Growth and stem extension issues have become very apparent in some winter-sown spring oat crops. Around six week ago, I wrote about my concerns regarding potential frost damage in winter crops and oats in particular. I was afraid that the cold could have killed the developing ears in many stems as thay moved into stem extension.

I have no evidence that this is widespread, but another related problem has emerged. Over the past few weeks, there have been numerous issues reported from fields up and down the country of crops having somewhat similar symptoms, but the cause is much less easy to diagnose.

Damage symptoms

Crops can look OK from a distance, but things change when you walk in. The symptom is difficult to show on a photograph, but very easy to see.

The crop is at two levels. Depending on the field, up to 80% to 95% of the heads were emerging at 8in to 12in on distorted stems and the panicles may not even fully emerge from those leaf sheaths.

The remaining 5% to 20% of stems are at normal crop height, have healthy-looking leaves and normal big stems.

That lower-level crop is dense and tight. Some have a proportion of dead/red leaves, others are largely green.

Stem extension of the lower internodes was badly affected in all crops, but there was no clean death of the developing ear within the stem post-stem extension. This was the classic damage I was expecting to find.

It would seem that the problem relates to some form of interaction between the use of growth regulators, low temperatures and the specific development stage of individual stems.

I say this because in one crop, the field edge under a tall hedge was completely unaffected and it all got the same sprays.

This may have been something to do with shelter from frost, a slower thawing process or more advanced plants. A field of the same variety and sprayed the same day showed no symptoms.

In affected fields, the majority of the crop was very short, with only some normal stems.

Some of those low tillers were now dead, as were the ears that had developed inside the sheaths. Most had heads emerging, but there was a high level of abortion or blasting on both the top and bottom of those ears.

So, it is difficult to be optimistic about grain numbers, or indeed the ability of these plants to fill these grains.

The common symptom was, not surprisingly, the short internodes. But as well as being short, they were solid and narrow, rather than robust and extremely fragile. Once the leaf sheaths were stripped away, they would easily snap with a gentle shake.

Differing symptoms

While all fields had similar shortening symptoms, there were other issues observable too. The visually short stems had one other worrying and confusing symptom – the presence of white spots and streaks on leaves.

These suggested potential symptoms of oat mosaic virus (OMV). This is a very troublesome and damaging soil-borne virus disease, which would have a permanent impact on rotation if it proved to be the cause of the dwarfing and mottling problems.

Where a crop is infected with OMV, the symptoms would normally be present on all leaves of all tillers on that plant and it is possible that a neighbouring plant would have no symptoms.

However, the healthy stems on these plants had perfectly healthy leaves, suggesting that this is not the virus.

Also, when the fronts of plots at Seedtech were trimmed to even up plot lengths, the regrowth that occurred on those same plants was perfectly healthy, again indicating that this symptom is unlikerly to be OMV.

A proportion of the affected plants and tillers at all sites showed significant stem infection by fusarium.

Watery brown leaf sheaths were very common, but the associated slitting of the stems was not present in every field.

It was impossible to know if this was the chicken or the egg – the cause or the consequence, but my money is on consequence. The weaker, unhealthy stems were fair game for the fungus.

One other symptom that I saw looked entirely like sharp eyespot. This showed the sharply defined lesions on the leaf sheath, plus the proliferation of fungus material inside that leaf sheath, on the stem.

Again, I feel that this happened because the stem was unhealthy, rather than it being the cause of it being unhealthy. And it was not widespread.

It was common on stems to find an area that was totally dead. This may have been tissue kill or something else.

Lessons

It seems to me that at least part of the problem here is caused by the over-regulation of the leaves as well as the stems, which was the objective.

On quite a lot of the dead or rotting stems, it was obvious that both leaf growth and ear emergence were prevented by the fact that the wrap-around sheaths had not grown to allow the emerging leaves and ears to grow, extend or emerge.

This problem, which seems to exist from Cork to Louth, will cause a mess in these crops, as they will mature unevenly and will most likely have very variable grain quality and colour.

If there is one lesson to be learned from this experience, it is the need to be careful with growth regulation in oats.

While specific location is an issue in the occurrence of this problem, we should remember that early spring is a far more sensitive time for PGRs in winter-sown spring varieties than in true winter varieties or the other cereals.

This could be clearly seen at trial sites where winter varieties, sown beside spring varieties, look to be completely unaffected.

Growth regulators work best in times of growth and we have now learned that their use in ‘warm windows’ in periods of no growth can have serious development consequences.