger (poor soil / lack of nitrogen) is an even, all-over pale colour with no mosaic pattern and no puckering — feed the plant and it greens up. Drought or sun scorch dries leaf edges brown. Yam mosaic shows that patchy light-and-dark mosaic and leaf distortion that stays no matter how well you water or feed — that is the tell.

watch hardest while the crop is young — through sprouting and early vine growth. This is when the virus does the most damage and when you can still pull out sick plants before they seed the whole field.

every week or two, walk a big "W" shape across the plot so you cover all four corners and the middle — not just the row nearest the path. Make about 10 stops along the way.

look closely at the young, newly-opened leaves of a few plants (they show the mosaic first). Turn a leaf over and check the undersides and growing tips for aphids — tiny soft insects, often pale green or black, clustered in groups. An aphid (a small sap-sucking insect) is the vector — the living carrier that moves the virus from a sick plant to a healthy one.

because there is no cure, the threshold is low. If you find even 1 plant in your 10 stops showing the mosaic pattern, mark it and pull it out (rogue it) that same day. "Rogue" means uproot and remove the whole sick plant so it can't infect its neighbours.

if you planted setts from an unselected or bought-in source you're not sure about and you see aphids active on the crop, treat the field as high-risk — scout more often and be ready to rogue at the first mosaic leaf.

at the very first mosaic leaf, uproot the whole plant and carry it out of the field (don't just cut the vine — the tuber and roots stay infected). Every sick plant you remove early is one less source of virus for the aphids to spread. Also clear weeds in and around the plot: some weeds are hidden reservoirs that keep the virus and the aphids alive between crops.

there is no chemical cure for this virus, and no spray is recommended against it. No spray can remove a virus from an infected plant. Insecticides also do a poor job of stopping this virus, because the aphids pass it on in seconds of feeding (non-persistent spread) — faster than a poison can kill them, and killing an aphid after it has already probed does not un-infect the plant. Spend your effort and money on clean seed and roguing, not on chemicals that will not fix a virus.

if mosaic is showing on many plants and roguing can no longer keep up, accept this season's loss on the worst plants and focus on protecting your seed supply. Harvest and eat or sell the tubers from clearly sick plants — do NOT keep any of them as setts. Mark your few remaining clean, strong vines and save only those for next year's planting, or start fresh next season with certified clean seed. Breaking the sick-seed cycle is the one move that stops the disease getting worse year after year.