Cotton Seed Distributors - Web on Wednesday

Dr. Stephen Allen, CSD Plant Pathologist discusses how to identify the key early season plant disorders and steps that can be taken to avoid them.

Steve what sort of seedling disorders should people be finding this time of the season?

As the plants are just coming up, there’s a whole range of things that can be causing problems. You only want to plant once and there’s a range of things that can effect the stand establishment. If you’ve prepared your beds right, you haven’t planted too early and the temperature has been kind to you, the plants are off to a good start.

A couple of the things that can cause problems; one is fertiliser placement. If you’ve had it dry between the time you’ve put your fertilizer in and the band is under the plant line you can have problems.

Herbicides are another product we use at planting time that can cause problems. If there’s any looseness at all in the beds and you get rain after planting, you can get herbicide washed in the root zone and that can cause problems.


Then you’ve got the seedling disease pathogens themselves. We talk about a seedling disease complex. People have looked and found about 20 or 30 different fungi that can cause seedling diseases but generally we talk about a seedling disease complex and that’s mainly Pythium (right) or Rhizoctonia (below) with black root rot and some other Fusariums, not the bad fusarium but other Fusariums being an issue as well. All of these things can take advantage of seedlings that are growing slowly or cool conditions in cool wet conditions early in the season.

So that we know what we’re looking at, we'll start with the pathogens first. Can you explain what do Rhizoctonia and Pythium look like?

You’d probably be very brave to try and pick up a plant and say this definitely one or the other but there are some basic principles. Rhizoctonia is generally a chestnutty, brown, dry, sunken lesion. It sometimes gurdles the tap root. If you very carefully lift the seedling out with the screwdriver or a knife sometimes they talk about “dancing soil” where the soil is attached to the root and you can get little bits of soil bouncing around the root as you hold it up and look at it against the sky. So, Rhizoctonia is generally a dry lesion, a dry spot, a dry mark, generally chestnutty brown on the roots.

Pythium usually comes in a bit later and it’s more of a wet rot, so you’ll get a complete collapse of the stem. Often you’ll pull up the seedling that’s wilting and when you apply some pressure it’ll bend in one spot and you’ll see that the stem is completely fallen apart in a particular zone and that’s often a sign of Pythium but not always, so you get plants that are sometimes lodging right over.

Then you move onto black root rot (right) and I guess most of you would be familiar with Black Root Rot. The root system is blackened and you can grab it with the thumb and forefinger and just strip off the blackened layer and reveal a white more or less healthy center to the root. That’s typical of black root rot.

As I say, there’s a range of other things that can cause diseases and its possible sometimes to get two or three acting at the same time. You can get Rhizoctonia, Pythium and Black Root Rot all in the same field plus numerous other things as well. Sometimes these seedling disease pathogens can actually take advantage and attack the seedling before it emerges so sometimes you get seed rots.

Generally what you see though is plants wilting, has very small leaves, is just not developing and eventually fading away and dying. All these things are aggravated, if you’ve got problems with herbicides washing into the root zone and cooler water logged conditions.

While we’re on that, what symptoms are typical of herbicide damage?

I guess when you’ve got a good stand, you get rain and within a week or so of the rain you start seeing plants deterioating. In some cases you’ll see parts of the cotyledons developing brown areas, lesions developing on the cotyledons, the cotyledons sort of falling off and plants dying completely. Depending on what herbicide you are using you’ll see a range of different symptoms. Some of the herbicides like Trifluralin will cause some root pruning, you get a swelling on the lower parts of the stem and on the upper part of the roots and you won’t get second root development. Sometimes you’ll see short lateral roots and you’ll find a little blackened spear point on the lateral roots all indicating either herbicide burn or in some cases fertiliser burn.

What about some of the mechanical type damages; fertiliser burn and smearing. What sort of symptoms do you look for there?

Perhaps if we do a fertilizer burn first. One of the things you’ll find  there is that all the roots are terminated at the same level, so if you dig up 20 plants and line them up together you’ll find that at the same point in the roots at a certain depth they are all pinched off, which is indicating that there’s something at a consistent depth and that’s usually a good sign of a fertiliser problem.

Compaction is an interesting one. If you’re tempted to plant when it’s a bit wet and particularly if you get smearing of the slot, there’s a couple of problems. One, the young emerging radical can’t get out of the slot and it does all sorts of curls and loops trying to get through the bottom of the slot and get going down into the soil.

One of the other problems associated with planting under the wet conditions is you might smear the sides of the slot but if you then get hot, dry conditions the slot tends to reopen and you get the so called Kinze Crack where it splits right open down the center of the bed and you need to flush water to close up the crack. So it’s important to be patient.

One, be patient and wait for the conditions to be warm enough to plant and two, be patient enough and let it dry out enough so that it’s not too wet when you are planting.

One of the general principles of establishment in all of the seedling diseases; Black Root Rot, Fusarium and all of those things is don’t go too early. I know it doesn’t always work out that way but generally warmer weather follows cooler weather and those one to two weeks of patience at the beginning of the season sometimes pays off in terms of less seeding disease, less damage to the root system and a better start to the planting season.

All those things we’ve talked about. Is there anything you can do after the stand has come up and then starts getting damaged to try and alleviate those problems?

No, once you’ve planted and watered and it’s all going, it’s either stick with the stand you’ve got or replant. That’s the only decision, there’s nothing you can come along with afterwards and apply to control seedling diseases and Black Root Rot after you’ve planted, so that’s why it’s so important to be patient, do it right the first time and not have to redo it over and over again.

Further Information:  Robert Eveleigh, John Marshall, Craig McDonald or David Kelly