Potato virus

is the leaves change colour and shape. Look for a mosaic — a patchy mix of light and dark green blotches on the leaf, like sun and shadow, instead of one even green. Leaves may also look mottled, crinkled or puckered, and you may see small dead brown flecks (this is the PVY virus).

the top leaves roll upward into tubes, like a rolled cigarette, and go stiff, dry and leathery — they crackle if you squeeze them. The whole plant often stands short and stunted, paler than its healthy neighbours.

the give-away is the harvest itself: the plants get smaller and the tubers get fewer and smaller each year from the same saved seed. A run-down, weak, low-yielding crop is the disease showing its full cost.

with hunger or drought. A nitrogen-hungry plant goes evenly pale-yellow all over, not blotchy; drought or heat wilts the leaves but they recover in the cool evening. Virus mosaic stays patchy green-on-green and never recovers, and rolled virus leaves stay stiff and leathery even after rain — a thirsty plant's leaves go soft again once watered.

watch hardest from emergence through the fast-growing early and mid-season — while the plants are young and the vines are still bulking up. This is when aphids move in and when catching a sick plant early does the most good.

every week walk a big "W" across the field so you cross all four corners and the middle — not just the row by the path. Make about 10 stops and check 10 plants at each stop.

at each plant look at the new top leaves for mosaic, mottling, crinkling or upward rolling, and note any plant that is short and stunted next to its neighbours. Pay special attention to plants grown from your own home-saved seed — that is where the virus hides.

turn a few leaves over and look on the underside for aphids — tiny soft green, pink or black insects clustered on the veins. A yellow pan of water or a yellow sticky card in the field will catch flying aphids so you can see when they arrive.

because there is no cure, act on the very first plants you find. As soon as you see even 1 plant showing mosaic, rolled leaves or bad stunting — or you notice aphid numbers climbing — pull those plants out that same day and act on the aphids. Do not wait for a percentage; with virus, one sick plant is a seed store for the next.

as soon as you spot a plant with mosaic, rolled leaves or stunting, pull the whole plant out, tubers and all, and remove it from the field (burn or bury it). This is called roguing. Doing it early, before aphids spread the virus off that plant, protects the rest of the crop — the healthy plants around it are what you are saving.

an insecticide can knock down aphids, and a light mineral-oil spray on the leaves can slow the virus passing from aphid to plant — but no spray cures an infected plant or stops the disease on its own (it is the weakest tool here). If you do spray: cover your skin, eyes, nose and mouth, spray in the cool of morning or evening with the wind at your back, keep it away from food, water sources, children and bees, respect the pre-harvest interval (the number of days you must wait between spraying and digging, printed on the label), and wash yourself and your clothes after. Rotate to a different chemical group each time so the aphids don't build up resistance.

step up roguing and aphid control, and accept that this crop cannot be un-infected. The most important move is to protect next season — do not save seed from an infected field. Buy fresh certified seed, plant it far from this year's ground, and if a field is badly run down, cut your losses on saved seed and start clean rather than carrying the sickness forward.