Fungicide Reistance Parts 1&2 Nov 2012

Fungicide Resistance

This is a topic close to me as when I develop programs up for turf managers I always ask what their problems are and when do they occur, how there budget is etc. Pretty much what GCI Magazine just published with their “Get with the plan” story in the September 2012 issue. I go away and devlop a program and usually come back with a varied program to what they are already doing. One thing I always try to do is educate turf managers on is resistance and the need to be aware of it and steps to minimise the onset of it especially with less than less newer fungicide groups being developed. In Australia we have no documented cases of resistance (in fungicides, herbicides or insecticides) in turf, not that there isn’t any it’s just that work has not been done. For those in the US and Canada reading this I’m envious of this service you guys have where you can easily get samples tested for a nominal fee from local universities. We don’t have that service in Australian turf.

With this post I wanted to outline the basics of resistance and give you a few helpful tips when choosing the product for the job. You can relate this back to herbicides and insecticides but I will be concentrating on fungicides. 


On a side note there was an interesting blog post by one of our horticulture retailers on herbicide reistant rye grass. Have a quick read on this as in turf we use the SU Group B herbicides that they talk about. Click here

What is resistance?

Resistance occurs when a fungus which was sensitive to a fungicide becomes resistant to it (Vargas). Another definition is “resistance is a genetic adjustment by a fungus that results in reduced sensitivity to a fungicide. “(Damicone)

There are 2 strains of fungus in turf. These are the:
•    Wild Type Strain &
•    Resistant Strain

The Wild Type strain is the natural fungus in the turf that has been present before any fungicide has ever been used. The fungus is sensitive to the fungicide and thus the fungus is eliminated.

The Resistant Strain is the fungus that is not eliminated by the fungicide.The build up of the resistant strain is caused by repeated use of the fungicide and the selectivity of the fungicide against the wild type strains and for the mutant resistant strain. Thus the fungicide only works on the sensitive strain and not the resistant strain, which in turn becomes an increasing proportion of the total fungus population, as long as that fungicide is continually used as a selection agent.

Keep in mind that it is the mutation of genes that causes resistance. The fungicide applied works on the fungus that is the wild type strain thus allowing an increase in the resistant strain. Once the resistant strain is dominant and the wild type strain is the minority the fungicide will no longer be able control the fungus, hence resistance. Another way of putting it is “The fungicide selectively inhibits sensitive strains (Wild Type) but allows the increase of resistance strains (Damicone).”


Chooisng the right product for the job

There are many products out there to control the same disease in most cases. Some are better than others.

What’s important here is when choosing a products is ask yourself the following:

  • Is it turf registered on the disease you want to target  
  • Is there more than one disease you need to treat
  • Is it curative or preventative spray
  • Do you need a systemic or contact fungicide or both
  • Is it worth doing a tank mix
  • Will you need a reapplication of another productin 10-14 days time
  • What else are you doing to get recovery from the turf
  • What resistance group is it- am I applying too much of this group  

One of the most common complaints of fungicides is that “the product did not work or work as well as expected”.There are many factors that are the more likely to cause this rather than resistance.Resistance can only be proven by scientific means.

Keep in mind: 

  •  Right rates are used
  • The fungicide is applied correctly with the correct equipment, water volume and  timing. Understand how the chemical you are using works.
  • The spray equipment is calibrated correctly and running efficiently. Especially make sure the nozzles are in good working order and they are the correct type.
  • The more established the disease the harder it is to eradicate it-  hence  there may not be as long residual as expected from the fungicide and follow up applications at shorter intervals will need to be made.
  • If you have resistance to a  fungicide group on one disease you can still use that fungicide group on other diseases. For example if you have resistance to Ippon (Iprodione) on dollar spot you can still use it for brown patch control and other diseases on the label.
  • If the grass is too weak not even the best fungicide will revive it-hence recovery is essential to minimise re-occurrence of the disease.

I will have part 2 a little later on

Any questions as always please email me at



Nadeem Zreikat


Part 2

In this 2nd part I will concentrate on  and differences between contact and systemic fungicides and different strategies you can use.

Contact Fungicides 

Contact fungicides are multi site fungicides and have a minimal chance of resistance due to the fungicide attacking many different vital systems of the fungus and have multiple modes of action. They form a protective barrier around the plant tissue (i.e. chemical barrier between the fungus and the plant). They do not penetrate the plant. They generally last only 7-14 days depending on the physical removal by mowing, physical wear by players, sunlight and rainfall/watering New shoots are not protected. Contact fungicides generally only work on a preventative basis. Examples are Dacogreen WeatherShield (not prone to washing off due to formulation), Flowable TMTD, Mancozeb

Systemic Fungicides

Systemic fungicides are absorbed by the plant. The fungicide works inside the plant to control the fungus and stop the plant from being infected and will also protect new growth. Hence systemic fungicides work on both a curative and preventative basis. The residual effect comes from the fact that the plant has absorbed the fungicide and, once absorbed water and sunlight is not an issue. However, degradation by the plant metabolism may still occur. 

Systemic fungicides are classified into 4 groups

Full systemic fungicides move up and down the plant. The only product available is Signature.

Basipetal systemic
• Basipetal systemic fungicides are translocated throughout the plant in a downwards direction through the phloem (sap). There are no products currently available.

Acropetally systemic

• Acropetally systemic fungicides are translocated throughout the plant in an upwards direction through the xylem (water transport). Hence it is important to wash these fungicides off the leaf surface so they can be absorbed by the roots. Examples are Tridim and Proplant.

Locally systemic or meso systemic
• Locally systemic fungicides move below the plant surface but will only move very short distances. They have similar characteristics to contact fungicides in that they protect the plant at the point of contact but, unlike contact fungicides, they move into the plant tissue. These are also commonly known as translaminar because, when applied to one surface of the leaf, they are able to move through the leaf to the other surface of the leaf. Examples are Ippon and Protak.

Be aware, even though systemic fungicides have a residual of up to 28 days they may last much less than this depending on disease pressure at the time. If disease pressure is high with wet day/nights, high night/day temperatures and high humidity, the fungicide may not control the disease for 28 days and subsequent applications may be needed even at possibly 10-14 days intervals. If spraying on a curative basis the fungicide is less likely to last the full 28 days as well. Bare in mind using the lower label rate (if available) of the product will also shorten residual and may not have curative properties.

On a side note – if you are continuously spraying and not sure why the issue is not going away look at your plant health. Is there other things at work here such as insect damage, nematodes, heat stress etc.

Strategies for Resistance

There is no wrong or right strategy here. What is important is mixing up your resistance groups. Not just your active ingredients. For example we have Tridim (triadimenol) & Protak (prochloraz) for dollar sport control. Both have different active ingredients but are the same Group 3 (DMI) Fungicides. So if you were using Tridim then followed by Protak you are not doing anything to combat resistance you are only increasing the risk.

My suggestion would be to to do the following to really mix up your groups.Here is an example for dollar spot control with our fungicides.

App 1: Tridim (Group 3)

App 2: Dacogreen (Group M5)

App 3: Ippon or 250GT (Group 2)

App 4: Protak (Group 3) 

App 5: Vorlon (Group 1)

 As you can see in the 5 applications for dollar spot I have used 4 different resistance groupings

Limit the use of high risk groupings. In turf Group 11 (e.g Azoxystrobin, Trifloxystrobin-this is active comes in a pre mix fungicide) and Group 1 (Vorlon Thiabendazole) have a higher risk of resistance. Use these products mainly for preventative measures rather a curative application. There is worldwide resistance documented to Group 11 fungicides in various crops and turf.   

On a side note – when filling out spray records always include what group you used.This will allow you to monitor and recall what groups you have been using throughout the pressure times.

What about Pre Mix fungicides:

Pre Mix fungicides are good tools to help combat a broad range of diseases and can help with resistance management as there are usually 2 (can be more) different groups in a product. For example Headway and Dedicate have the groups 11 and 3 in it and combat a wide range of diseases. However you still need to be aware even if using them you are still applying that group on the disease (so limit the amount of sprays in your program to what is recommended) and these products may have less active ingredient in the product hence you may get a shorter residual control period than the stand alone product.

With fungicides always look to apply them at the right time on a prevention basis in pressure times. If the disease has taken hold it may take multiple applications to get the disease under control and increase resistance along the way. As well being weakened other diseases which are not normally an issue start to become prevelant. I have seen many tests come back with diseases such as phoma and bipolaris from the samples. These are more secondary diseases. You have to ask yourself why are these diseases there in the first place.

Cultural Practices

This is one of the most useful tools in resistance management. If utilised cultural practices can reduce the instance of disease and thus putting less pressure on your fungicide and even reduce the number of applications in a season.

The following cultural practices will help in aiding disease management and improve the turf surface:

Removing dew (use DewCure here or dew brooms) to reduce leaf diseases

  • Rolling greens (this aids especially with dollar spot and anthracnose management)
  • Frequent dusting
  • Spoon feeding with a balanced NPK
  • Raise mowing heights in stressful times
  • Increase air flow and sunlight to diseased prone areas
  • Reduce thatch
  • Improve drainage
  • Keep a balanced soil profile (get a soil test do not guess)
  • Keep mower blades sharp to reduce injury 

All in all this is just a short summary on resistance management. There is plenty of information out there which can be utilised.

If you are interested in reading more on the subject there are two great book that I use on a regular basis:

  • A Practical guide to Turfgrass Fungicides by Richard Latin (one a side note I will be attending Dr Latin’s class at the GCSAA conference and will bring you up to date information here)
  • Management of Turfgrass Diseases by Joe Vargas

Take care and if you want to contact me please don’t hesitate to email me


Nadeem Zreikat


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