CSD Plant Pathologist Dr Stephen Allen discusses the factors that influence seedling survival.

What are the key factors that influence seedling mortality?

The two key things are temperature and moisture.  You don’t want it too cool and you want enough moisture to plant without having problems with compaction if it is too wet or not enough moisture if its too dry.

Some people will be planting on rain moisture this year. What are the implications of this?

There are several potential problems.  You have got to make sure you have got enough moisture.  Rainfall moisture may not be as much as a pre-watering.  On the other hand, another thing to consider is that rainfall moisture can be variable; you can have more at one end of the paddock than the other end of the paddock.  Another thing that we have seen also is where there has been weed patches the ground is dryer and it takes up more moisture and you get more variability in the field with rain moisture.

You mentioned temperature; CSD has got a network of soil thermometers across the industry and the information for that is on our web site. Most of the thermometers at this stage are showing temperatures in the high teens.  What are the implications of this?

Well that’s generally taken as a good indication; high teens and increasing is good.  Just avoid going too early and look for those high teens when you plant.  The indications are that it should be good for planting.

Is there a critical level in terms of temperature (if there was a number)?  

Some people have used that 16-17° and rising but I think it depends on what time of the month it is or what time of the year it is and what the outlook is in terms of weather forecasts for the next 10 days or so.

Another factor of seedling mortality that we have seen a lot in the past has been herbicide damage.  With the adoption of Roundup Ready® cotton, there has been a lot less planting herbicide used, have you seen a difference in seedling mortality as a result of the reduction in planting herbicides?

Not directly.  It will be interesting just to look at disease survey results over the next few years and see if there is a tie-up between the reduced use of those planting herbicides and Roundup Ready® technology and less seedling disease.  One would expect an improvement because certainly those herbicides can cause problems particularly with rain after planting.

A couple of the diseases that we deal with every year are Rhizoctonia and Pythium; the ‘damping off’ type diseases.  What are the conditions that make these diseases more prevalent generally?

Basically cold conditions or cold shock weather, slow growing seedlings. Cold and wet conditions favour those pathogens and if you have got lots of residues carrying over from a previous crop that can add to the problem as well.

What can growers do to avoid those ‘damping off’ diseases?

Well it is all a matter of timing.  You need high firm beds, avoid those low loose beds. You need the moisture to be right and the temperature right.  If you can get everything right and get your timing right then you should have minimal problems.

In the last couple of years you have done some good work showing that Bion seed treatment has some really good affects on Fusarium and Black Root Rot. It hasn’t had the same influence on those seedling diseases like Rhizoctonia and Pythium; why is this so?

I will have to think about that.  One of the possible explanations would be that Pythium and Rhizoctonia are ubiquitous pathogens; they attack virtually everything; nearly all crops that you grow, even vegetables. Nearly all plants are susceptible to Pythium and Rhizoctonia, they don’t have specialised protection processes and I think the fact that they have developed that ability to be all embracing and attack everything means that they are able to overcome the defence mechanisms on a whole range of plants which means that they are able to overcome the defences that are turned on by the Bion.

Black Root Rot has appeared to be a lot less evident in the last couple of years than it has been previous to that. Can you give us a bit of an idea of why that might be the case?

Black Root Rot certainly is affected by temperature so with warm conditions at planting and following planting you are going to see less of it.  You say that it has been less evident; it certainly has still been around and last year in our trials we had no problem finding whole fields affected with black root rot but certainly once the conditions are warmer, plants grow away from it fairly quickly and it’s not as obvious.

In closing, obviously there is a number of factors that influence seedling mortality and some of them that growers can control and some of them they can’t.  Can you go through just some key points that people can do, what is in their control to try and maximise the level of seedling survival?

Well formed beds, well drained fields, good fertilizer placement (make sure it is not straight underneath the plant where it is going to cause problems).  The Roundup Ready technology gets away from those potential herbicide problems, getting the moisture right, not going too early and being patient and waiting for the temperature to be right. Then in terms of moisture, the critical thing is ‘not too early not too late’, getting the timing right.  Going too early creates the potential problems with smearing slots and compaction as plants are emerging.  Going too late (particularly with these hot days) the moisture may dry back too quickly and the plants won’t make it in time.  So timing is critical.

MORE INFORMATION: http://www.csd.net.au/default.asp?file=/factsonfriday.asp?doc=2251