Rub a wheat leaf between your fingers.
If they come away yellow, orange or rusty brown, you have rust.
Rusts are among the oldest and most destructive diseases of wheat.
Stem rust has caused regional food shortages in history,
and yellow rust can destroy much of a crop's leaf area before the grain fills.
Rust fungi are unusual.
They feed only on living plant cells, keeping the host alive while draining it. They are highly specialised, down to particular wheat varieties.
And they make enormous numbers of wind-borne spores:
a heavily infected field releases billions of spores a day, which can travel hundreds to thousands of kilometres.
In this video: how to tell the three wheat rusts apart, yellow rust, leaf rust and stem rust in turn, how rusts spread and survive, resistant varieties, field practices and scouting, fungicides chosen by FRAC group, and safety.
The advice is general and global.
Your local extension service and the product label always come first.
There are three rusts of wheat, and each has its own look and its own weather.
Yellow rust, also called stripe rust, makes small yellow-orange pustules lined up in stripes between the leaf veins.
It is the coolest of the three.
Leaf rust, or brown rust, makes small, round, orange-brown pustules scattered over the leaf, mostly on the upper surface. It likes mild weather.
And stem rust, or black rust, makes large, dark reddish-brown pustules with torn, ragged edges, on stems and leaf sheaths as well as leaves.
It is the most heat-loving.
All three need hours of leaf wetness from dew or rain to infect.
And all three rub off on your fingers as powder.
Septoria and nitrogen deficiency also yellow wheat leaves, but they make no powdery pustules.
Yellow rust is caused by Puccinia striiformis. On young plants in autumn and winter, the pustules appear in patches rather than stripes, often in small hot spots of infected plants in a field. From stem elongation to the flag leaf, they form long yellow-orange stripes along the veins, and leaves turn yellow and dry from the tips.
Later, pustules reach the ears, black stripes form, and the grain shrivels. Infection is favoured at about seven to fifteen degrees Celsius with dew, and needs about three to six hours of leaf wetness. Then the fungus grows hidden inside the leaf for about ten to fourteen days, longer in winter. That is why infections that started weeks earlier seem to appear suddenly in warm spring weather.
Losses of ten to seventy per cent are reported in susceptible varieties in epidemic years, and total loss is possible when infection starts early.
Since about two thousand, new aggressive strains adapted to warmer temperatures have made yellow rust the most widespread wheat rust in many regions.
Leaf rust, caused by Puccinia triticina, is the most common and widespread rust of wheat. Its pustules are round to oval, about one and a half millimetres across,
orange-brown and scattered, mostly on the upper leaf surface, often with a pale halo.
It infects from about ten to thirty degrees, best at fifteen to twenty-two with dew, and makes new pustules in about seven to ten days in warm weather. So it runs through many cycles in one season.
Leaf rust rarely destroys a crop outright.
Annual losses are typically about five to fifteen per cent in affected areas,
rising to thirty to fifty per cent in susceptible varieties when infection is early and severe. The loss comes from fewer and smaller grains, because the flag leaf dies early. But it occurs almost every year in so many regions that it causes more total loss worldwide than the other rusts.
Stem rust, caused by Puccinia graminis, is historically the most feared disease of wheat. In a warm, humid season, it can turn a healthy-looking crop into a tangle of broken, shrivelled stems within weeks.
Look for large, elongated, brick-red pustules on stems, leaf sheaths, leaves and ears, bursting through the surface with ragged edges.
Both sides of the leaf are pustuled. Late in the season, the pustules turn black.
Stems weaken and break, and the grain is shrivelled and light.
Stem rust spreads fastest at about twenty to thirty degrees with dew or rain,
and it needs about six to eight hours of moisture to infect.
Epidemics can cause losses of fifty to seventy per cent, and total loss of a susceptible crop.
For decades, resistant varieties kept it in check. Then the Ug99 race group, found in Uganda in nineteen ninety-eight,
overcame widely used resistance and spread through Africa and the Middle East. Other new races caused the first major epidemic in decades in Sicily in twenty sixteen.
Rusts have up to five spore stages, but one drives the epidemics: the repeating stage, the powdery spores you rub off a leaf. They complete a cycle in a week or two, so numbers grow very fast while the weather suits them, and the wind carries them between fields, regions and even continents.
Between seasons, the rusts survive mainly on green volunteer wheat and grasses. This is called the green bridge.
Some rusts also have an alternate host.
Stem rust completes its sexual cycle on barberry,
and yellow rust was shown to use barberry too, in twenty ten.
Leaf rust uses meadow rue in some regions.
On the alternate host, sexual recombination creates new races,
and that is why barberry removal campaigns greatly reduced stem rust epidemics in North America and Europe.
Resistant varieties are the backbone of rust control.
Resistance genes are named by rust: Sr for stem rust, Lr for leaf rust, and Yr for yellow rust.
Race-specific genes give strong protection, but a new race can overcome them. That is what Ug99 did to the stem rust gene Sr31. Adult-plant, or slow-rusting, resistance is partial but durable.
One gene, Lr34, also known as Yr18 and Sr57,
gives partial resistance to several rusts at once, and has stayed effective for decades.
Breeders stack several genes together.
For the farmer, that means three things.
Choose varieties with good resistance ratings for your region.
Do not grow a single susceptible variety over a large area.
And remember that ratings can go out of date within a few seasons, when new races arrive.
Break the green bridge: destroy volunteer wheat and grass hosts before sowing.
Sow at the recommended time. In regions where autumn yellow rust is common,
avoid very early sowing, and late-sown crops are exposed longer to warm-weather stem rust.
Avoid excess nitrogen.
And where common barberry grows near wheat fields, remove it.
Scout from late winter for yellow rust patches,
and from stem elongation inspect the top three leaves,
and the stems and sheaths too, for stem rust.
Estimate how much leaf area is covered, and compare your variety with its resistance rating.
Follow national rust advisories and surveillance alerts.
And if you suspect stem rust, report it to your national plant protection service, so samples can be race-typed.
Laisha names fungicides only by their FRAC group. There is no universal numeric threshold for rusts. The decision depends on the variety's resistance, the crop stage, and local advice.
For yellow rust, advisory services act at the first active patches in susceptible varieties during stem elongation. For stem rust, fungicides are an emergency tool, most effective at first detection.
And in every case, protecting the flag leaf and the ear gives the most benefit.
Three groups are used against wheat rusts. FRAC group three, the demethylation inhibitors, or triazoles, are systemic, with some curative activity while the fungus is still hidden in the leaf.
Group eleven, the quinone outside inhibitors, or strobilurins, are mainly preventive and usually used in mixtures.
And group seven, the succinate dehydrogenase inhibitors.
Fungicide resistance in rusts has been rare so far, but use mixtures of groups, and limit the number of applications per season.
Use only products registered for wheat in your country, at the label dose, and respect the pre-harvest interval.
Now, safety.
Read the label before you buy, mix or spray.
Wear what the label asks for: chemical-resistant gloves, long sleeves and trousers, boots and eye protection.
Gloves matter most, because most farm poisoning happens through the skin,
and handling the concentrate while mixing is the riskiest step. Mix outdoors, and do not eat, drink or smoke while you work.
Calibrate your sprayer, spray when the wind is light, and leave a buffer zone near water.
Store pesticides locked, in their original containers, away from food, feed, children and animals.
Triple-rinse empty containers, puncture them, and never reuse them. If someone is exposed, remove contaminated clothing, wash the skin with plenty of water, and get medical help at once, taking the label with you.
To recap.
Yellow rust makes stripes in cool weather, leaf rust scatters round brown pustules in mild weather, and stem rust tears brick-red pustules on stems in warm weather. All spread by wind-borne spores in repeated cycles,
and survive on the green bridge of volunteers and grasses. Grow resistant varieties and diversify them.
Destroy volunteers, sow on time, avoid excess nitrogen and remove barberry.
Scout the top leaves and the stems from stem elongation,
and report stem rust.
If you spray, choose by FRAC group, mix groups, protect the flag leaf, and follow the label.
For every pest and disease, and full grow guides, visit laisha.ai. Thank you for watching.
