Stem rust

a rusty, reddish-brown powder on the stems and leaf sheaths that rubs off orange-brown on your fingers, clothes and hands as you walk through the crop — like brick dust or paprika.

look on the stems and leaf sheaths for raised, oval to long brick-red to reddish-brown blisters, roughly the size of a grain of rice, that have torn open the skin of the stem with ragged, ripped edges and shed the rusty powder. They feel rough and bumpy under your thumb.

the pustules turn dark brown to black as the crop ripens, the infected stems weaken and bend or fall over (lodge), and the grain in the head comes out small, light and shrivelled.

with the other rusts: leaf rust makes smaller, round, orange spots mostly on the upper side of the leaf blade (not the stem) and barely tears the surface; stripe (yellow) rust makes bright yellow dots in neat lines/stripes between the leaf veins. Stem rust is the one that is on the stem and sheath, bigger, darker brick-red, and rips the stem skin open.

stem rust hits during the mid and late season — from stem elongation (jointing), through heading, into grain fill. Start checking as the stems begin to lengthen and keep checking through flowering and while the grain fills.

check at least twice a week, and more often during warm days with heavy dew or rain — that is when rust explodes. Also find out from your extension officer or neighbours whether an aggressive race such as Ug99 is around, because it can hit varieties that used to be safe.

walk a large "W" pattern across the field so you cover all four corners and the middle — not just the edge by the path where you always pass.

at about 10 stops, bend down and look closely at the lower and middle stems, the leaf sheaths, and both sides of the leaves. Rub a suspect blister with your thumb — if rusty reddish-brown powder comes off, it is rust.

stem rust has no safe count — the moment you find even the FIRST rusty pustule on a stem or leaf sheath, between jointing and heading, act now. In warm wet weather one pustule becomes thousands within days, so do not wait for it to spread.

if you find rust and the crop is still filling grain, spray a triazole fungicide (a systemic fungicide — one that is taken up inside the plant and protects it from within, unlike a contact spray that only coats the surface). Spray as soon as the first pustules appear, and if warm wet weather keeps the pressure up, spray again around heading.

cover your skin, eyes, nose and mouth; spray in the cool of the morning or evening with the wind at your back so it blows away from you; keep the spray off food, drinking water, children, animals and bees; and after spraying, wait the full pre-harvest interval (the number of days on the label between spraying and harvest) before you cut or graze the crop, then wash yourself and your clothes.

if you must spray more than once, rotate to a fungicide from a different chemical group each time (not just a different brand of the same triazole), so the fungus does not learn to survive the spray.

if rust has already spread through most of the field, the stems are turning black and starting to lodge, and it is too late in the season for spraying to save the grain, stop spending on chemicals. Instead harvest early, before the stems fall flat, or cut the crop for hay or livestock fodder to salvage some value — then next season start clean with a resistant variety, early planting, and no volunteer cereals so the rust has nowhere to survive.