Tobacco mosaic virus

on the young top leaves, a pattern of light-green and dark-green patches mixed together — like a blotchy map or the skin of an unripe pawpaw. This uneven "mosaic" is the giveaway; a healthy leaf is one even green all over.

leaves get puckered, crinkled, curled, or narrow and thread-like (fern-leaf), and the whole plant grows short and stunted with less fruit. On tomato and pepper the fruit sets poorly and comes out mottled, uneven, or brown-streaked inside when you cut it.

, so a plant may look almost normal at midday and show the pattern again in the cool. Don't assume it has recovered.

with a feeding shortage (that yellows the whole leaf evenly, often oldest leaves first, with no green-on-green patchwork), with spray/herbicide drift (cupping and burning that hits many plants in one patch at once, not a spreading mosaic), or with spider-mite damage (fine pale speckling with tiny webs under the leaf). TMV's mark is the two-tone green mosaic plus twisted, stunted growth.

This virus can strike from the seedling and nursery stage right through early growth and into mid-season, so begin checking in the nursery bed and keep checking every week until the crop is well grown. A single infected seedling in a tray can seed a whole field.

Bend down and look at the newest top leaves of the young plants for the light-and-dark green mosaic — young leaves show it first and clearest. One sick source here spreads to many when you handle the seedlings.

so you cross all four corners and the middle, not just the edge by the path. Make about 10 stops, and at each stop look closely at the young leaves of a few plants.

Because it spreads by touch, look for sick plants strung out along the row in the order you transplanted, staked or pruned — a line of affected plants points to spread on tools and hands.

There is no cure and no "wait and see" number — as soon as you spot a symptomatic plant, pull it and tighten your hygiene (see control). Every day it stands, your hands can carry it further.

the moment you see a plant with the mosaic, pull the whole plant out and destroy it (burn it or bury it far from the field — never drop it on the ground or the compost near your crop). Do the sick plants last, or wash up before touching healthy ones, so you don't wipe the virus down the row.

No fungicide, no insecticide, no chemical cures it, so don't waste money on sprays hoping to treat it. (If you do spray for other pests, still follow safety: cover your skin, eyes and nose, spray in the cool with the wind at your back, keep it off food, water, children and bees, respect the pre-harvest interval — the wait between spraying and picking — and wash afterwards.) For TMV itself, prevention and roguing are the only real tools.

go back over your hygiene — most spread that continues comes from un-disinfected tools, hands or tobacco use — and rogue every new sick plant the day you see it. If most of a nursery bed is already infected, don't nurse it along — destroy the whole bed and start again with clean seed and a resistant variety, because saving a few plants isn't worth seeding your whole field. Next season, plant resistant varieties from the start so the virus has nothing to catch.