By: Dr. Mary K. Hausbeck, Michigan State University

Pathogen resistance is a real threat to the most successful of disease management programs. In the laboratory, resistance can be seen on petri plates when pathogens grow despite the addition of a fungicide. In the greenhouse, pathogen resistance can be seen when disease becomes uncontrollable despite the use of fungicides that have been previously proven to work. Another term for fungicide resistance is control failure. Control failure is the result of the fungicide no longer working and the resulting plant loss from disease.

Tips to avoid control failure and keep effective fungicide tools working:

• Attack pathogens when they are most vulnerable. Control should be aimed at the pathogen’s “weak link.”
• Consider how the pathogen is being introduced to the growing area. If the pathogen is coming in as a hitchhiker, how is the pathogen being managed before you receive it? There are lots of examples of a grower unknowingly receiving a pathogen on a crop and then that pathogen is already resistant to the key fungicides!
• Consider fungicide effectiveness. What fungicides have been proven to be effective in controlled studies? Sometimes growers experience control failure and think they have a pathogen resistance problem when the real problem is that they aren’t using a proven effective fungicide.
• Check your spray intervals. If a fungicide isn’t working, the problem may not be fungicide resistance but could be the wrong spray interval. Fungicides suppress the pathogens for a period of time, after which the pathogen will resume its destructive activity. If the interval between fungicide sprays is too long then the pathogen will not be limited and will continue to cause problems.
• Alternate fungicide sprays. Make sure that the active ingredients in each spray differ enough so that the pathogen cannot readily adjust and overcome one particular fungicide type. Relying only on your “favorite fungicide” may give the pathogen ample opportunity to mutate resulting in a pathogen that no longer responds to your fungicide program.
• Use the labeled rates of fungicides. Using less than the labeled amount of fungicide results in a sub-lethal dose that is thought by some to condition a pathogen to tolerate the active ingredient in that fungicide.
• Do not over-apply fungicides. When disease is rampant and a crop is at stake, it is tempting to overdo fungicide rates and the number of applications. This tactic rarely works and tends to cause further problems, including plant burn.
• Keep up-to-date on the newest fungicides as they become registered. If they’ve been proven to be effective, incorporate them into your fungicide rotation program if they offer a new and unique means of managing your pathogen problems.

Choosing the Right Fungicides for Pythium

Pythium is a water mold and “nibbles” the feeding roots of plants, resulting in stunted growth and death. Root rot disease is favored by growing conditions that are “too wet,” such as when media does not drain quickly or when weather doesn’t allow rapid drying. Pythium can be introduced into a greenhouse via plant plugs or other pre-finished plant material. This pathogen can also be a greenhouse “resident” that hibernates on dirty plant containers, benches and greenhouse walkways, ready to become activated by the right plant and weather conditions. Although Pythium can be a problem on many annuals and perennials, it seems to favor geraniums and poinsettias. Sanitation is especially important in limiting root rot. Conditions that favor good plant growth and minimize stress make the plant less vulnerable to attack by a root rot. But if you’ve done everything right and still find yourself with a Pythium problem, choosing the fungicide tools that work can minimize your losses.

If Pythium is diagnosed as the problem, fungicides that are specific for Pythium can be used and could include mefenoxam, propamocarb, etridiazole (Everris Truban®), and aluminum tris. Some greenhouse growers who have struggled with Pythium problems have determined that mefenoxam does not control the disease and have had to rely on other fungicides. While mefenoxam has long been an effective tool against root rot, in some cases Pythium has developed resistance and is no longer controlled by this fungicide. A laboratory screen is available from some diagnostic clinics and can readily determine whether a particular Pythium strain is sensitive to mefenoxam. If Pythium is sensitive to mefenoxam, then this fungicide will be effective in halting root rot. Pythium should be tested each year that root rot is a problem to determine whether mefenoxam can be part of an effective fungicide program. The problem with Pythium and fungicide resistance may also extend to propamocarb. Some Pythium strains that are resistant to mefenoxam may also be resistant to propamocarb. Truban is often used by growers when their particular Pythium is resistant to mefenoxam. Other growers successfully rotate Truban with mefenoxam in a program to control root rot and delay potential problems with fungicide resistance.

Choosing the Right Fungicides for Black Root Rot

While root rots caused by Pythium and Rhizoctonia are the most common among greenhouse crops, black root rot is a serious threat to pansies, petunias and vinca. Black root rot is caused by the fungus, Thielaviopsis, and may also infect cyclamen, poinsettia, primula, impatiens, snapdragon, verbena, phlox, begonia and nicotiana. Plants with black root rot often show symptoms that mimic nutrient deficiencies such as stunting with older leaves shriveling. Leaves may turn yellow and the youngest leaves become stunted and tinged with red. In mild infections, older leaves are yellow-green with the veins retaining their green color. Black root rot may also affect the lower stem on crops such as poinsettia, causing cracks that appear black.

Sanitation is the best preventive measure against black root rot. Once this fungus is established in a crop or in a greenhouse, an effective fungicide program is needed. Based on studies conducted in my lab, I recommend that fungicides with thiophanate-methyl (such as AllBan® or Banrot®) as the primary active ingredient be used frequently at the high labeled rate.

Good rotational products include Terraguard 50W and Medallion since they have different modes of action and have been shown to be effective in my studies against black root rot.

Choosing an effective fungicide to control black root rot is critical because a misstep early in the disease epidemic may result in an unsalable crop. If the crop is treated for Pythium root rot when black root rot caused by Thielaviopsis is really the problem, not only will time and money have been wasted but the disease will have a head start in causing damage to the crop before it can be halted with the correct fungicide.