Sunflower rust

patches of leaf that yellow, then brown and dry out and die early — starting on the lower, older leaves and creeping upward. A badly hit plant looks scorched from the bottom up while the head is still filling.

turn a leaf over and rub it. You'll find cinnamon to rust-brown powdery bumps (pustules), about the size of a pinhead, that smudge orange-brown on your finger like brick dust. They sit mostly on the underside of the leaf, often ringed by a pale yellow halo, and later spread up onto the stem, the green bracts and the back of the flower head.

late in the season the same spots turn dark brown to almost black and no longer rub off — these are the tough winter spores that survive on the stubble.

with harmless orange pollen dusting the leaves at flowering (pollen wipes clean and leaves no raised bump), with downy mildew (pale, powdery-white or greyish growth on the under side with yellow blotches on top, no rusty brown), or with plain nutrient yellowing (even paling with no powder and nothing that rubs off). Rust is the only one that leaves a raised, rusty, brick-dust smudge.

begin checking from the development stage (knee-high, actively growing) right through flowering and into grain-fill — these are the vulnerable stages, and the danger is highest in warm, damp weather (around 18–30°C) after nights of heavy dew, mist or rain that leave the leaves wet for hours.

once a week, walk a large "W" across the field so you cover all four corners and the middle, not just the edge by the path. Make about 10 stops along the way.

pick a plant and look at the underside of the lower and middle leaves first (rust starts low), then check the newer leaves higher up. Rub any brown spots — real rust smudges rust-brown on your finger.

the key danger sign is rust moving up the plant toward the top leaves and the head. Note how high the pustules have reached each week; fast upward spread in humid weather means act now.

the trigger is about 3% of the leaf covered in pustules on the upper leaves at or just before early flowering — in plain terms, when the top leaves of most plants (say 4 or more in every 10 you check) are clearly carrying scattered pustules and the weather stays warm and wet, it is time to act, especially on a susceptible variety. A few old pustules stuck low on dying bottom leaves is not a reason to spray.

rust spreads fastest once it reaches the upper leaves, so combine the steps above and keep scouting — the earlier you catch the upward climb, the fewer green leaves you lose. There is no leaf to save once it has browned off.

if a susceptible crop hits the threshold at or before early flowering under warm, wet weather, spray a triazole fungicide. A triazole is systemic — it moves inside the leaf and protects a little into the future, unlike a contact spray that only coats the surface. Cover the leaves well, follow the label rate, and repeat only if pressure stays high.

cover your skin, eyes, nose and mouth; spray in the cool of morning or evening with the wind behind you so the mist blows away from your face; keep the spray off food, drinking water, children and grazing animals, and away from flowering weeds where bees are working. Respect the pre-harvest interval — the number of days the label says you must wait between spraying and harvest — and wash yourself and your clothes afterwards.

if you must spray more than once, rotate to a fungicide from a different chemical group each time (for example alternate the triazole with a different mode of action) so the fungus does not learn to survive one product.

a resistant variety plus clean rotation should hold rust to a dull roar. If, despite spraying, rust still races up into the top leaves and the head early in grain-fill, one more well-timed triazole spray on a different group is your last useful move. If the crop is already near maturity and the top leaves are browning off, stop spending on sprays — you cannot revive dead leaves; harvest on time to save what seed has filled, then clear and destroy the stubble and switch to a resistant variety and a longer rotation next season.