Late blight

dark green to brown water-soaked blotches on the leaves — they look wet, oily or greasy, as if the leaf was scalded, and they spread outward fast. They often start at the leaf tip or edge and are ringed by a pale, sickly halo where the healthy green fades.

on a cool, damp morning turn the leaf over — under the blotch you will find a soft white downy mould, like a fine dusting of flour or cobweb, right at the edge between the dead brown patch and the living green. This white fuzz is the disease making new spores.

the brown-black rot climbs onto the stems and leaf stalks, which go dark and collapse. On tomato fruit you get greasy brown patches that stay firm, not soft; on potatoes, the tubers show a reddish-brown firm rot under the skin that you only see when you cut them.

with early blight, which makes dry, dark-brown spots with rings inside them like a target or an archery board, starts on the old lower leaves, and has no white mould underneath. Late blight looks wet and greasy, spreads fast, and grows white fuzz on the underside. Frost, sunburn and drought can also brown a leaf, but they never grow that white downy mould.

from when the crop is growing fast and the leaves start to close over the rows (the development and mid-season stages) — this tender, leafy growth is what the disease loves. Step up your checks the moment the weather turns cool and wet (about 10-24 degrees C), foggy, or after rain or a heavy dew, because those are the conditions it explodes in.

every few days, walk a big "W" shape so you cross all four corners and the middle — not just the edge nearest the path. Make about 5 to 10 stops along the way.

look at the lower and shaded leaves first (they stay damp longest), and turn leaves over in the early morning while the dew is still on them, hunting for water-soaked blotches and the white downy mould underneath.

the threshold is the very first plant you find with a spreading water-soaked lesion — 1 sick plant is enough to trigger action, because this disease doubles so fast there is no safe waiting. And when cool, wet weather is forecast to settle in, protect the crop before any spots appear rather than waiting to find them.

the moment you find a lesion, pull or cut the infected plants (and the ones touching them) and destroy them — burn or bury them deep, well away from the field. Do this on a dry day so you don't spread wet spores by hand, and never throw the sick plants on the compost or the cull pile.

if the weather is against you and the disease is moving, use a protectant (contact) fungicide such as mancozeb. "Protectant" means it coats the leaf and stops spores landing on it — it shields healthy leaves but cannot cure a leaf already infected, so it must go on before or at the very first sign, and be re-applied on a tight schedule (about weekly) through wet spells and after rain washes it off.

cover your skin, eyes, nose and mouth; spray in the cool of the morning or evening with the wind at your back; keep the spray off food crops you'll eat soon, off drinking water and rivers, and away from children and bees. Respect the pre-harvest interval — the number of days the label says you must wait between the last spray and eating or selling the crop. Wash yourself and your clothes afterwards.

don't lean on one product forever — rotate to a different chemical group each time so the disease can't get used to it, and never rely on a curative/systemic product alone. Protectant sprays like mancozeb used preventively are the backbone.

when the tops are collapsing and you can't hold it, cut off and remove all the potato foliage (the haulm) and let the spores on the ground die. Then wait about 2 to 3 weeks before lifting the potatoes, so the tubers harden their skins and any spores on the soil surface die off — this stops the rot following your crop into the store. Never bag or store soft, rotting or blighted tubers; sort them out, because one rotten potato will rot the whole sack.