9 October, 2020
What is causing white heads in your wheat?
BY Paul MacIntosh, Pulse Australia/ AHRI
THE GROWING end for winter crops is drawing closer to many of us in southern Queensland. Many of our cereal crops have died, rather than attained natural physiological maturity.
In many CQ areas, producers have been harvesting their mixed bag of yielding crops for a few weeks now, and no doubt all of us are waiting for a considerable amount of rain to produce a decent summer crop.
Meanwhile, plenty of farmers are growing their cereal crops on primary root growth, which is quite an amazing ability of the wheat or barley plant.
Many times, lately, I have witnessed large cracks in the very dry soil and the tiny cereal plant roots crisscrossing down in the cracked earth from the 45cm (18 inch) wide sowing rows.
These thread-like fibrous roots are also penetrating down into the soil to quite unbelievable depths of 30 to 40cm.
So how does that happen and what happened to our usual secondary roots?
When primary roots develop, they have a root cap on the very tip of the new root. This cap protects the area of new growth and cell division in the meristematic region, and so simply put, the root develops and grows with this cell division occurring every 12 to 36 hours.
However, where does it grow? Does it smell the fertiliser or the moisture pockets and develop toward them?
No, roots do not have noses. It is mostly gravity sensed by the root cap, for a predominantly downwards fashion.
That all makes sense doesn’t it? Except for the question of why those little side roots are going bananas as the plant ages, with our primary root growth only cereal crops.
Like us humans, who when we find a place we like, spread out, roots proliferate or branch below the ground when they accidentally encounter a moisture band with plenty of nutrients in the soil solution. Alternatively, when they run into a hard, cold rock they detour around the obstacle.
So-called ‘secondary roots’ are caused by an increase in the thickness or girth of the plant. They are mostly in our grass crops like wheat, barley and sorghum.
They are very pronounced in maize and many actually describe them as ‘prop roots’, as they provide extra stability to taller and bigger plants. The secondary root development is aided by some decent moisture levels around the plant base, which we have sadly missed in recent times. This has caused increased lodging (falling over) of our cereal crops.
As you drive by or inspect the cereal growing paddocks, you may observe these whitish coloured wheat or barley heads. Now, with the rest of the crop mostly green, these white heads do stand out when doing your regular agronomic scouting duties.
So, for those younger players, these white heads could have three major causes and possibly some minor ones.
Our first consideration should be mice. These damaging pests may be climbing up the cereal plant and chewing under the extended head, just above the top node where there is still a bit of moisture.
The second causal reason for whitish heads is the dry weather.
The third is crown rot. Crown rot is a fusarium type soil borne disease which infects the cereal plant through the coleoptile, crown tissue or sub crown internode. Yield loss can be considerable, and these white heads become very visible during stressful periods of the plant’s reproductive age, especially when combined with dry soils or high temperatures.
Furthermore, frost can cause a bleaching of the grain head and whilst it is not white, it can certainly turn a pale, lucent colour.
I have observed in years gone by a complete tiller including the grain head turning white with white ants, and yes, it is not a common occurrence.
My last reason is drought stress. Essentially, there is simply not enough soil moisture to fill the cereal plant grain positions, at the top of the seed head.
Quite possibly, there are other side issues that can cause this white head syndrome, but mice are probably the only factor you can do something about. However if you have an irrigation set up, it then becomes prevention of drought stressed grain heads rather than a cure.