Grey leaf spot

small, pale, watery spots on the lower, older leaves — the ones nearest the ground. These start low because the fungus splashes up from the old maize trash on the soil, and the bottom of the plant stays wettest and mistiest the longest.

the spots grow into long, narrow, straight-sided grey-to-tan rectangles, like little matchsticks or grey bricks laid end-to-end along the leaf, running the same direction as the veins. The leaf veins act like walls, so the spots stay narrow and boxy instead of round.

many rectangles join side-by-side and the whole leaf turns grey and dries up as if scorched, working upward toward the ear leaf and the top. Badly hit plants dry out early and the stalks go soft and lodge (fall over).

with the round or oval, ragged spots of other leaf blights (northern leaf blight makes long cigar-shaped grey-green blotches with rounded ends, not straight-sided boxes), and not with sun-scorch or fertiliser burn, which hits leaf tips and edges evenly, not in vein-bounded rectangles. Rub a fresh grey lesion: grey leaf spot feels dry and papery with a faint grey fuzz on the underside in the morning dew.

start walking your field from tasselling (when the maize throws its top flower) and keep checking through grain-fill — the mid-season and late-season stages are when this disease strikes and does its harm. Look most closely after a run of warm, misty, humid days with heavy dew, which is the weather it loves.

cross the field in a big "W" shape so you cover all four corners and the middle, not just the row by the path. Pay extra attention to low, damp corners, valley bottoms and misty patches where dew sits longest.

make about 10 stops along your "W". At each stop look at a few plants — check the lower and middle leaves first, then look at the important ear leaf (the leaf that carries the cob) and note how high up the plant the grey rectangles have climbed.

act if you find the grey rectangular lesions on the leaves below the ear at, or just before, tasselling — especially on a variety known to be susceptible in a humid, minimum-till field. In plain terms: if 1 in 5 of the plants you check already show these spots creeping up toward the ear leaf, treat now — waiting until the top leaves grey over is too late.

the spots reaching and passing the ear leaf during grain-fill is the danger point — that leaf feeds the cob, and losing it is what turns a few spots into lost yield.

if you grow a susceptible variety and the spots are climbing toward the ear leaf around tasselling, a fungicide can protect the crop. Use a fungicide of the triazole or strobilurin group (ask your agro-dealer or extension officer to name a registered product) and apply it early, at the first lesions around tasselling — a spray works far better as a shield before the top leaves are lost than as a rescue afterward.

cover your skin, eyes, nose and mouth, spray in the cool of the morning or evening with the wind behind you, and keep the spray away from food, drinking water, children and bees. Respect the pre-harvest interval — the number of days the product says you must wait between spraying and picking the maize — so no chemical is left on the grain. Wash yourself and your clothes afterward.

if you must spray in back-to-back seasons, rotate to a different fungicide group each time (don't use the same one over and over) so the fungus can't get used to it and stop responding.

if lesions still race up past the ear leaf despite spraying, focus on saving the stalks — the weakened plants will lodge, so harvest early any cobs that have filled before the plants fall and rot on the wet ground. Then treat next season as a fresh start: plant a resistant variety, rotate out of maize, and bury every bit of the infected trash so you break the cycle instead of feeding it.