Cotton Root Rot

Tom Isakeit, Extension Plant Pathologist, Texas A & M University
(Interviewed by Bill Tywrhitt, Auscott Warren, ACGRA)

Tom what are we looking at here today?

This disease is called cotton root rot.  It has other names ‘Texas Root Rot’, or Phymatotrichum root rot.

Where does it originate from?

This is a very unique fungus that is present in the American south west, primarily Texas and Arizona. It is also present in a few of the adjacent states and also in Mexico. As far as we know that’s the only places in the world where this fungus is present.

What sort of yield losses can a grower expect if they have this disease?

Depending on how extensive the fungus is distributed within the fields it can be up to ¼ of the field being affected by it and every infected plant can represent a total loss.

How does the environment interact with the fungus?

This particular fungus likes very high temperature so you start seeing the onset of symptoms when the temperatures approach about 40° or higher and that starts the symptoms. The fungus spreads within a field with water either in dryland conditions when there is a lot of rainfall during the season or under irrigated conditions. It can spread from plant to plant down a row.

Is there an increase in area with the spread each year, does it spread quickly?

Yes, you can get spread within a row, you can get adjacent plants becoming infected and dying just within the space of a few weeks.  It is not going to take out a whole field. It tends to occur in patches within a field and if a grower has a monoculture of cotton, in the following year, if the weather conditions are favourable for it, if there is enough moisture, the affected area will increase in size but this isn’t necessarily a permanent state of affairs. If the grower rotates for several years thereafter, the amount of the fungus in this large area decreases but the fungus always remains in the soil, at least in that initial spot where they have first seen it. 

Do you see this fungus as being a threat to the Australian Cotton Industry?

No I do not. From all of our experience in the United States over 100 years, this fungus is not really spread outside of the areas where it has been first identified. In fields that have been under cotton production for a long time in an area where root rot is known to be present, no one has ever reported seeing root rot all of a sudden. So if this disease is going to be introduced, you are talking about physically moving the resting bodies of this fungus, their survival structure is known as sclerotia. These sclerotia are small, about the size of mouse droppings and for disease spread to occur, they would have to be introduced into a soil. This would preferably be an alkaline soil in a warm area and there should be some moisture present.

Therefore it is not likely to happen as a result of an accidental introduction. We are talking moving large, large quantities of soil containing this survival structure. In other words, there is not a large threat of spreading. It hasn’t really spread in the United States and it hasn’t spread to adjacent fields where there is not history of it. 

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