Barley yellow dwarf

What you are looking at

Roughly circular patches in a cereal crop where the plants are shorter than everything around them and a different colour. The patch is most intense at the centre and fades outward into healthy crop, so the edges are gradual rather than sharp.

In barley and wheat the colour is a bright, almost golden yellow, starting at the leaf tip and along the margins and working down. In oats it is red, purple or bronze instead, which is why the disease was called red leaf in oats for years before anyone connected the two.

The oldest leaves go first and the newest leaf stays greener. The plants are stiff and upright, tiller poorly, and carry small heads that fill badly or fail to emerge.

Patches show up in early spring on a crop that looked perfectly well in November. The outer rounds of a field and the ground next to grassy margins are usually the worst of it.

Infected in October, visible in April

Aphids carry the virus and they do it in a particular way. A cereal aphid feeding on infected grass or a volunteer picks the virus up, and after a short period it can transmit for the rest of its life. Bird cherry-oat aphid and grain aphid are the two that matter in most regions.

Autumn is when the damage is done. A crop that emerges in September or early October is standing there as the last aphid flights of the year come off the grass, and every plant infected in those few weeks is a plant that will be stunted next spring. Infection after the turn of the year costs far less, because the plant has already built most of what it is going to build.

So the disease you see in April was settled in October, and by the time the patches are visible nothing can be done about that crop. The whole of the control sits before and around drilling.

Why the sowing date carries the advice

Aphid activity falls off steeply with temperature. Delaying an autumn sowing by two or three weeks moves the crop’s emergence past the bulk of the flight, and it routinely halves the infection without costing anything but patience.

The other half is the green bridge. Volunteer cereals and grass weeds standing between harvest and drilling keep the aphids fed and the virus circulating through the gap, and they deliver both straight into the new crop when it comes up alongside. Clearing that growth several weeks before drilling breaks the chain.

How much the sowing date is worth depends on the winter. Where it turns cold enough to stop aphid flight, the exposure window is short and moving the drill date closes most of it. Where winters stay mild the aphids remain active and keep spreading the virus through the season, so a later sowing helps less and the tolerant varieties carry more of the weight.

What to do about it

Sow later in the autumn where the winter is cold enough for that to matter.

Cultivate off the volunteers and grass weeds at least five weeks before drilling.

Choose a variety carrying Bdv2 in wheat or Ryd2 and Ryd3 in barley on land with a history.

Inspect for aphids in the weeks after emergence rather than waiting for the patches.

Expect the headlands and the ground beside grass to be worst.

Keep the beetle banks and rough margins that feed the aphid predators.

Judge the crop in spring and plan the next sowing, since nothing rescues the current one.

What it gets mistaken for

Nitrogen shortage yellows the oldest leaves across the whole field and corrects when you feed.

Waterlogging follows the low ground and the wheelings.

Rust raises pustules that rub off onto your fingers.

Cereal cyst nematode stunts patches too, and the roots show excessive branching and small white cysts.

Aphids visible on the crop are the carrier rather than the cause of the loss.

Frost and wind scorch the leaf tips across a whole exposed area following the wind.