Dry weather does not equal disease-free crops

So far, this has been a relatively dry year giving lower than normal pressures from our usually troublesome wet season diseases like septoria, rhyncho and net blotch. This could change but for the moment yellow rust and now mildew in spring crops need special attention. This is still not a disease-free year.

Elsewhere, I have written about the real problems that yellow rust is posing in some winter wheat crops in particular but it is important to remember that this disease was also in some winter barley crops during winter so if you have a crop that is not fully protected against rusts, problems could still occur.

Brown rust, should it occur in barley, is a bit easier to control than yellow rust but it is also a potential late season scourge.

While winter barley crops are now all finished with disease sprays, growers are still advised to keep an eye out for the appearance of rusts – just in case.

Mildew

Early indications in 2017 indicate that a few spring barley varieties, especially Propino and Mickle, are picking up a lot of mildew. This disease is also evident in spring wheat and oats. Occasional warmth coupled with bursts of moisture on warm soil provide good humid conditions for its propagation. And the fact that many or most winter barley crops had mildew through the winter provided the inoculum to kick-start spring infection.

So we had the disease source and now we have good growth conditions in spring crops to drive infection. Mildew will need to be controlled in infected crops and this must be done at the earliest possible opportunity. Mildew may not kill a lot of foliage but it can significantly reduce fertile tiller numbers and yield potential.

Resistance will weaken

First, a note about the factors that drive mildew infection. It is ubiquitous in that it is always hanging around on plants somewhere, whether that be on volunteers, stray plants in ditches, regrowth in the fields, etc. Wherever there is a susceptible barley plant there is likely to be mildew on it.

New varieties that initially exhibit good resistance to mildew can quickly begin to fade given the ability of the disease to reproduce and to keep reinventing itself. Varieties can have good resistance but, as with fungicides, continuous exposure of the variety to the disease is likely to result in the production of variants that can overcome that specific resistance.

This happens with all diseases but mildew is always one of the first to overcome the controlling power of a new variety or a new fungicide active.

It is also important to make this same point with regard to yellow rust. This disease has individual races that can be controlled by specific genetic resistance characteristics in individual varieties. So a new variety can be very resistant to all known races but evolution within the fungus can quickly make a very resistant variety totally susceptible.

The situation with regard to disease resistance never stands still. Sooner or later, new strains will emerge that can infect a previously resistant variety.

The situation with mildew is broadly similar but it does not have clear races in its population.

Mildew can occur at any time and in any crop but it is important to remember that it is crop-specific. So just because you have mildew in oats does not necessarily mean that you will get mildew in an adjoining wheat or barley crop. However, when the conditions are favourable for mildew in any crop, they are suitable in all crops.

One of the big factors which influences spring mildew infection is the carryover of disease from autumn to spring. Autumn infection does not necessarily guarantee spring infection but spring infection is less likely where autumn infection did not occur. So sensible crop management in autumn can be key to minimising spring infection risk.

The two main factors here are avoiding crop conditions that favour the disease and not having susceptible volunteers in stubbles that can carry over disease infection from autumn to spring.

Husbandry can favour mildew

Most diseases depend on having living plant material to continue their life cycle. The presence of volunteers helps do this – the green bridge. This cannot be totally prevented but the amount of carryover is proportionate to the likely level of subsequent infection.

Certain husbandry conditions increase the likelihood of mildew carryover in crops. The most obvious is a susceptible variety and early planting adds to that risk. Season also has an impact because you can plant a mildew susceptible variety early and still not get mildew.

High seeding rates also increase the pressure for mildew as the conditions in such crops can be more humid. However, low seeding rates still cannot be used to prevent mildew.

Perhaps one of the most predisposing factors is autumn fertility and especially nitrogen. Mildew is much more likely to be a problem on fresh ground close to grass. Ongoing mineralisation in these situations gives more autumn N, softer growth and a more humid canopy which favours mildew. This is less likely on worn ground and mildew has been much less of an issue in recent decades.

As we move to improving our soils to deliver higher yield potential, they will deliver more autumn mineralisation and this may again improve conditions for mildew. Higher seed rates then add to this pressure.

Autumn application of high rates of high-N organic manures – poultry litter or pig slurry – add to the problem. Large amounts of either of these will drive autumn growth and soften foliage to help disease attack. That said, there may be no issue with lower rates of either. But the improved fertility and soil health generated from frequent application will naturally grow more and softer autumn foliage and give higher yield potential.

Control

Many spring crops now have mildew present. Some varieties are particularly bad, like Propino, Mickle, Barra, etc, and where mildew is already well established you will need both an eradicant morpholine containing either fenpropimorph, (Corbel), fenpropidin (Tern) or spiroxamine (contained in Helix etc).

One of these will need to be used with Talius or Flexity which will help to keep mildew controlled for longer. Morpholines tend to be quick to act but short on persistence and mildew can be a very persistent disease once it is established.

Mildew treatment should be done once the disease is evident. If your first fungicide is already applied don’t wait for the next spray – treat for the mildew now. If your crop has only an occasional mildew pustule you might use either straight Talius or Flexity.

These treatments are additional to the other actives that will be applied to provide general disease control targeting rhyncho, net blotch and rusts.

Read more

Tackling a yellow rust epidemic