Sugarcane mosaic virus

hold a young leaf up to the light and look near the base — you'll see a mosaic, a mix of pale-green, yellow-green and normal dark-green patches, blotches or short stripes running along the leaf, like a shadow-pattern that doesn't belong. It shows clearest on the youngest leaves and fades as the leaf ages.

infected stools grow short and stunted, the leaves may look narrow, and on some varieties you'll see reddish or dead (necrotic) streaks on the leaf or a general lack of vigour — the plant is simply smaller and paler than its healthy neighbours.

with hunger signs: nitrogen shortage yellows the whole leaf evenly (usually the older, lower leaves first), while mosaic is a patchy light-and-dark pattern on the young leaves. Iron or other shortages give long even yellow stripes between the veins, not the broken, blotchy mix of mosaic. When you're unsure, look at the youngest leaf near its base in good light — that is where mosaic shows best.

the virus is favoured by infected setts, high aphid numbers, susceptible varieties, and nearby maize, sorghum, grass and weeds that quietly harbour it.

check hardest while the crop is young — from emergence through the early growing (development) stage, because plants infected young lose the most. Also inspect every batch of seed cane before you cut setts from it.

Walk a large "W" across the field so you cover all four corners and the middle, not just the row beside the path. Make about 10 stops as you go.

At each stop, pick the youngest fully-open leaves on a few stools and hold them to the light, checking the leaf base for the pale-and-dark mosaic pattern. Look at roughly 10 stools per stop.

there is no spray to wait for — so act on the very first symptomatic stool. Find even 1 mosaic stool in 10 and you rogue (dig out and destroy) it the same day, and mark that block as not fit for cutting seed cane.

While you scout, note aphid activity on the leaves (small soft insects clustered on the underside), because warm weather with active aphid flights is when spread speeds up — but remember the real cure is clean setts and resistant cane, not chasing aphids.

no spray kills a virus inside the plant, and spraying insecticide to stop aphids does little, because aphids pass the virus in seconds (non-persistent spread — they infect a plant almost the instant they touch it, long before any poison acts). Don't waste money or risk your health chasing them. If you ever spray for another reason, still protect yourself: cover your skin, eyes and nose, spray in the cool of the day with the wind at your back, keep it off food, water, children and bees, respect the pre-harvest interval (the required wait between spraying and harvest), and wash well afterwards.

step up roguing and switch entirely to certified clean setts and a resistant variety for your next planting. Where a block is heavily infected, don't ratoon it and never cut seed from it — plough it out, rest or rotate the land, and replant clean, resistant cane to start fresh.